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Your Source for HVAC&R Professional Development

Fundamentals of Heating Systems (I-P Edition)

A Fundamentals of HVAC&R Series Self Directed Learning Course

1791 Tullie Circle NE • Atlanta, GA 30329 • www.ashrae.org

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1791 Tullie Circle, NE • Atlanta, GA 30329-2305 USA • Tel 404.636.8400 • Fax 404.321.5478 • www.ashrae.org

Karen M. Murray

Email: [email protected]

Manager of Professional Development

Dear Student, Welcome to the ASHRAE Learning Institute (ALI) Fundamentals of HVAC&R Series of self-directed or group learning courses. We look forward to working with you to help you achieve maximum results from this course. You may take this course on a self-testing basis (no continuing education credits awarded) or on an ALI-monitored basis with credits (PDHs, CEUs, or LUs) awarded. ALI staff will provide support and you will have access to technical experts who can answer inquiries about the course material. For questions or technical assistance, contact us at 404-636-8400 or [email protected]. Skill Development Exercises at the end of each chapter will test your comprehension of the course material. These exercises allow you to apply the principles you have learned and develop a deeper mastery of the subject matter. If you take this course for credit, please complete the exercises in the workbook and send copies from each chapter to [email protected] (preferred method) or ASHRAE Learning Institute, 1791 Tullie Circle, Atlanta, GA 30329-2305. Please include your student ID number with each set of exercises submitted. Your student ID is composed of the last five digits of your Social Security number or other unique five-digit number you create. We will return answer sheets to the Skill Development Exercises and maintain records of your progress. Please keep copies of your completed exercises for your own records. When you finish all exercises, please submit the course evaluation, which is located at the back of your course book. Once we receive all chapter exercises and the evaluation, we will send you a Certificate of Completion indicating 35 PDHs/LUs or 3.5 CEUs of continuing education credit. Please note: The ALI does not award partial credit for self-directed learning courses. All exercises must be completed to receive full continuing education credit. You will have two years from the date of purchase to complete each self-directed learning course. We hope your educational experience is satisfying and successful. Sincerely,

Karen M. Murray Manager of Professional Development

ASHRAE Continuing Education

Fundamentals of Heating Systems (I-P Edition)

Prepared by William E. Murphy, Ph.D., P.E. University of Kentucky

ASHRAE 1791 Tullie Circle NE • Atlanta, GA 30329

ISBN 978-1-931862-31-8 ©2000 ASHRAE All rights reserved. ASHRAE is a registered trademark in the U.S. Patent and Trademark Office, owned by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. No part of this book may be reproduced without written permission from ASHRAE, except by a reviewer who may quote brief passages or reproduce illustrations in a review with appropriate credit; nor may any part of this book be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, photocopying, recording or other) without written permission from ASHRAE. Requests for permission should be submitted at www.ashrae.org/permissions. ASHRAE has compiled this publication with care, but ASHRAE has not investigated, and ASHRAE expressly disclaims any duty to investigate, any product, service, process, procedure, design or the like that may be described herein. The appearance of any technical data or editorial material in this publication does not constitute endorsement, warranty or guaranty by ASHRAE of any product, service, process, procedure, design or the like. ASHRAE does not warrant that the information in this publication is free of errors. The entire risk of the use of any information in this publication is assumed by the user.

ASHRAE Education Department: Tanya R. Fisher, Manager of ASHRAE Learning Institute/Education Bruce Kimball, Managing Editor Halcyone Williams, Secretary

For course information or to order additional materials, please contact: ASHRAE Learning Institute 1791 Tullie Circle NE Atlanta, GA 30329 Telephone: 404/636-8400 Fax: 404/321-5478 Email: [email protected]

Comments, criticism and suggestions regarding the subject matter are invited. Any errors or omissions in the data should be brought to the attention of Bruce Kimball, Managing Editor.

Updates/errata for this publication will be posted on the ASHRAE Web site at www.ashrae.org/publicationupdates. Errata noted in the list dated 12/15/11 have been corrected.

x: 1

Table of Contents

Chapter 1 • • • • •

Introduction

Instructions 1.1 Course Overview 1.2 Terminology 1.3 Reference Materials Bibliography

Chapter 2

Overview of Heating Systems

• Instructions • Study Objectives of Chapter 2 • • • • • •

2.1 Basic Heating System Components 2.2 Fuel Source 2.3 Energy Conversion Plant 2.4 Energy Distribution System Summary Skill Development Exercises

Chapter 3

Basic Selection Criteria

• Instructions • Study Objectives of Chapter 3 • 3.1 Basic Selection Criteria • 3.2 Occupancy and Comfort Considerations • 3.3 Thermal Envelope • 3.4 Ventilation Requirements • 3.5 Regional Preferences • 3.6 Availability of Fuels • Summary • Bibliography • Skill Development Exercises

Fundamentals of Heating Systems

Table of Contents

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Chapter 4 • • • • • • • •

Instructions Study Objectives of Chapter 4 4.1 Commercial Building Types 4.2 Central Multizone Systems 4.3 Forced Air Furnaces and Unitary Heating Systems Summary Bibliography Skill Development Exercises

Chapter 5 • • • • • • • • •

Industrial Heating Systems

Instructions Study Objectives of Chapter 5 5.1 Basic System Considerations 5.2 District Heating and Cooling 5.3 Waste Heat Recovery 5.4 High Temperature Water and Steam Systems Summary Bibliography Skill Development Exercises

Chapter 6 • • • • • • • • •

Commercial Heating Systems

Residential Heating Systems

Instructions Study Objectives of Chapter 6 6.1 System Types 6.2 Single-Family Systems 6.3 Multifamily Systems 6.4 Cooling Towers Summary Bibliography Skill Development Exercises

Table of Contents

Fundamentals of Heating Systems

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Chapter 7

Heating Cost Calculations

• Instructions • Study Objectives of Chapter 7 • 7.1 Energy Estimation Methods • 7.2 Installation Costs • 7.3 Operating and Maintenance Costs • 7.4 Simple Payback Calculations • 7.5 Lifecycle Cost Calculations • Summary • Bibliography • Skill Development Exercises Chapter 8 • • • • • • • • •

Instructions Study Objectives of Chapter 8 8.1 What Are Codes and Standards? 8.2 Safety Codes and Standards 8.3 Performance Standards 8.4 Code- and Standards-Writing Organizations Summary Bibliography Skill Development Exercises

Chapter 9 • • • • • • • •

Codes and Standards

Building Commissioning and Maintenance

Instructions Study Objectives of Chapter 9 9.1 Heating System Design Summary 9.2 Commissioning of Heating Systems 9.3 Maintenance Requirements Summary Bibliography Skill Development Exercises

Fundamentals of Heating Systems

Table of Contents

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Appendices • Appendix A • Appendix B

Terminology Bibliography

Skill Development Exercises for All Chapters

Table of Contents

Fundamentals of Heating Systems

1: 1

Chapter 1 Introduction

Contents of Chapter 1 • Instructions • 1.1

Course Overview

• 1.2

Terminology

• 1.3

Reference Materials

• Bibliography

Instructions This chapter is introductory in nature and contains little technical material beyond the introduction of terminology and the units that will be used in this course. You should review the terminology in Appendix A to learn the terms that are used. Also, an extensive list of references is included in Appendix B if you desire more in-depth analysis of this material.

1.1

Course Overview

This course begins with an overview of the various options available for heating system design, including the distribution system within the building, the energy conversion plant and the fuel source. While these three components are discussed separately, often the selection of one of the three will limit the number of options available for the other two components in the system. While some solid fuel heating systems will be discussed, most heating plants in use today utilize gas (natural or propane), electricity or oil as the fuel. Large industrial systems (which may utilize coal or other solid fuels) are discussed insofar as they may be used for space heating. Their application for process heating or high pressure steam generation is beyond the scope of this course, which addresses primarily residential and commercial building heating systems. Before a heating system can be designed, numerous other factors must be considered. These include the building occupancy, the building envelope, outdoor air ventilation requirements, regional preferences for certain types of systems, and the availability of fuels. These parameters will dictate the size of the system, the type of distribution systems that are appropriate, and the ultimate annual operating cost. The amount of outdoor air that is specified will usually depend on the occupancy of the space and local building codes. Most ventilation Fundamentals of Heating Systems

Chapter 1 Introduction

1: 2

codes in the United States are based on ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality.1 The 1989 revision of this standard increased the minimum outdoor air requirement from 5 cfm per person to 15 cfm per person. This increase in the ventilation air has a significant impact on system design, as it makes heat recovery systems more economically attractive in heating dominated climates while the efficiency of the heating plant has more impact in lifecycle analyses. A significant portion of this course addresses the many types of heating systems that are used in commercial buildings. The great variety of commercial systems attests to the many applications for which heating systems are needed, and to the fact that these systems are practically always designed by engineers. These are two primary reasons why commercial heating systems are often far more complex than those found in residential applications. Commercial systems will usually be designed to take advantage of certain features in the distribution system or the heating plant to more closely match the requirements of the occupants or the building envelope. A large class of heating equipment falls in the category of unitary systems. These are systems that are fabricated at the factory and are installed largely intact at the site. Unitary systems encompass practically all types of distribution systems, heating plants and fuel types, but are usually smaller in size because of practical limitations in transportation and erection. The very large systems will normally be field-erected from many large components according to the engineer’s design. On the basis of sheer numbers, there are many more residential heating systems than commercial, industrial and all other types combined. While larger commercial buildings will typically have a space cooling system installed, a significant number of houses, particularly in Europe, will only have a heating system. Air-conditioning of houses in the United States exceeds 80% in new construction, and so must be a primary consideration in the type of distribution system that is selected for the heating system. The vast majority of air-conditioning systems utilize a forced air delivery system, making a forced air heating system a logical choice to keep installed costs reasonable. Solar energy does hold some promise for certain residential applications, although it is quite difficult to justify economically for larger applications. The economics of heating systems – addressing installation, operating and maintenance costs – are covered in Chapter 7, which also addresses lifecycle costs. It is becoming increasingly important for customers to understand the total costs associated with their heating systems, not simply the installed cost at the time of construction or renovation. The final two chapters in this course address codes, standards and the commissioning and maintenance of heating systems. Building codes are revised approximately every five years, so this course will simply attempt to make you aware of the sources of these codes and the implications they have on the design of heating systems. When designing heating systems, you should always be familiar with the current local codes that apply to that particular job. Chapter 1 Introduction

Fundamentals of Heating Systems

1: 3

Commissioning and maintenance have received more attention in ASHRAE as heating systems have become more complex. Large commercial HVAC systems do not come with an owner’s manual for complete operating and maintenance instructions, so it is not always obvious whether they are operating as they were designed. Independent companies that can conduct system performance analysis and/or system balancing are often an important step in turning over the completed project to the new building owner or operator.

1.2

Terminology

Many of the terms used in this course are listed in Appendix A. An understanding of the HVAC&R terminology is important for a clear understanding of heating system operation. These terms are taken from Terminology of Heating, Ventilation, Air-Conditioning & Refrigeration.2 The engineering units used throughout this course are inch-pound (I-P) units. While the United States is steadily converting to the metric-based SI system, progress is slow in the HVAC&R industry where equipment life typically exceeds 30 years. Table 1-1 lists the most common units that are used for the many variables in this text, while Table 1-2 gives factors to convert between the I-P and SI units.

1.3

Reference Materials

A list of general references is included in Appendix B if you want to go into depth in any of the topics covered in this course. This is only intended as an introductory course to the subject of heating systems. Once completing this course, you should review some of the references for further study in these areas.

Bibliography 1. ASHRAE. 1989. ANSI/ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality. Atlanta, GA: ASHRAE. 2. ASHRAE. 1991. Terminology of Heating, Ventilation, Air-Conditioning & Refrigeration. Atlanta, GA: ASHRAE.

Fundamentals of Heating Systems

Chapter 1 Introduction

1: 4

Table 1-1. Dimensions and Units Commonly Used in Air-Conditioning Applications Dimension

SI Unit

IP Unit

Acceleration

m/s 2

ft/sec2

Area

m2

ft 2

Density

kg/m3

lbm /ft3

Energy

N⋅m, Joule (J)

Btu, ft⋅lb f

Force

(kg⋅m)/s2 , Newton (N)

pound (lbf )

Length

m, meter (m)

foot (ft)

Mass

kg, kilogram (kg)

pound mass (lbm )

Power

J/s, Watt (W)

Btu/h

Pressure

N/m2 , Pascal (Pa)

psi

Specific Heat

J/(kg⋅°C)

Btu/(lbm ⋅°F)

Time

second (s)

second (sec)

Temperature (absolute)

degree Kelvin (K)

degree Rankine (R)

Temperature

degree Celsius (°C)

degree Fahrenheit (°F)

Thermal Conductivity

W/(m⋅°C)

Btu/(h⋅ft⋅°F)

Thermal Flux Density

W/m2

Btu/(h⋅ft 2 )

Velocity

m/s

ft/sec, ft/min, fpm

Volume

m

ft 3

Volume Flow Rate

m3 /s

ft3 /sec, ft3 /min, cfm

Chapter 1 Introduction

3

Fundamentals of Heating Systems

1: 5

Table 1-2. Unit Conversion Factors Dimension

SI Unit

IP Unit

Length

1 m = 3.281 ft

1 ft = 0.305 m

Area

1 m2 = 10.76 ft 2

1 ft2 = 0.0283 m2

Volume

1 m3 = 35.32 ft 3

1 ft3 = 0.0929 m3

Mass

1 kg = 2.205 lb m

1 lbm = 0.435 kg

Force

1 N = 0.2248 lb f

1 lbf = 4.448 N

Energy

1 kJ = 0.9478 Btu 1 J = 0.7376 ft⋅lb f 1 kWh = 3.412 ×10 3 Btu

1 Btu = 778.2 ft ⋅lb f = 1.055 kJ 1 ft⋅lb f = 1.356 J 1 Btu = 2.930×10 -4 kWh

Specific Energy Specific Enthalpy

1 kJ/kg = 0.4298 Btu/lb m

1 Btu/lb m = 2.326 kJ/kg

Power

1 W = 3.412 Btu/h 1 kW = 1.341 hp 1 kW = 0.2844 ton refrigeration

1 Btu/h = 0.293 W 1 hp = 2545 Btu/h = 0.746 kW 1 ton = 12,000 Btu/h = 3.516 kW

Pressure

1 Pa = 1.450×10 -4 psi 1 atm = 101 kPa

1 psi = 6.897×103 Pa 1 atm = 14.7 psi = 29.92 in. Hg

Temperature

1ºC ΔT = 9/5ºF ΔT yºC = [(9/5)y + 32]ºF K = ºC + 273.15

1ºF ΔT = 5/9ºC ΔT yºF = (y – 32)(5/9)ºC R = ºF + 459.67

Velocity

1 m/s = 1.969 ×10 2 ft/min

1 ft/min = 5.079 ×10 -3 m/s

Mass Density

1 kg/m3 = 6.243 ×10 -2 lbm/ft 3

1 lbm /ft3 = 16.02 ×10 1 kg/m 3

Mass Flow Rate

1 kg/s = 2.205 lb m/sec 1 kg/s = 7.937×10 3 lb m/h

1 lb m/sec = 0.4535 kg/s 1 lb m/h = 1.260 ×10 -4 kg/s

Volume Flow Rate

1 m3/s = 2.119×10 3 cfm 1 m3/s = 1.585 ×10 4 gal/min

1 cfm = 4.719×10 -4 m 3/s 1 gal/min = 6.309×10 -5 m 3/s

Thermal Conductivity

1 W/(m ⋅°C) = 0.5778 Btu/(h⋅ft⋅°F)

1 Btu/(h ⋅ft⋅°F) = 1.731 W/(m⋅°C)

Heat Transfer Coefficient

1 W/(m 2⋅°C) = 0.1761 Btu/(h ⋅ft 2⋅°F)

1 Btu/(h ⋅ft 2⋅°F) = 5.679 W/(m 2⋅°C)

Specific Heat

1 J/(kg⋅°C) = 2.389×10 -4 Btu/(lb m⋅°F)

1 Btu/(lb m⋅°F) = 4.186×10 3 J/(kg ⋅°C)

Fundamentals of Heating Systems

Chapter 1 Introduction

2: 1

Chapter 2 Overview of Heating Systems

Contents of Chapter 2 • Instructions • Study Objectives of Chapter 2 • 2.1

Basic Heating System Components

• 2.2

Fuel Source

• 2.3

Energy Conversion Plant

• 2.4

Energy Distribution System

• Summary • Skill Development Exercises

Instructions Read the material in this chapter for general content, and re-read the parts that are emphasized in the summary. Complete the skill development exercises without consulting the text, then review the text as necessary to verify your solutions.

Study Objectives of Chapter 2 Chapter 2 introduces the basic structure of heating systems in their simplest form: the source, plant and system. It addresses the issue of how selecting one component in the system may limit the options for the other parts of the system. The various options that are in common usage for each component in the heating system will be discussed briefly. You should complete this chapter with an appreciation of the many options that are available to heating system designers.

Fundamentals of Heating Systems

Chapter 2 Overview of Heating Systems

2: 2

2.1

Basic Heating System Components

While there are dozens, and in some cases hundreds, of components that comprise heating systems, they can be categorized into three basic functions: • Fuel Source. This includes the fuel itself and also the infrastructure needed to deliver it to the heating plant at the building site. • Energy Conversion Plant. This is the collection of equipment needed to convert the fuel into a useful form of heat energy that can be distributed to the different parts of the conditioned space. • Energy Distribution System. The system of pipes, ducts, fans, pumps, controls, coils, mixing boxes and dampers is perhaps the most complicated part of the heating system, and the part most likely to fail to meet its design goals. The energy distribution system is usually designed exclusively for the particular project in which it is installed.

2.2

Fuel Source

The choice of fuel is perhaps the most fundamental decision that must be made before the heating system can be designed. For instance, a small industrial plant located in rural Pennsylvania may have access to cheap coal that it uses to generate steam for its process applications. For this application, the specification of a steam heating system for the office areas that uses the plant's coal-fire generated steam may be a very good economic selection. In contrast, if that same industrial plant was located in downtown Pittsburgh, the fuel of choice may be natural gas or fuel oil due to space limitations, surface water runoff regulations, air pollution restrictions, fuel transport logistics, or other factors. As another example, there may not be many fuel options for the condominium owner who lives on the fifth floor of a Pittsburgh high-rise building. Coal is obviously out of the question, although its basic cost per million Btu may be the most economical by far. The major difficulties with solid fuels are handling, storage, ash removal and transportation to the plant. Of possibly equal concern today would be pollution control. Several states have restricted the burning of wood or coal in new residential homes strictly on the basis of pollution control. Commercial and industrial users may also be subject to stringent restrictions on particulate emissions. Few new commercial establishments are designed to use solid fuels for their heating needs due to fuel and ash handling problems as well as air pollution concerns. Coal and wood heating plants are primarily limited to large basic industries as well as plants where the burning of wood waste is an economically attractive disposal option.

Chapter 2 Overview of Heating Systems

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The delivery of electricity and natural gas to the heating plant is practically invisible to the end-user. While not every facility will have a natural gas line buried in its front yard, a growing percentage of commercial establishments are choosing natural gas as the primary heating fuel. The gas curtailments of the 1970s, when US natural gas supplies were artificially limited due to federal regulations, have been replaced by abundant supplies and stable prices in the 1990s after the industry was deregulated in the 1980s. Dual fuel options are sometimes pursued to minimize the utility component of plant operating costs. Variable rate structures add complexity to the fuel selection decision as electric utilities often give a preferred rate for all-electric facilities. In facilities with large lighting or motor loads and modest heating loads, the cost savings from the all-electric rate on these other electrical loads could offset any cost savings from using the cheaper natural gas for only the heating load component. The use of solar energy as a heating option in the United States greatly diminished once the Department of Energy demonstration programs of the 1970s were concluded. Solar energy is a viable option for domestic water heating where gas and electricity costs are high and there is a high availability of sunlight. However, the added complexity of solar space heating systems (requiring thermal storage and having highly variable daily or monthly load profiles) makes the economics of such systems difficult to justify in the stable fuel environment of the 1990s. While the development of solar thermal systems has shown few significant improvements since the 1970s, the development of photovoltaic (PV) technologies has progressed significantly. Pricing for PV systems was still not competitive with conventional energy sources in 1996, but the cost has been dropping rapidly to the point that some common applications may begin to look attractive in higher fuel cost areas by the year 2000. Table 2-1 lists the fuel source options that are available to heating system designers and gives the advantages and disadvantages of each. Because few facilities ever change their fuel source during their useful life, it is important to closely examine the fuel options and perform appropriate cost analyses (including expected replacement and maintenance costs) before committing that facility to a particular fuel source. Because of local variations in fuel cost and availability, you should become familiar with the fuel options in the area where the facility is located. When evaluating fuel options for a particular installation, you should not fall into the trap of simply weighing installation cost versus annual fuel costs. Maintenance costs can factor heavily into the total owning and operating costs for many of these systems, especially the solid fuel and solar heating systems. Generally, the more mechanically complex the system, the larger the system will need to be to justify the added maintenance costs.

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Chapter 2 Overview of Heating Systems

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Another consideration not listed in Table 2-1 is owner convenience. Wood heating systems were popular with homeowners in the United States in the 1970s during the so-called energy crisis. However, the inconvenience of having to cut and transport the wood, continually load the stove, and remove the ashes caused many wood stoves to be removed or left idle after just a few years of operation. In industrial or commercial solid fuel systems, the added cost of a part-time or full-time operator who handles fuel loading and ash removal must be factored into overall system cost.

Chapter 2 Overview of Heating Systems

Fundamentals of Heating Systems

2: 5

Fundamentals of Heating Systems

Chapter 2 Overview of Heating Systems

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2.3

Energy Conversion Plant

The heart of any heating system is its energy conversion plant, where the fuel is converted into thermal energy for distribution to the conditioned space. The most common conversion processes can be summarized in four basic categories: • Chemical energy conversion to thermal energy by combustion; • Electrical energy conversion to thermal energy; • Electrical energy driving a heat pump which moves thermal energy; and • Solar (radiation) energy conversion to thermal energy.

The combustion process will vary with the fuel type and its characteristics. In general, solid fuel furnaces must be properly designed by the manufacturer and carefully operated to provide proper fuel combustion while minimizing emissions of ash and other pollutants. Oil, natural gas and propane systems generally burn much cleaner, with simpler burner configurations. All types of combustion systems must be provided with adequate air for the combustion process. Oil and gas systems, due to their simplicity, can even be located in the middle of a conditioned space. In these cases, combustion air may be required by code to be ducted into the furnace closet or enclosure. In situations where outdoor combustion air is not provided to indoor furnace systems, you should be careful that the heating system produces adequate draft to exhaust the combustion products. Special consideration must be given to waste incinerators, whether solid waste, waste oil or biomass (agricultural waste). The fuel will often include a variety of materials not intended for ordinary combustion. The systems must be designed to prevent air pollution by appropriately high combustion temperatures, particulate removal from the flue gases, and toxic gas scrubbing. The solid waste systems are probably the most difficult to control in this regard, because the fuel source is difficult to regulate or control reliably. These heating plants must be large in size to justify the special controls and pollution control systems while still able to take advantage of the low cost fuel. Electricity is a very high grade manufactured energy source that can be converted into heat with no losses at the site using very compact resistance elements. These elements are small enough to be inserted into water heaters or air ducts without special enclosures or cabinets. They have no combustion products, so they require no flue or outdoor air vent. Resistance heat requires heavy gauge electrical conductors and additional electrical transformers and breakers to carry the extra current consumed by these systems. Because of the very high cost per Btu of electricity, a heat pump system is often used where an electric heating system is advantageous. As its name implies, a heat pump "pumps" heat

Chapter 2 Overview of Heating Systems

Fundamentals of Heating Systems

2: 7

from a low temperature heat source to the conditioned space at a higher temperature, usually by means of a vapor-compression refrigeration cycle. A domestic refrigerator is a type of heat pump, where the heat source is the cold cabinet (which gets its heat by conduction through the cabinet walls) and the unit discharges its warm air directly into the conditioned space. Figure 2-1 depicts the basic elements of a vapor-compression system. When the cooling effect is the desired output, the unit would be termed an air conditioner or chiller. When the heating effect is desired, it is called a heat pump. A heat pump is not an energy conversion device, like an electric resistance heater or a combustion device. It simply moves heat from an area of low temperature and dissipates it in an area of high temperature as a result of the circulating refrigerant undergoing changes in its working pressure. Because it is not an energy conversion device, it is not limited by conventional thermodynamic efficiencies of 100% or less. Heat pumps can deliver two to four units of heat energy for every unit of energy needed to power them. The efficiency of a heat pump is usually referred to as its Coefficient of Performance or COP.

Figure 2-1. Components in a Vapor-Compression Refrigeration Cycle

Fundamentals of Heating Systems

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The COP is calculated by: Heating COP = Heat Output Rate/Power Consumption where the numerator and denominator on the right side of the equation are expressed in the same units. The COP of a particular unit will vary with the temperature of its heat source and the conditioned space temperature. The heat source of a heat pump can be the outside air, or any other “infinite” heat source such as a large body of water or the earth. Additional piping and an intermediate circulating fluid are usually required to exchange heat with these more remote heat sources. There are heat pumps that are driven by reciprocating engines with heat recovery from the hot exhaust gases, as well as heat pumps that utilize absorption technology. If you are interested in learning more about heat pumps, refer to any introductory engineering thermodynamics text and the ASHRAE Handbook–Fundamentals.

2.4

Energy Distribution System

Just as the fuel type affects the type of heating plant that is utilized, so will the energy distribution system. There are four broad categories of energy distribution systems: • Hot water systems; • Steam systems; • Warm air systems; and • Radiant systems.

There are typically several options within each of these categories that are in widespread use. Hot water systems utilize the good heat transfer and economical cost of water to move heat from the plant to the different utilization points within the conditioned space. The boilers for these systems usually operate at temperatures and pressures such that the water never vaporizes. Circulating pumps move the hot water through the remote heat exchanger devices in the various zones and back to the boiler. Practical limitations on hot water temperatures will keep the boiler temperature below 250°F, which is adequate for any space heating application. Steam systems also utilize a boiler, although one that vaporizes the water. The design of a steam boiler is very different from a simpler hot water boiler, where water treatment and makeup water may be needed if steam is bled off for process applications. A feedwater Chapter 2 Overview of Heating Systems

Fundamentals of Heating Systems

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pump injects the liquid water into the steam boiler, where the liquid is then vaporized into steam. The steam pressure generated in the boiler causes the steam to flow to its remote heat exchanger devices. The steam will normally condense in the heat exchangers at the point of ultimate use, releasing its latent heat of vaporization at a constant temperature. The condensate is returned to the boiler feedwater pump by either gravity flow or by condensate pumps to repeat the cycle. Higher temperatures are possible with steam systems, although these heating systems are less popular than in the past for space conditioning applications due to potential problems with control, safety and noise. Steam systems are most common where steam is being generated for process applications anyway, as well as in older buildings. Warm air systems transport their heat using air ducts distributed through the space. One advantage of the warm air system is that it is easily combined with air-conditioning (comfort cooling), which almost always uses forced air distribution. This combination permits separate zones to be heated or cooled simultaneously. In residential applications, the forced air system is usually the lowest cost to install because the heating and cooling systems can utilize the same fan and duct system. Ducted air systems have the heat exchange devices located at or near the main heating plant, and so tend to make repair or replacement easier to perform and less disruptive of activities in the conditioned spaces. A major advantage in commercial spaces is the ease with which outdoor makeup air can be introduced into the system and distributed to the various zones in the space. For spaces with many occupants, this amount of outdoor air can be substantial and would need a separate duct and conditioning system (to preheat or precool the outdoor air before introducing it into the space) for non-ducted heating systems. One last benefit is that the central air stream can be used to clean or humidify the air in the space without having small air moving systems in each zone. Combination forced air and hot water systems are common, usually where cooled air produced at the central air handler is ducted to zone mixing boxes where tempering or reheating is provided by a hot water coil. Radiant heating systems utilize large, warm surfaces to heat the space by raising the mean radiant temperature of the space. The radiating surfaces can be the floor, walls or ceiling, or smaller, high temperature combustion surfaces. The term radiator applied to a hot water or steam heat exchanger located in the space is actually a misnomer, as it transfers most of its heat to the space by convection. Radiant surfaces can be heated by electric resistance elements, by plastic tubing through which hot water flows, or by metal tubing through which combustion gases flow. The electric resistance type may have the elements embedded in gypsum wallboard panels that are integral parts of the ceiling or walls, or they may be long flat elements placed adjacent to the

Fundamentals of Heating Systems

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ceiling or floor. Electric radiant panels are also available in various forms that are standalone devices for retrofit applications. The hot water tube types are usually placed under the floor or cast within the concrete slab. Radiant systems should be perfectly quiet as they need no forced air movement to perform their heating operation. They transfer their heat by radiation only (ceiling type) or by convection and radiation (wall or floor type). The floor type provides a particularly uniform temperature distribution in the space as the warmed air at the floor gradually rises to the ceiling by natural convection. There are also radiant heating systems that utilize combustion to produce a high temperature surface. These may have a tube through which combustion gases flow (usually vented to outdoors) or a red-hot burner face (often unvented). Because of the higher temperatures of these devices, less surface area is needed to provide the required radiant heat transfer. These devices are typically located overhead to eliminate the risk of burns to occupants. Radiant systems on their own cannot address outdoor makeup air, air cleaning or humidification. Certain types of radiant heating systems are often used in spaces where no open flame or spark can be permitted due to the presence of dust or other flammable or explosive materials. Solar heating systems can be in either the hot water or the warm air category, depending on the type of solar collector that is used. The solar collector essentially replaces the heating plant used in other conventional systems. The hot water type must incorporate drain-back controls or use an antifreeze mixture to prevent freezing of the circulating fluid in the collectors and piping outside of the heated space at night during cold weather. Installation and operating costs are obviously important concerns when selecting any type of heating system. There are several methods available to compare or rank the overall cost effectiveness of one system to another. The simplest technique is referred to as simple payback. In its most general sense, simple payback can be calculated from: Simple Payback (in years) = Cost Differential/Annual Operating Cost Savings where the payback is expressed as the number of years that it takes for the cost savings to repay the extra cost of the more efficient system. This rather simplistic way to rank the performance of systems usually does not account for maintenance costs or equipment life in the operating cost savings. A more realistic method to compare actual costs of various systems is called lifecycle cost analysis. This method will be covered more extensively in Chapter 7, but it involves converting all estimated future costs into a single present-day cost. This technique can then

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account for high recurring maintenance costs, low or escalating fuel costs, equipment replacement at some future time, the time value of money, etc. The single cost figure is equivalent to the amount of money you would need to have available today to install and operate a system for either a fixed number of years or for the life of the equipment.

Summary There are three main components to consider when designing a heating system: the fuel source, the energy conversion plant, and the energy distribution system. The three are not completely independent of each other, as certain fuels or distribution systems are more appropriate with specific types of plants. Solid fuels are most appropriate for use with large industrial or commercial plants where maintenance and operating staff are available to service the fuel and ash handling systems. Waste fuels (such as used lubricants or agricultural waste products) can be used where their low costs will compensate for possibly higher heating plant installation and maintenance costs and where costs would likely be incurred for their disposal. Most residential and commercial buildings are heated with fossil fuels (natural gas, propane or LP gas or fuel oil) or electricity largely because of their ease of handling and their suitability for automated systems. LP gas or fuel oil are often used where natural gas is not available. Electric resistance heating systems are very small and reliable, but the direct conversion of electricity to heat is quite expensive. Heat pump systems provide two to four units of heat energy for every unit of energy consumed. The electric heat pump makes the economy of electric heat more competitive with the fossil fuel options, as well as providing capabilities for comfort cooling in hot weather. Solar heating systems have no fuel cost, but the limited availability of sunlight requires storage and backup systems that make the overall cost of these systems quite high despite having a free energy source. The energy conversion plants can be divided into four categories: combustion systems, electric resistance, heat pumps and solar energy. Combustion systems convert the chemical energy in the fuel into thermal energy during combustion. Electric resistance is a direct conversion of electric energy into heat energy. Heat pumps move heat from a lower temperature area to a higher temperature area. Solar (radiant) energy is converted to thermal energy when it is absorbed by the collector absorber plate. The energy distribution systems can be divided into four categories: hot water, steam, warm air and radiant systems. The hot water and steam systems circulate water or steam through pipes to remote heat exchangers in the different zones in the space. These steam or hot

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water heat exchangers may use convection, radiation or forced warm air to deliver the heat to the space. Warm air systems use air ducts to move heated air either directly from the heating plant to the individual zones, or from local zone air handlers to the individual spaces. Warm air systems are very common in residential applications because the same ducts can be used for both heating and cooling applications. Radiant systems use either large surfaces at low to moderate temperatures or smaller surfaces at high temperatures to provide the radiant heat flux to the space. One major disadvantage of radiant systems is that they cannot provide for outdoor air to comply with building codes. Lifecycle cost analysis is a more representative method of comparing the total installation and operating costs of different systems. Simple payback analysis normally uses only energy cost differences and so does not account for recurring maintenance or equipment replacement costs.

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Skill Development Exercises for Chapter 2 Complete these questions by writing your answers on the worksheets at the back of this book.

2-01. Name the three major components of any heating system. 2-02. Coal costs $50 per ton and has a heating value of 14,000 Btu/pound. Oil costs $0.90 per gallon and has a heating value of 135,000 Btu/gallon. Electricity costs $0.075 per kWh. Calculate the cost per million Btu of heat supplied from a coal boiler that is 85% efficient, from an oil furnace that is 70% efficient, and from an electric resistance duct heater that is 100% efficient. 2-03. An electric heat pump has an overall COP of 3.0 at a certain operating point. If it delivers 50,000 Btu/h of heating capacity, what is its power consumption in kW? If electricity costs $0.07 per kWh, what is the cost per million Btu of heat supplied by this heat pump? 2-04. You are given the task of renovating a small six-room doctor’s office that has 8-ft ceilings and no plenum above the ceilings. It is on the ground floor of a three-floor building. The office is located in northern Minnesota where air-conditioning is not needed. Would you choose a hot water distribution system or a warm air distribution system? Explain your answer. 2-05. A high efficiency gas furnace is 95% efficient while your existing furnace is 78% efficient. Last year, you spent $620 on gas to heat your house during an average winter. If it would cost $1,800 to have a high efficiency furnace installed, what is the simple payback period, in years? 2-06. A school building will need 15 cfm of outdoor air per pupil to meet current building codes. Which distribution systems would make it easiest to provide this outdoor air to the space? 2-07. A high bay garage is used to work on large trucks. What types of heating and distribution systems might you choose to heat this space if no combustion system can be used because of the possibility of gasoline spills?

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Chapter 3 Basic Selection Criteria

Contents of Chapter 3 • Instructions • Study Objectives of Chapter 3 • 3.1

Basic Selection Criteria

• 3.2

Occupancy and Comfort Considerations

• 3.3

Thermal Envelope

• 3.4

Ventilation Requirements

• 3.5

Regional Preferences

• 3.6

Availability of Fuels

• Summary • Bibliography • Skill Development Exercises

Instructions Read the material in this chapter for general content, and re-read the parts that are emphasized in the summary. Complete the skill development exercises without consulting the text, then review the text as necessary to verify your solutions.

Study Objectives of Chapter 3 When selecting a heating system for a certain application, there are three criteria that must always be remembered: • The system must provide comfortable conditions for the occupants as they use the space; • The system must provide the heat output that is necessary for the building; and • The system should be cost effective.

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Another important consideration in selecting and sizing the heating equipment is the amount of outdoor air that will be required. Using 15 cfm/person as specified by ASHRAE Standard 62-1989, the outdoor air load will typically be 15% to 25% of the annual HVAC cost for commercial buildings. Some final selection criteria deal with fuel availability and what may be called regional preferences. The objectives of this chapter are to discuss these selection criteria and look at the impact on lifecycle costs. You should develop an appreciation for the necessity of accurate building load calculations and an understanding of comfort requirements. Outdoor air requirements will be covered as well as their impacts on indoor air quality and operating costs. The chapter will conclude with a discussion of fuel selection and regional preferences for heating systems, so you should develop an understanding of why some systems are commonly found in certain parts of the country and not in others.

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3.1

Basic Selection Criteria

While it is rare that only one type of heating system can adequately satisfy the comfort conditioning needs of a building’s occupants, many systems have been unsatisfactory because the heating system was not properly matched to the building load. This load matching does not merely account for total system capacity or output, but also includes how the heat is distributed and the air quality within the space. A completely successful system must also be economical to operate because few applications can afford heating comfort at any cost. Design engineers must make system design and equipment selection decisions based on their experience with certain types of systems in similar applications, their understanding of how a certain system would perform in a different application, and the installation and operating budgets and maintenance personnel that will be available. Many heating system designs get modified (or shortcuts are taken) after the initial design was finalized but before installation, to keep total building costs within budget. Such steps often result in an overall unsatisfactory system, however, it may be difficult to convince the building owner that a system they never see is critical to the building's operation. Many other heating systems are modified at some point after occupancy to reduce excessive operating costs, usually compromising system performance for the sake of necessary operating economy. Schools are a prime example, where the HVAC system first-cost is usually constrained by a tight construction budget, while HVAC operating costs represent a large part of the total building operating budget (including personnel salaries). With such tight economic constraints, it is critical for you to understand the appropriate system selection criteria and their economic consequences. With so many options to choose from, how can you make an intelligent decision about which heating system is best for your application? Selection of a heating system is somewhat like buying an automobile. There are many models of cars to choose from and they all provide the same basic function of transporting you from point A to point B. However, every car buyer places different values on the different features that are offered in different cars. If everyone had a common set of values, we would all drive the same car model. Building heating systems also have different features that we can choose from. Some have ductwork and some do not. Some circulate water and some circulate air. Some burn gas and some use electricity to run a compressor. With so many options, it is not surprising that we have so many different types of heating systems in common usage. One quantifiable characteristic of all heating systems is their lifecycle cost. (If you are not familiar with the concept of lifecycle cost, refer to any text on engineering economics.) Briefly, lifecycle cost analysis is a relatively simple way to combine all the costs associated with owning and operating a building or a heating system into a single cost number. Each

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type of heating system will have its own lifecycle cost based on its installed cost and debt service, annual operating costs, recurring maintenance costs, replacement cost and scrap value. Some of these cost items are quite difficult to evaluate when you are at the design phase of a project, and in some cases, you must rely on the experience of seasoned engineers or building operators. Some parameters (such as interest rates over the life of the equipment) can never be known in advance. However, if reasonable, honest representations of these cost parameters are used in the lifecycle cost analysis, meaningful comparisons can be made of the overall owning and operating costs of the different systems being compared. There will still be many tangible features whose values must be assessed by the building owner or the design engineer without being able to assign dollar values to them. Just as we can quickly zero in when car shopping by looking at the prices of Cadillacs, Chevrolets and Geos, a lifecycle cost analysis of several heating system options lets the building owner or engineer focus on a short list of systems from which to choose. In practice, detailed lifecycle cost comparisons are not often performed. While it may be likened to buying a car by shopping at only one dealer, there are several cases where lifecycle costs may not reflect the actual cost situation, such as speculation construction where low first-cost is a primary consideration for quick sale upon completion, and landlord/tenant situations where the landlord pays for the heating system while the tenants pay the operating costs. In a perfect world, the operating costs of the heating system would be reflected in the price of the building and the rents that are charged. However, such is not the case in the real world, so we have many buildings with inefficient heating systems where the occupants pay high monthly utility costs.

3.2

Occupancy and Comfort Considerations

Except for special cases of industrial process control, the primary reason for heating buildings is to maintain acceptable occupant comfort. The designer should develop an understanding of the following occupancy considerations for the space: • Hours of occupancy; • Number of occupants in each zone; • Total number of occupants in the building; • Special considerations (such as age, activity, manner of dress, etc.); and • Desired indoor conditions.

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The hours of occupancy will indicate whether automatic setback control options would be economical. Buildings occupied only during normal business hours from 8:00 am to 5:00 pm are unoccupied nearly 75% of the time, and so present opportunities for heating energy conservation during those unoccupied periods. The use of automatic setback controls impacts the required system capacity as well as the overall system control characteristics. Thermostat setback produces a lower temperature difference between indoors and outdoors, resulting in proportionately less heating requirements during the setback period. Figure 3-1 illustrates how the mean space temperature may vary during the setback period and the following recovery period. Obviously, the indoor air temperature does not immediately drop down at the time of setback, as shown in Figure 3-1. However, the actual transient cool down and the transient recovery portions of the indoor temperature curve would somewhat cancel each other out, yielding approximately a rectangular temperature response curve as representative of the overall process. The time it takes for the space temperature to drop down to the setback temperature will depend on the outdoor conditions as well as the thermal capacitance of the structure. During mild weather, the space temperature may never reach the setback temperature and the heating system will not need to operate at all. An important consideration when using setback is the length of time needed for recovery. The system should be set to begin the recovery process at an appropriate time in advance of building occupancy. Some energy management systems can perform this setback function according to preselected schedules, and can even adjust the recovery schedule based on

Figure 3-1. Thermostat Setback for Heating Energy Savings

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outdoor temperature conditions. Heating system capacity may need to be increased by 15% to 20% to ensure a reasonable recovery time when at or near design outdoor temperature conditions. Intelligent energy management control systems can identify such severe outdoor conditions when setback should not be used to avoid the 15% to 20% oversizing needed for setback recovery at design conditions. Such controls can also be applied to existing systems to avoid setback when the system capacity is not adequate for recovery. It is a popular misconception that thermostat setback does not produce a net savings of heating energy. The amount of energy saved will be in proportion to the amount of setback and how long the space is kept at the lower setback temperature. A steady-state energy balance on the heated space would yield:

q&sensible = ∑ (UA)(Ti − To ) + m& o c p,air (Ti − To ) − q& internal

3-1

where the summation term on the right side of the equation accounts for all the conduction losses through the components of the building envelope, the middle term accounts for air infiltration, and the final term subtracts off the internal heat being generated in the space. From Equation 3-1, it is obvious that the sensible heating requirements of a structure will be reduced if the indoor to outdoor temperature difference is reduced. If the internal heat gains are negligible, the reduction in heat losses will be directly proportional to the reduction in the temperature difference. As long as the heating requirements are greater than zero during setback conditions, Equation 3-1 indicates that the actual reduction in space heating requirements will be the same regardless of the value of internal heat generation, although the percentage reduction will depend on the relative magnitude of the internal heat sources. Setback energy savings are illustrated in Example 3-1.

EXAMPLE 3-1 Problem: The thermostat in a small office building is set back from 72°F to 62°F during the hours of 6:00 pm to 7:00 am Monday through Friday, and all day Saturday and Sunday. Estimate the percentage savings from thermostat setback at average outdoor temperatures of 30°F and 50°F. First assume that there are no internal heat gains during the setback period, then repeat the calculations for the case of constant internal heat gains equal to 30% of the total heat loss when it is 30°F outdoors. Solution: A simple analysis of the setback situation illustrated in Figure 3-1 indicates that for the case of no internal heat generation, the heating energy savings would be proportional to the reduction in area between the Ti and the To curves. Total hours of setback are 13

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hours per day on Monday through Friday, plus 48 hours on the weekend. Using the concept of areas between the two temperature curves, the setback area reduction during one week is: ⎡ hours ⎤ Area reduction = ⎢13 × 5 days + 48 hours⎥ × 10° F = 1130 ° F ⋅ hours ⎣ day ⎦

The total area between the Ti and the To curves, without setback and with no heat generation, would be:

⎤ ⎡ hours Total area = ⎢24 × 7 days⎥ × (72 − 30)° F = 7056 ° F ⋅ hours day ⎦ ⎣ In the absence of any internal heat gains, the percent savings will be the percentage reduction in the area between the indoor and outdoor temperature curves: Percent heat savings =

1130 = 16% when 30° F outdoors 7056

For the milder temperature conditions of 50°F outdoors, ⎡ hours ⎤ Total area = ⎢24 × 7 days⎥ × (72 − 50)° F = 3696 ° F ⋅ hours day ⎣ ⎦

The percent savings at this milder heating condition is then: Percent heat savings =

1130 = 31% when 50° F outdoors 3696

If the internal heat gains reduce the heating requirements by 30% when it is 30°F outdoors, that is an equivalent area reduction of 30% of 7056 °F⋅hours, or 2117 °F⋅hours. The percent savings at 30°F outdoors for the case with internal heat generation is then: Percent heat savings =

1130 = 23% at 30° F outdoors 7056 − 2117

At the milder outdoor conditions:

Percent heat savings =

Fundamentals of Heating Systems

1130 = 72% at 50° F outdoors 3696 − 2117

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This example shows the order of magnitude of savings that can be expected from thermostat setback during unoccupied periods. From a percentage viewpoint, the savings are dramatic during mild heating conditions. The actual seasonal percentage savings will be closer to the percentages at the more severe heating conditions because much more heat is used at lower outdoor temperatures, and relatively little is used at the mild heating conditions. Proportionately greater savings are produced if lower setback temperatures can be used while still providing a reasonable recovery time. Human comfort is a condition in which the body’s heat losses match its heat generation rate at a neutral deep body temperature of approximately 98.6°F. Comfort is a difficult quantity to measure because it incorporates many elements: most importantly, temperature, humidity, radiation exchange and air movement. Efforts to quantify human comfort date back to the early 1900s and extensive work continues to this day. Based on work by Fanger1 and others, comfort is generally considered to be a condition where 80% of a group of people are satisfied with their environment, recognizing the virtual impossibility of achieving 100% satisfaction due to differences in metabolism, dress and other factors. Maintaining occupant comfort has been a challenge ever since a caveman built the first fire. An individual’s personal comfort depends on air temperature and humidity, mean radiant temperature, air velocity, amount of clothing, activity level and their current individual metabolic rate. Comfort conditions are often shown on a psychrometric chart, as given in Figure 3-2 from ASHRAE Standard 55a-1995.2 The almost vertical bounding lines of the comfort regions are based on effective temperatures, which attempt to account for temperature, humidity and air movement effects. Indices have been developed to qualitatively estimate the thermal resistance of clothing and the metabolic rate as a function of activity level. The clo is defined in terms of the thermal resistance of the clothing ensemble between the surface of the skin and the outer surface of the garment. Table 3-1 lists clo values for different dress ensembles. The met is defined as the metabolic rate that produces 18.4 Btu/h per ft2 of body surface area. For an average person, met = 1.0 for a seated, at rest, level of activity. Table 3-2 lists several values of met for various activities. While most office buildings and dwellings are designed for occupants considered to be either sedentary or performing light work, there are applications where higher metabolic rates may be maintained, such as with assembly lines or exercise rooms. For these applications, temperatures may be maintained below the normal comfort conditions shown in Figure 3-2. An empirical relation can be used to estimate the effect of clothing and metabolic levels (for 1.1