Chapter 12—Heating, Air Conditioning, and Ventilating

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-1

Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-4

Standard Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-4

Central Heating Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-7

Space Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-12

Hydronic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-14

Direct Vent Wall Furnaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-15

Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-15

Air Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-15

Circulation Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-16

Evaporation Coolers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-16

Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-17

Chimneys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-17

Fireplaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-18

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-19

Additional Sources of Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-19

Figure 12.1. Heat Pump in Cooling Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-5

Figure 12.2. Piping Hookup for Inside Tank Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-6

Figure 12.3. Piping Hookup for Buried Outside Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-6

Figure 12.4. Minimum Clearance for Pipeless Hot Air and Gravity Warm Air Furnace . . . . . . .12-7

Figure 12.5. Minimum Clearance for Steam or Hot Water Boiler and Mechanical Warm-air Furnace .12-7

Figure 12.6. Heating Ducts Covered With Asbestos Insulation . . . . . . . . . . . . . . . . . . . . . . . . .12-7

Figure 12.7. Typical Underfeed Coal Stoker Installation in Small Boilers . . . . . . . . . . . . . . . . .12-8

Figure 12.8. Cutaway View of Typical High-pressure Gun Burner . . . . . . . . . . . . . . . . . . . . . . .12-9

Figure 12.9. Gas-fired Boiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-9

Figure 12.10. Typical Gravity One-pipe Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-10

Figure 12.11. One-pipe Gravity Water Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-11

Figure 12.12. Two-pipe Gravity Water Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-11

Figure 12.13. Warm-air Convection Furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-11

Figure 12.14. Cross-sectional View of Building Showing Forced-warm-air Heating System . . . . .12-12

Figure 12.15. Perforated-sleeve Burner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-13

Figure 12.16. Condition of Burner Flame With Different Rates of Fuel Flow . . . . . . . . . . . . . . .12-13

Figure 12.17. Wall and Ceiling Clearance Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-14

Figure 12.18. Draft in Relation to Height of Chimney . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-14

Figure 12.19. Location and Operation of Typical Backdraft Diverter . . . . . . . . . . . . . . . . . . . . . .12-15

Figure 12.20. Split-system Air Conditioner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-16

Figure 12.21. External Air-conditioning Condenser Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-16

Figure 12.22. Chimney Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-17

Figure 12.23. Fireplace Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-18

Healthy Housing Reference Manual 1

Contents
“Our climate is warming at a faster rate than ever before recorded.” 

D. James Baker 

NOAA Administrator, 1993–2004 

Introduction

The quotes below provide a profound lesson in the need for

housing to provide protection from both the heat and cold. 

“France heat wave death toll set at 14,802: The

death toll in France from August’s blistering heat

wave has reached nearly 15,000, according to a

government-commissioned report released

Thursday, surpassing a prior tally by more than

3,000.” USA Today, September 25, 2003.

“In the study of the 1995 Chicago heat wave, those

at greatest risk of dying from the heat were people

with medical illnesses who were socially isolated

and did not have access to air conditioning.”

Centers for Disease Control and Prevention,

Morbidity and Mortality Weekly Report, July 4, 2003.

“3 Deaths tied to cold . . .The bitter cold that

gripped the Northeast through the weekend and

iced over roads was blamed for at least three deaths,

including that of a Philadelphia man found inside a

home without heat.” Lexington [Kentucky] Herald

Leader, January 12, 2004.

“In many temperate countries, death rates during

the winter season are 10%–25% higher than those

in the summer.” World Health Organization,

Health Evidence Network, November 1, 2004.

This chapter provides a general overview of the heating

and cooling of today’s homes. Heating and cooling are

not merely a matter of comfort, but of survival. Both

very cold and very hot temperatures can threaten health.

Excessive exposure to heat is referred to as heat stress and

excessive exposure to cold is referred to as cold stress.

In a very hot environment, the most serious health risk is

heat stroke. Heat stroke requires immediate medical

attention and can be fatal or leave permanent damage.

Heat stroke fatalities occur every summer. Heat

exhaustion and fainting are less serious types of illnesses.

Typically they are not fatal, but they do interfere with a

person’s ability to work.

At very cold temperatures, the most serious concern is

the risk for hypothermia or dangerous overcooling of the

body. Another serious effect of cold exposure is frostbite

or freezing of exposed extremities, such as fingers, toes,

nose, and ear lobes. Hypothermia can be fatal if

immediate medical attention is not received.

Heat and cold are dangerous because the victims of heat

stroke and hypothermia often do not notice the

Chapter 12: Heating, Air Conditioning, and Ventilating

Healthy Housing Reference Manual 12-1

Air duct—A formed conduit that carries warm or cold air from the furnace or air-conditioner and back again.

Antiflooding Control—A safety control that shuts off fuel and ignition when excessive fuel oil accumulates in the

appliance.

Appliance: 

High heat—A unit that operates with flue entrance temperature of combustion products above 1,500°F (820°C).

Medium heat—Same as high-heat, except above 600°F (320°C).

Low heat—same as high-heat, except below 600°F (320°C).

Boiler:

High pressure—A boiler furnishing pressure at 15 psi or more.

Low pressure (hot water or steam)—A boiler furnishing steam at a pressure less than 15 psi or hot water 

not more than 30 psi.

Burner—A device that mixes fuel, air, and ignition in a combustion chamber.

Definitions of Terms Related to HVAC Systems
symptoms. This means that family, neighbors, and

friends are essential for early recognition of the onset of

the conditions. The affected individual’s survival depends

on others to identify symptoms and to seek medical help.

Family, neighbors, and friends must be particularly

diligent during heat or cold waves to check on

individuals who live alone.

Although symptoms vary from person to person, the

warning signs of heat exhaustion include confusion and

profuse and prolonged sweating. The person should be

removed from the heat, cooled, and heavily hydrated.

Heat stroke signs and symptoms include sudden and

severe fatigue, nausea, dizziness, rapid pulse,

lightheadedness, confusion, unconsciousness, extremely

high temperature, and hot and dry skin surface. An

individual who appears disorientated or confused, seems

euphoric or unaccountably irritable, or suffers from

malaise or flulike symptoms should be moved to a cool

location and medical advice should be sought

immediately.

Carbon monoxide (CO) detector—A device used to detect CO (specific gravity of 0.97 vs. 1.00 for oxygen, a colorless

odorless gas resulting from combustion of fuel). CO detectors should be placed on each floor of the structure at eye level

and should have an audible alarm and, when possible, a digital readout at eye level.

Chimney—A vertical shaft containing one or more passageways.

Factory-built chimney—A tested and accredited flue for venting gas appliances, incinerators and solid or liquid 

fuel-burning appliances.

Masonry chimney—A field-constructed chimney built of masonry and lined with terra cotta flue or firebrick.

Metal chimney—A field-constructed chimney of metal.

Chimney connector—A pipe or breeching that connects the heating appliance to the chimney.

Clearance—The distance separating the appliance, chimney connector, plenum, and flue from the nearest surface of

combustible material.

Central cooling system—An electric or gas-powered system containing an outside compressor, cooling coils, and a

ducting system inside the structure designed to supply cool air uniformly throughout the structure. 

Central heating system—A flue connected boiler or furnace installed as an integral part of the structure and designed to

supply heat adequately for the structure.

Collectors—The key component of active solar systems and are designed to meet the specific temperature requirements

and climate conditions for different end uses. Several types of solar collectors exist: flat-plate collectors, evacuated-tube

collectors, concentrating collectors, and transpired air collectors.

Controls:

High-low limit control—An automatic control that responds to liquid level changes and will shut down if they 

are exceeded. 

Primary safety control—The automatic safety control intended to prevent abnormal discharge of fuel at the burner 

in case of ignition failure or flame failure.

Combustion safety control—A primary safety control that responds to flame properties, sensing the presence 

of flame and causing fuel to be shut off in event of flame failure.

Convector—A radiator that supplies a maximum amount of heat by convection, using many closely spaced metal fins

fitted onto pipes that carry hot water or steam and thereby heat the circulating air.

Conversion—A boiler or furnace, flue connected, originally designed for solid fuel but converted for liquid or gas fuel.

Damper—A valve for regulating draft on coal-fired equipment. Generally located on the exhaust side of the combustion

chamber, usually in the chimney connector. Dampers are not allowed on oil- and gas-fired equipment. 

Draft hood—-A device placed in and made a part of the vent connector (chimney connector or smoke pipe) from an

appliance, or in the appliance itself. The hood is designed to a) ensure the ready escape of the products of combustion in

the event of no draft, back draft, or stoppage beyond the draft hood; b) prevent backdraft from entering the appliance;

and c) neutralize the effect of stack action of the chimney flue upon appliance operation.

Definitions of Terms Related to HVAC Systems

12-2 Heating, Air Conditioning, and Ventilating
Healthy Housing Reference Manual 12-3

Warning signs of hypothermia include nausea, fatigue,

dizziness, irritability, or euphoria. Individuals also

experience pain in their extremities (e.g., hands, feet,

ears) and severe shivering. People who exhibit these

symptoms, particularly the elderly and young, should be

moved to a heated shelter and medical advice should be

sought when appropriate.

The function of a heating, ventilation, and air

conditioning (HVAC) system is to provide for more than

human health and comfort. The HVAC system produces

heat, cool air, and ventilation, and helps control dust and

moisture, which can lead to adverse health effects. The

variables to be controlled are temperature, air quality, air

motion, and relative humidity. Temperature must be

maintained uniformly throughout the heated/cooled

area. There is a 6°F to 10°F (-14°C to -12°C) variation

in room temperature from floor to ceiling. The adequacy

of the HVAC system and the air-tightness of the

structure or room determine the degree of personal safety

and comfort within the dwelling.

Draft regulator—A device that functions to maintain a desired draft in oil-fired appliances by automatically reducing the

chimney draft to the desired value. Sometimes this device is referred to in the field as air-balance, air-stat, or flue velocity

control or barometer damper. 

Fuel oil—A liquid mixture or compound derived from petroleum that does not emit flammable vapor below a

temperature of 125°F (52°C).

Heat—The warming of a building, apartment, or room by a furnace or electrical stove.

Heating plant—The furnace, boiler, or the other heating devices used to generate steam, hot water, or hot air, which

then is circulated through a distribution system. It typically uses coal, gas, oil or wood as its source of heat.

Limit control—A thermostatic device installed in the duct system to shut off the supply of heat at a predetermined

temperature of the circulated air.

Oil burner—A device for burning oil in heating appliances such as boilers, furnaces, water heaters, and ranges. A burner

of this type may be a pressure-atomizing gun type, a horizontal or vertical rotary type, or a mechanical or natural draftvaporizing type.

Oil stove—A flue-connected, self-contained, self-supporting oil-burning range or room heater equipped with an integral

tank not exceeding 10 gallons; it may be designed to be connected to a separate oil tank.

Plenum chamber—An air compartment to which one or more distributing air ducts are connected.

Pump, automatic oil—A device that automatically pumps oil from the supply tank and delivers it in specific quantities

to an oil-burning appliance. The pump or device is designed to stop pumping automatically if the oil supply line breaks.

Radiant heat—A method of heating a building by means of electric coils, hot water, or steam pipes installed in the

floors, walls, or ceilings.

Register—A grille-covered opening in a floor, ceiling, or wall through which hot or cold air can be introduced into a

room. It may or may not be arranged to permit closing the grille.

Room heater—A self-contained, freestanding heating appliance (space heater) intended for installation in the space

being heated and not intended for duct connection.

Smoke detector—A device installed in several rooms of the structure to warn of the presence of smoke. It should provide

an audible alarm. It can be battery powered or electric, or both. If the unit is battery powered, the batteries should be

tested or checked on a routine basis and changed once per year. If the unit is equipped with a 10-year battery, it is not

necessary to replace the battery every year. 

Tank—A separate tank connected, directly or by pump, to an oil-burning appliance.

Thimble—A metal or terra cotta lining for a chimney or furnace pipe.

Valve (main shut-off valve)—A manually operated valve in an oil line used to turn the oil supply to the burner on or off.

Vent system—The gas vent or chimney and vent connector, if used, assembled to form a continuous, unobstructed

passageway from the gas appliance to the outside atmosphere to remove vent gases. 

Definitions of Terms Related to HVAC Systems
Gas, electricity, oil, coal, wood, and solar energy are the

main energy sources for home heating and cooling. Heating

systems commonly used are steam, hot water and hot air.

A housing inspector should have knowledge of the various

heating fuels and systems to be able to determine their

adequacy and safety in operation. To cover fully all

aspects of the heating and cooling system, the entire area

and physical components of the system must be considered.

Heating

Fifty-one percent of the homes in the United States are

heated with natural gas, 30% are heated with electricity,

and 9% with fuel oil. The remaining 11% are heated with

bottled fuel, wood, coal, solar, geothermal, wind, or solar

energy [1]. Any home using combustion as a source of

heating, cooling, or cooking or that has an attached

garage should have appropriately located and maintained

carbon monoxide (CO) gas detectors. According to the

U.S. Consumer Product Safety Commission (CPSC),

from data collected in 2000, CO kills 200 people and

sends more than 10,000 to the hospital each year. 

The standard fuels for heating are discussed below.

Standard Fuels

Gas

More than 50% of American homes use gas fuel. Gas

fuels are colorless gases. Some have a characteristic pungent

odor; others are odorless and cannot be detected by smell.

Although gas fuels are easily handled in heating equipment,

their presence in air in appreciable quantities becomes a

serious health hazard. Gases diffuse readily in the air,

making explosive mixtures possible. A proportion of

combustible gas and air that is ignited burns with such a

high velocity that an explosive force is created. Because of

these characteristics of gas fuels, precautions must be

taken to prevent leaks, and care must be exercised when

gas-fired equipment is lit. 

Gas is broadly classified as natural or manufactured. 

• Natural gas—This gas is a mixture of several

combustible and inert gases. It is one of the richest

gases and is obtained from wells ordinarily located

in petroleum-producing areas. The heat content

may vary from 700 to 1,300 British thermal units

(BTUs) per cubic foot, with a generally accepted

average figure of 1,000 BTUs per cubic foot.

Natural gases are distributed through pipelines to the

point of use and are often mixed with manufactured

gas to maintain a guaranteed BTU content. 

• Manufactured gas—This gas, as distributed, is

usually a combination of certain proportions of

gases produced from coke, coal, and petroleum.

Its BTU value per cubic foot is generally closely

regulated, and costs are determined on a guaranteed

BTU basis, usually 520 to 540 BTUs per cubic foot.

• Liquefied petroleum gas—Principal products of

liquefied petroleum gas are butane and propane.

Butane and propane are derived from natural gas

or petroleum refinery gas and are chemically

classified as hydrocarbon gases. Specifically,

butane and propane are on the borderline

between a liquid and a gaseous state. At ordinary

atmospheric pressure, butane is a gas above 33°F

(0.6°C) and propane a gas at -42°F (-41°C).

These gases are mixed to produce commercial gas

suitable for various climatic conditions. Butane

and propane are heavier than air. The heat content of

butane is 3,274 BTUs per cubic foot, whereas that

of propane is 2,519 BTUs per cubic foot. 

Gas burners should be equipped with an automatic

shutoff in case the flame fails. Shutoff valves should be

located within 1 foot of the burner connection and on the

output side of the meter.

Caution: Liquefied petroleum gas is heavier than air;

therefore, the gas will accumulate at the bottom of

confined areas. If a leak develops, care should be taken to

ventilate the appliance before lighting it.

Electricity

Electricity has gained popularity for heating in many

regions, particularly where costs are competitive with

other sources of heat energy, with usage increasing from

2% in 1960 to 30% in 2000. With an electric system, the

housing inspector should rely mainly on the electrical

inspector to determine proper installation. There are a few

items, however, to be concerned with to ensure safe use of

the equipment. Check to see that the units are approved

by an accredited testing agency and installed according to

the manufacturer’s specifications. Most convector-type

units must be installed at least 2 inches above the floor

level, not only to ensure that proper convection currents

are established through the unit, but also to allow

sufficient air insulation from any combustible flooring

material. The housing inspector should check for curtains

that extend too close to the unit or loose, long-pile rugs

that are too close. A distance of 6 inches on the floor and

12 inches on the walls should separate rugs or curtains

from the appliance.

12-4 Heating, Air Conditioning, and Ventilating
Heat pumps are air conditioners that contain a valve that

allows switching between air conditioner and heater.

When the valve is switched one way, the heat pump acts

like an air conditioner; when it is switched the other way,

it reverses the flow of refrigerants and acts like a heater.

Cold is the absence of energy or calories of heat. To cool

something, the heat must be removed; to warm

something, energy or calories of heat must be provided.

Heat pumps do both.

A heat pump has a few additions beyond the typical air

conditioner: a reversing valve, two thermal expansion valves,

and two bypass valves. The reversing valve allows the unit

to provide both cooling and heating. Figure 12.1 shows a

heat pump in cooling mode. The unit operates as follows: 

• The compressor compacts the refrigerant vapor

and pumps it to the reversing valve. 

• The reversing valve directs the compressed vapor

to flow to the outside heat exchanger (condenser),

where the refrigerant is cooled and condensed to a

liquid. 

• The air blowing through the condenser coil

removes heat from the refrigerant. 

• The liquid refrigerant bypasses the first thermal

expansion valve and flows to the second thermal

expansion valve at the inside heat exchanger

(evaporator) where it expands into the evaporator

and becomes vapor. 

• The refrigerant picks up heat energy from the air

blowing across the evaporator coil and cool air

comes out at the other side of the coil. The cool

air is ducted to the occupied space as airconditioned air.

• The refrigerant vapor then goes back to the

reversing valve to be directed to the compressor to

start the refrigeration cycle all over again. 

Heat pumps [3] are quite efficient in their use of energy.

However, heat pumps often freeze up; that is, the coils in

the outside air collect ice. The heat pump has to melt this

ice periodically, so it switches itself back to air conditioner

mode to heat up the coils. To avoid pumping cold air into

the house in air conditioner mode, the heat pump also

uses electric strip heaters to heat the cold air that the air

conditioner is pumping out. Once the ice is melted, the

heat pump switches back to heating mode and turns off

the burners.

Radiant heat warms objects directly with longwave

electromagnetic energy. The heating panels diffuse heating

energy rays in a 160° arc, distributing warmth evenly. The

goal is to achieve no more than a 4°F (-16°C) difference

in temperature between floor level and ceiling level.

When properly installed, radiant heat warms a room

sooner and at lower temperature settings than do other

kinds of heat. Extreme care must be taken to protect

against fire hazards from objects in close proximity to the

infrared radiation reflectors. Inspectors dealing with this

heat source should have specialized training. Radiant

heating is plastered into the ceiling or wall in some homes

or in the brick or ceramic floors of bathrooms. If wires are

bare in the plaster, they should be treated as open and

exposed wiring. The inspector should be knowledgeable

about these systems, which are technical and relatively new.

Fuel Oil

Fuel oils are derived from petroleum, which consists

primarily of compounds of hydrogen and carbon

(hydrocarbons) and smaller amounts of nitrogen and

sulfur. Domestic fuel oils are controlled by rigid

specifications. Six grades of fuel oil—numbered 1

through 6—are generally used in heating systems; the

lighter two grades are used primarily for domestic heating: 

• Grade Number 1—a volatile, distillate oil for use

in burners that prepare fuel for burning solely by

vaporization (oil-fired space heaters).

• Grade Number 2—a moderate weight, volatile,

distillate oil used for burners that prepare oil for

burning by a combination of vaporization and

atomization. This grade of oil is commonly used

in domestic heating furnaces.

Healthy Housing Reference Manual 12-5

Figure 12.1. Heat Pump in Cooling Mode [2]
Heating values of oil vary from approximately 152,000 BTU

per gallon for number 6 oil to 136,000 BTU per gallon for

number 1 oil. Oil is more widely used today than coal

and provides a more automatic source of heat and

comfort. It also requires more complicated systems and

controls. If the oil supply is in the basement or cellar area,

certain code regulations must be followed (Figure 12.2)

[4–7]. No more than two 275-gallon tanks may be

installed above ground in the lowest story of any one

building. The IRC recommends a maximum fuel oil

storage of 660 gallons. The tank shall not be closer than

7 feet horizontally to any boiler, furnace, stove, or

exposed flame(s). 

Fuel oil lines should be embedded in a concrete or

cement floor or protected against damage if they run

across the floor. Each tank must have a shutoff valve that

will stop the flow if a leak develops in the line to or in the

burner itself. A leak-tight liner or pan should be installed

under tanks and lines located above the floor. They

contain potential leaks so the oil does not spread over the

floor, creating a fire hazard.

The tank or tanks must be vented to the outside, and a

gauge showing the quantity of oil in the tank or tanks

must be tight and operative. Steel tanks constructed

before 1985 had a life expectancy of 12–20 years. Tanks

must be off the floor and on a stable base to prevent

settlement or movement that may rupture the

connections. Figure 12.3 shows a buried outside tank

installation. In 1985, federal legislation was passed

requiring that the exterior components of underground

storage tanks (USTs) installed after 1985 resist the effects

of pressure, vibration, and movement. Federal regulations

for USTs exclude the following: farm and residential tanks

of 1,100 gallons (420 liters) or less capacity; tanks storing

heating oil used on the premises; tanks on or above the

floor of basements; septic tanks; flow-through process

tanks; all tanks with capacity of 110 gallons or less; and

emergency spill and overfill tanks [8]. A review of local

and state regulations should be completed before

installing underground tanks because many jurisdictions

do not allow burial of gas or oil tanks. 

Coal

The four types of coal are anthracite, bituminous, subbituminous, and lignite. Coal is prepared in many sizes

and combinations of sizes. The combustible portions of

the coal are fixed carbons, volatile matter (hydrocarbons),

and small amounts of sulfur. In combination with these

are noncombustible elements composed of moisture and

impurities that form ash. The various types differ in heat

content. Heat content is determined by analysis and is

expressed in British thermal units per pound.

Improper coal furnace operation can result in an

extremely hazardous and unhealthy home. Ventilation of

the area surrounding the furnace is very important to

prevent heat buildup and to supply air for combustion.

Solar Energy

Solar energy has gained popularity in the last 25 years as

the cost of installation of solar panels and battery storage

has decreased. Improved technology with panels,

installation of panels, piping, and batteries has created a

much larger market. Solar energy largely has been used to

heat water. Today, there are more than a million solar

water-heating systems in the United States. Solar water

heaters use direct sun to heat either water or a heattransfer fluid in collectors [3]. That water is then stored

for use as needed, with a conventional system providing

any necessary additional heating. A typical system will

reduce the need for conventional water heating by about

two-thirds, minimizing the cost of electricity or the use of

fossil fuel and thus the environmental impact associated

12-6 Heating, Air Conditioning, and Ventilating

Figure 12.2. Piping Hookup for Inside Tank Installation [4] Figure 12.3. Piping Hookup for Buried Outside Tank [4]
with their use. The U.S. Department of Housing and

Urban Development and the U.S. Department of Energy

(DOE) have instituted initiatives to deploy new solar

technologies in the next generation of American housing

[3]. For example, DOE has the Million Solar Roofs

Initiative begun in 1997 to install solar energy systems in

more than 1 million U.S. buildings by 2010.

Central Heating Units

The boiler should be placed in a separate room whenever

possible, which is usually required in new construction.

In most housing inspections, however, the inspector is

dealing with existing conditions and must adapt the

situation as closely as possible to acceptable safety

standards. In many old buildings, the furnace is located in

the center of the cellar or basement. This location does

not lend itself to practical conversion to a boiler room.

Consider the physical requirements for a boiler or

furnace.

• Ventilation—More circulating air is required for

the boiler room than for a habitable room, to

reduce the heat buildup caused by the boiler or

furnace and to supply oxygen for combustion.

• Fire protection rating—As specified by various

codes (fire code, building code, and insurance

underwriters), the fire regulations must be strictly

adhered to in areas surrounding the boiler or

furnace. This minimum clearance for a boiler or

furnace from a wall or ceiling is shown in

Figures 12.4 and 12.5. 

Asbestos was used in numerous places on furnaces to

protect buildings from fire and to prevent lost heat.

Figure 12.6 shows asbestos-coated heating ducts, for

example. Where asbestos insulation is found, it must be

handled with care (breathing protection and protective

clothing) and care must be taken to prevent or contain

release into the air [10]. 

The furnace or boiler makes it difficult to supply air and

ventilation for the room. Where codes and local authority

permit, it may be more practical to place the furnace or

boiler in an open area. The ceiling above the furnace

should be protected to a distance of 3 feet beyond all

furnace or boiler appurtenances, and this area should be

free of all storage material. The furnace or boiler should

be on a firm foundation of concrete if located in the cellar

or basement. If codes permit furnace installations on the

first floor, then they must be consulted for proper setting

and location.

Heating Boilers

The term boiler is applied to the single heat source that

can supply either steam or hot water (a boiler is often

called a heater).

Healthy Housing Reference Manual 12-7

Figure 12.5. Minimum Clearance for Steam or Hot Water Boiler and

Mechanical Warm-air Furnace [4]

Figure 12.6. Heating Ducts Covered With Asbestos Insulation

Figure 12.4. Minimum Clearance for Pipeless Hot Air and Gravity Warm Air

Furnace [4]
Boilers may be classified according to several kinds of

characteristics. They are typically made from cast iron or

steel. Their construction design may be sectional,

portable, fire-tube, water-tube, or special. Domestic

heating boilers are generally of low-pressure type with a

maximum working pressure of 15 pounds per square inch

(psi) for steam and 30 psi for hot water. All boilers have a

combustion chamber for burning fuel. Automatic fuelfiring devices help supply the fuel and control the

combustion. Hand firing is accomplished by the provision of

a grate, ash pit, and controllable drafts to admit air under

the fuel bed and over it through slots in the firing door. A

check draft is required at the smoke pipe connection to

control chimney draft. The gas passes from the

combustion chamber to the flue passages (smoke pipe)

designed for maximum possible transfer of heat from the

gas. Provisions must be made for cleaning flue passages.

Cast-iron boilers are usually shipped in sections and

assembled at the site. They are generally classified as 

• square or rectangular boilers with vertical

sections; and

• round, square, or rectangular boilers with

horizontal pancake sections.

Most steel boilers are assembled units with welded steel

construction and are called portable boilers. Large boilers

are installed in refractory brick settings on the site. Above

the combustion chamber, a group of tubes is suspended,

usually horizontally, between two headers. If flue gases

pass through the tubes and water surrounds them, the

boiler is designated as the fire-tube type. When water

flows through the tubes, it is termed water-tube. Fire-tube

is the predominant type.

Heating Furnaces

Heating furnaces are the heat sources used when air is the

heat-carrying medium. When air circulates because of the

different densities of the heated and cooled air, the

furnace is a gravity type. A fan may be included for the

air circulation; this type is called a mechanical warm-air

furnace. Furnaces may be of cast iron or steel and burn

various types of fuel.

Some new furnaces are as fuel efficient as 95%. Furnaces

with an efficiency of 90% or greater use two heat

exchangers instead of one. Energy savings come not only

from the increased efficiency, but also from improved

comfort at lower thermostat settings.

Fuel-burning Furnaces

Some localities throughout the United States still use coal

as a heating fuel, including residences, schools, colleges

and universities, small manufacturing facilities, and other

facilities located near coal sources.

In many older furnaces, the coal is stoked or fed into the

firebox by hand. The single-retort, underfeed-type

bituminous coal stoker is the most commonly used

domestic automatic-stoking steam or hot-water boiler

(Figure 12.7). The stoker consists of a coal hopper, a

screw for conveying coal from hopper to retort, a fan that

supplies air for combustion, a transmission for driving

coal feed and fan, and an electric motor for supplying

power. The air for combustion is admitted to the fuel

through tuyeres (air inlets) at the top of the retort. The

stoker feeds coal to the furnace intermittently in

accordance with temperature or pressure demands. 

Oil burners are broadly designated as distillate, domestic,

and commercial or industrial. Distillate burners are

usually found in oil-fired space heaters. Domestic oil

burners are usually power driven and are used in domestic

heating plants. Commercial or industrial burners are used

in larger central-heating plants for steam or power

generation.

Domestic oil burners vaporize and atomize the oil and

deliver a predetermined quantity of oil and air to the

combustion chambers. Domestic oil burners operate

automatically to maintain a desired temperature.

Gun-type burners atomize the oil either by oil pressure or

by low-pressure air forced through a nozzle. The oil

system pressure-atomizing burner consists of a strainer,

pump, pressure-regulating valve, shutoff valve and

atomizing nozzle. The air system consists of a powerdriven fan and an air tube that surrounds the nozzle and

electrode assembly.

12-8 Heating, Air Conditioning, and Ventilating

Figure 12.7. Typical Underfeed Coal Stoker Installation in Small Boilers [4]
The fan and oil pump are generally connected directly to the

motor. Oil pressures normally used are about 100 psi, but

pressures considerably in excess of this are sometimes used.

The form and parts of low-pressure, air-atomizing burners

are similar to high-pressure atomizing (gun) burners

(Figure 12.8) except for addition of a small air pump and

a different way of delivering air and oil to the nozzle or orifice. 

The atomizing type burner, sometimes known as a

radiant or suspended flame burner, atomizes oil by

throwing it from the circumference of a rapidly rotating

motor-driven cup. The burner is installed so that the

driving parts are protected from the heat of the flame by a

hearth of refractory material at about the grate elevation.

Oil is fed by pump or gravity; the draft is mechanical or a

combination of natural and mechanical.

Horizontal rotary burners were originally designed for

commercial and industrial use but are available in sizes

suitable for domestic use. In this burner, fuel oil being

thrown in a conical spray from a rapidly rotating cup is

atomized. Horizontal rotary burners use electric gas or

gas-pilot ignition and operate with a wide range of fuels,

primarily Numbers 1 and 2 fuel oil. Primary safety

controls for burner operation are necessary. An

antiflooding device must be a part of the system to stop

oil flow if ignition in the burner should fail. Likewise, a

stack control is necessary to shut off the burner if the

stack temperatures are exceeded, thus cutting off all power

to the burner. This button must be reset before starting

can be attempted. The newer models now use electric eyetype control on the burner itself.

On the basis of the method used to ignite fuels, burners

are divided into five groups:

• Electric—A high-voltage electric spark in the path

of an oil and air mixture causes ignition. This

electric spark may be continuous or may operate

only long enough to ignite the oil. Electric

ignition is almost universally used. Electrodes are

located near the nozzles, but not in the path of

the oil spray.

• Gas pilot—A small gas pilot light that burns

continuously is frequently used. Gas pilots usually

have expanded gas valves that automatically

increase flame size when a motor circuit starts.

After a fixed interval, the flame reverts to normal

size (Figure 12.9). 

• Electric gas—An electric spark ignites a gas jet,

which in turn ignites the oil-air mixture.

• Oil pilot—A small oil flame is used.

• Manual—A burning wick or torch is placed in

the combustion space through peepholes and thus

ignites the charge. The operator should stand to

one side of the fire door to guard against injury

from chance explosion.

The refractory lining or material should be an insulating

fireproof bricklike substance, never ordinary firebrick.

The insulating brick should be set on end to build a

21⁄2-inch-thick wall from furnace to furnace. The size and

shape of the refractory pot vary from furnace to furnace.

Healthy Housing Reference Manual 12-9

Figure 12.8. Cutaway View of Typical High-pressure Gun Burner [4] Figure 12.9. Gas-fired Boiler

Source: HUD
The shape can be either round or square, whichever is

more convenient to build. It is more important to use a

special cement having properties similar to that of the

insulating refractory-type brick.

Steam Heating Systems

Steam heating systems are classified according to the pipe

arrangement, accessories used, method of returning the

condensate to the boiler, method of expelling air from the

system, or the type of control used. The successful

operation of a steam heating system consists of generating

steam in sufficient quantity to equalize building heat loss

at maximum efficiency, expelling entrapped air, and

returning all condensate to the boiler rapidly. Steam

cannot enter a space filled with air or water at pressure

equal to the steam pressure. It is important, therefore, to

eliminate air and remove water from the distribution

system. All hot pipelines exposed to contact by residents

must be properly insulated or guarded. Steam heating

systems use the following methods to return the

condensate to the boiler:

• Gravity one-pipe air-vent system—One of the

earliest types used, this method returns

condensate to the boiler by gravity. This system is

generally found in one-building-type heating

systems. The steam is supplied by the boiler and

carried through a single system or pipe to

radiators, as shown in Figure 12.10. Return of the

condensate is dependent on hydrostatic head.

Therefore, the end of the stream main, where it

attaches to the boiler, must be full of water

(termed a wet return) for a distance above the

boiler line to create a pressure drop balance

between the boiler and the stream main.

Radiators are equipped with an inlet valve and an

air valve. The air valve permits venting of air from

the radiator and its displacement by steam.

Condensate is drained from the radiator through

the same pipe that supplies steam.

• Two-pipe steam vapor system with return trap—

The two-pipe vapor system with boiler return trap

and air eliminator is an improvement of the onepipe system. The return connection of the radiator

has a thermostatic trap that permits flow of

condensate and air only from the radiator and

prevents steam from leaving the radiator. Because

the return main is at atmospheric pressure or less,

a boiler return trap is installed to equalize

condensate return pressure with boiler pressure.

Water Heating Systems

All water heating systems are similar in design and

operating principle. The one-pipe gravity water heating

system is the most elementary of the gravity systems and

is shown in Figure 12.10. Water is heated at the lowest

point in the system. It rises through a single main because

of a difference in density between hot and cold water. The

supply rise or radiator branch takes off from the top of

the main to supply water to the radiators. After the water

gives up heat in the radiator, it goes back to the same

main through return piping from the radiator. This cooler

return water mixes with water in the supply main and

causes the water to cool a little. As a result, the next

radiator on the system has a lower emission rate and must

be larger.

Note in Figure 12.11 that the high points of the hot

water system are vented and the low points are drained.

In this case, the radiators are the high points and the

heater is the low point. 

• One-pipe forced-feed system—If a pump or

circulator is introduced in the main near the heater

of the one-pipe system, it becomes a forced

system that can be used for much larger

applications than can the gravity type. This system

can operate at higher water temperatures than the

gravity system can. When the water is moving

faster and at higher temperatures, it makes a more

responsive system, with smaller temperature drops

and smaller radiators for the same heating load.

• Two-pipe gravity system—A one-pipe gravity

system may become a two-pipe system if the return

radiator branch connects to a second main that

returns water to the heater (Figure 12.12). Water

temperature is practically the same in the entire

radiator.

12-10 Heating, Air Conditioning, and Ventilating

Figure 12.10. Typical Gravity One-pipe Heating System [4]
• Two-pipe forced-circulation system—This system

is similar to a one-pipe forced-circulation system

except that it uses the same piping arrangement

found in the two-pipe gravity system.

• Expansion tanks—When water is heated, it tends

to expand. Therefore, an expansion tank is necessary

in a hot water system. The expansion tank, either

open or closed, must be of sufficient size to

permit a change in water volume within the

heating system. If the expansion tank is open, it

must be placed at least 3 feet above the highest

point of the system. It will require a vent and an

overflow. The open tank is usually in an attic, where

it needs protection from freezing.

The closed expansion tank is found in modern

installations. An air cushion in the tank compresses

and expands according to the change of volume

and pressure in the system. Closed tanks are

usually at the low point in the system and close to

the heater. They can, however, be placed at almost

any location within the heating system. 

Air Heating Systems

Gravity Warm-air Heating Systems. These operate

because of the difference in specific gravity of warm air

and cold air. Warm air is lighter than cold air and rises if

cold air is available to replace it (Figure 12.13).

• Operation—Satisfactory operation of a gravity

warm-air heating system depends on three factors:

size of warm air and cold air ducts, heat loss of

the building, and heat available from the furnace.

• Heat distribution—The most common source of

trouble in these systems is insufficient pipe area,

usually in the return or cold air duct. The total

cross-section area of the cold duct or ducts must

be at least equal to the total cross-section area of

all warm ducts.

• Pipeless furnaces—The pipeless hot-air furnace is

the simplest type of hot-air furnace and is suitable

for small homes where all rooms can be grouped

about a single large register. Other pipeless gravity

furnaces are often installed at floor level. These are

really oversized jacketed space heaters. The most

common difficulty experienced with this type of

furnace is supplying a return air opening of

sufficient size on the floor. 

Forced-Warm-Air Heating Systems. The mechanical

warm-air furnace is the most modern type of warm-air

equipment (Figures 12.13 and 12.14). It is the safest type

because it operates at low temperatures. The principle of a

Healthy Housing Reference Manual 12-11

Figure 12.11. One-pipe Gravity Water Heating System [4]

Figure 12.12. Two-pipe Gravity Water Heating System [4]

Figure 12.13. Warm-air Convection Furnace [4]
forced-warm-air heating system is very similar to that of

the gravity system, except that a fan or blower is added to

increase air movement. Because of the assistance of the

fan or blower, the pitch of the ducts or leaders can be

disregarded; therefore, it is practical to deliver heated air

in the most convenient places. 

• Operation—In a forced-air system, operation of

the fan or blower must be controlled by air

temperature in a bonnet or by a blower control

furnace stat. The blower control starts the fan or

blower when the temperature reaches a certain

point and turns the fan or blower off when the

temperature drops to a predetermined point.

• Heat distribution—Dampers in the various

warm-air ducts control distribution of warm air

either at the branch takeoff or at the warm-air

outlet. Humidifiers are often mounted in the

supply bonnet to regulate the humidity within the

residence.

Space Heaters

Space heaters are the least desirable type of heating from

the viewpoint of fire safety and housing inspection. A

space heater is a self-contained, free-standing air-heating

appliance intended for installation in the space being

heated and not intended for duct connection. According

to the CPSC, consumers are not using care when

purchasing and using space heaters. Approximately

21,800 residential fires are caused by space heaters a year,

and 300 people die in these fires. An estimated 6,000

persons receive hospital emergency room care for burn

injuries associated with contacting hot surfaces of space

heaters, mostly in nonfire situations. 

Individuals using space heaters should use the heaters in

accordance with the following precautions:

• Read and follow the manufacturer’s operating

instructions. A good practice is to read aloud the

instructions and warning labels to all members of

the household to be certain that everyone

understands how to operate the heater safely.

Keep the owner’s manual in a convenient place to

refer to when needed.

• Choose a space heater that has been tested and

certified by a nationally recognized testing

laboratory. These heaters meet specific safety

standards.

• Buy a heater that is the correct size for the area

you want to heat. The wrong size heater could

produce more pollutants and may not be an

efficient use of energy.

• Choose models that have automatic safety

switches that turn off the unit if it is tipped over

accidentally.

• Select a space heater with a guard around the

flame area or heating element. Place the heater on

a level, hard, nonflammable surface, not on rugs

or carpets or near bedding or drapes. Keep the

heater at least 3 feet from bedding, drapes,

furniture, or other flammable materials.

• Keep doors open to the rest of the house if you

are using an unvented fuel-burning space heater.

This helps prevent pollutant build-up and

promotes proper combustion. Follow the

manufacturer’s instructions for oil heaters to

provide sufficient combustion air to prevent CO

production. 

• Never leave a space heater on when you go to

sleep. Never place a space heater close to any

sleeping person.

• Turn the space heater off if you leave the area.

Keep children and pets away from space heaters.

Children should not be permitted to either adjust

the controls or move the heater.

• Keep any portable heater as least 3 feet away from

curtains, newspapers, or anything that might burn.

• Have a smoke detector with fresh batteries on

each level of the house and a CO detector outside

the sleeping area. Install a CO monitor near oil

space heaters at the height recommended by the

manufacturer.

12-12 Heating, Air Conditioning, and Ventilating

Figure 12.14. Cross-sectional View of Building Showing Forced warm-air

Heating System [4]
• Be aware that mobile homes require specially

designed heating equipment. Only electric or

vented fuel-fired heaters should be used.

• Have gas and kerosene space heaters inspected

annually.

• Do not hang items to dry above or on the heater.

• Keep all heaters out of exit and high-traffic areas.

• Keep portable electric heaters away from sinks,

tubs, and other wet or damp places to avoid

deadly electric shocks.

• Never use or store flammable liquids (such as

gasoline) around a space heater. The flammable

vapors can flow from one part of the room to

another and be ignited by the open flame or by an

electrical spark. 

Coal-fired Space Heaters (Cannon Stove)

A coal stove is made entirely of cast iron. Coal on the

grates receives primary air for combustion through the

grates from the ash-door draft intake. Combustible gases

driven from the coal by heat burn in the barrel of the

stove, where they receive additional or secondary air

through the feed door. The side and top of the stove

absorb the heat of combustion and radiate it to the

surrounding space. Coal stoves must be vented to the flue.

Oil-fired Space Heaters

Oil-fired space heaters

have atmospheric

vaporizing-type

burners. The burners

require a light grade of

fuel oil that vaporizes

easily and in

comparatively low

temperatures. In

addition, the oil must

be such that it leaves

only a small amount of

carbon residue and ash

within the heater. Oil

stoves must be vented.

The burner of an oilfired space heater

consists essentially of a bowl, 8 to 13 inches in diameter,

with perforations in the side that admit air for

combustion. The upper part of the bowl has a flame ring

or collar. Figure 12.15 shows a perforated-sleeve burner.

When several space heaters are installed in a building, an

oil supply from an outside tank to all heaters is often

desirable. Figure 12.16 shows the condition of a burner

flame with different rates of fuel flow and indicates the

ideal flame height.

Electric Space Heaters

Electric space heaters do not need to be vented.

Gas-fired Space Heaters

The three types of gas-fired space heaters (natural,

manufactured, and liquefied petroleum gas) have a similar

construction. All gas-fired space heaters must be vented to

prevent a dangerous buildup of poisonous gases. Each

unit console consists of an enamel steel cabinet with top

and bottom circulating grilles or openings, gas burners,

heating elements, gas pilot, and a gas valve. The heating

element or combustion chamber is usually cast iron.

Caution: All gas-fired space heaters and their connections

must be approved by the American Gas Association

(AGA). They must be installed in accordance with the

recommendations of that organization or the local code.

Venting

Use of proper venting materials and correct installation of

venting for gas-fired space heaters is necessary to

minimize harmful effects of condensation and to ensure

that combustion products are carried off. (Approximately

12 gallons of water are produced in the burning of

Healthy Housing Reference Manual 12-13

Figure 12.15. Perforated-sleeve Burner 

Figure 12.16. Condition of Burner Flame with Different Rates of Fuel Flow [4]
1,000 cubic feet of natural gas. The inner surface of the

vent must therefore be heated above the dew point of the

combustion products to prevent water from forming in

the flue.) A horizontal vent must be given an upward

pitch of at least 1 inch per foot of horizontal distance.

When the smoke pipe extends through floors or walls, the

metal pipe must be insulated from the floor or wall

system by an air space (Figure 12.17). Sharp bends should

be avoided. A 9° vent elbow has a resistance to flow

equivalent to that of a straight section of pipe with a

length 10 times the elbow diameter. Be sure that vents are

of rigid construction and resistant to corrosion by flue gas

products. Several types of venting material are available

such as B-vent and other ceramic-type materials. A

chimney lined with firebrick type of terra cotta must be

relined with an acceptable vent material if it is to be used

for venting gas-fired appliances.

The same size vent pipe should be used throughout its

length. A vent should never be smaller than the heater

outlet except when two or more vents converge from

separate heaters. To determine the size of vents beyond

the point of convergence, one-half the area of each vent

should be added to the area of the largest heater’s vent.

Vents should be installed with male ends of inner liner

down to ensure condensate is kept within pipes on a cold

start. The vertical length of each vent or stack should be

at least 2 feet greater than the length between horizontal

connection and stack. Remember that the more

conductive the unit, the lower the temperature of

combustion and the more byproducts of combustion are

likely to be produced. These by-products are sometimes

referred to as soot and creosote. These by-products will

build up in vents, stacks, and chimneys. They are extremely

flammable and can result in fire in these units that is hot

enough to penetrate the heat shielding and throw burning

material onto the roof of the home. 

The vent should be run at least 3 feet above any

projection within 20 feet of the building to place it above

a possible pressure zone due to wind currents (Figure 12.18).

A weather cap should prevent entrance of rain and snow.

Gas-fired space heaters, as well as gas furnaces and water

heaters, must be equipped with a backdraft diverter

(Figure 12.19) to protect heaters against downdrafts and

excessive updrafts. Only draft diverters approved by the AGA

should be used.

The combustion chamber or firebox must be insulated

from the floor, usually with airspace of 15 to 18 inches.

The firebox is sometimes insulated within the unit and

thus allows for lesser clearance firebox combustibles.

Floors should be protected where coal space heaters are

located. The floor protection allows hot coals and ashes to

cool off if dropped while being removed from the ash

chamber. Noncombustible walls and materials should be

used when they are exposed to heated surfaces. For space

heaters, a top or ceiling clearance of 36 inches, a wall

clearance of 18 inches, and a smoke pipe clearance of

18 inches are recommended.

Hydronic Systems

Hydronic (circulating water) systems involving traditional

baseboards can be single-pipe or two-pipe. Radiant

systems are also an option. All hydronic systems require

an expansion tank to compensate for the increase in water

volume when it is heated (i.e., the volume of 50°F [10°C]

water increases almost 4% when it is heated to 200°F

[93°C]). Single-pipe hydronic systems are most

commonly used in residences. They use a single pipe with

hot water flowing in a series loop from radiator to

radiator. Massachusetts has a prototype set of hydronic

systems requirements [11].

12-14 Heating, Air Conditioning, and Ventilating

Figure 12.17. Wall and Ceiling Clearance Reduction [4] Figure 12.18. Draft Relation to Height of Chimney [4]
The drawback to this arrangement is that the temperature

of the water decreases as it moves through each radiator.

Thus, larger radiators are needed for those locations

downstream in the loop. A common solution to this is

multiple loops or zones. Each zone has its own

temperature control with circulation provided by a small

pump or zone valve in each loop. Two-pipe hydronic

systems use a pipe for supplying hot water to the radiators

and a second pipe for returning the water from the

radiators to the boiler.

There are also direct- and reverse-return arrangements.

The direct-return system can be difficult to balance

because the pressure drop through the nearest radiator

piping can be significantly less than for the farthest

radiator. Reverse-return systems take care of the balancing

problem, but require the expense of additional piping.

Orifice plates at radiator inlets or balancing valves at

radiator outlets can also be used to balance the pressure

drops in a direct-return system.

Direct Vent Wall Furnaces

Direct vent wall furnaces are specifically designed for

areas where flues or chimneys are not available or cannot

be used. The furnace is directly vented to the outside and

external air is used to support combustion. The air on the

inside is warmed as it recirculates around a sealed chamber.

Cooling

Air Conditioning

Many old homes relied on passive cooling-opening windows

and doors and using shading devices-during the summer

months. Homes were designed with windows on opposite

walls to encourage cross ventilation and large shade trees

reduced solar heat gains. This approach is still viable, and

improved thermal performance (insulating value)

windows are available that allow for larger window areas

to let in more air in the summer without the heat loss

penalty in the winter. However, increased outdoor noise

levels, pollution, and security issues make relying on open

windows a less attractive option in some locations today.

An air conditioning system of some kind may be installed

in the home. It may be a window air conditioner or

through-the-wall unit for cooling one or two rooms, or a

central split-system air conditioner or heat pump. In any

event, the performance of these systems, in terms of

providing adequate comfort without excessive energy use,

should be investigated. The age of the equipment alone

will provide some indication. If the existing system is

more than 10 years old, replacement should be considered

because it is much less efficient than today’s systems and

nearing the end of its useful life.

The refrigerant commonly used in today’s residential air

conditioners is R-22. Because of the suspicion that R-22

depletes the ozone layer, manufacturers will be prohibited

from producing units with R-22 in 2010. The leading

replacements for R-22 are R-134A and R-410A, and new

products are now available with these nonozone-depleting

refrigerants.

The performance measure for electric air conditioners

with capacities less than 65,000 BTU is the seasonal

energy efficiency ratio (SEER). SEER is a rating of

cooling performance based on representative residential

loads. It is reported in units of BTU of cooling per watt

per hour of electric energy consumption. It includes

energy used by the unit’s compressor, fans, and controls.

The higher the SEER, the more efficient the system.

However, the highest SEER unit may not provide the

most comfort. In humid climates, some of the highest

SEER units exhibit poor dehumidification capability

because they operate at higher evaporator temperatures to

attain the higher efficiency. A SEER of at least 10 is

required by the National Appliance Energy Conservation

Act of 1987 for conventional central split-system aircooled systems. The Department of Energy announced a

SEER of 13 effective January 2006.

Cooling system options vary widely, depending on the

level of control and comfort desired by the homeowner.

Fans can increase circulation and reduce cooling loads,

but they may be unsatisfactory in hot climates because

their cooling capability is directly limited by outdoor

conditions. Radiant barriers can reduce cooling loads in

very hot climates. Evaporative coolers can be a relatively

inexpensive and effective method of cooling in dry

climates, such as the Southwest. Electric air conditioning

maintains a comfortable indoor temperature and

humidity even under the most severe outdoor conditions.

Healthy Housing Reference Manual 12-15

Figure 12.19. Location and Operation of Typical Backdraft Diverter [4]
More than 75% of new homes in the United States are

equipped with some form of central air conditioning:

50% of the homes in the Northeast, 75% in the Midwest,

95% in the South, and approximately 60% in the West.

Electric air conditioning removes moisture from the air

and reduces its temperature. It can be a good investment

because, in most parts of the country, the payback is

significant when the house is sold.

Electric air conditioners that use the vapor-compression,

refrigeration cycle are available in a variety of sizes and

configurations, ranging from small window units to large

central systems. The most common form of central air

conditioning is a split-system with a warm air furnace

(Figure 12.20). The same ductwork is used for distributing

conditioned air during the heating and cooling seasons.

Supply air is cooled and dehumidified as it passes over an

A-shaped evaporator coil. The liquid refrigerant

evaporates inside the coil as it absorbs heat from the air.

The refrigerant gas then travels through refrigerant piping

to the outdoor unit, where it is pressurized in an

electrically driven compressor, raising its temperature and

pressure, and returned to a liquid state in the condenser

as it releases, or dumps, the heat to the outdoors. A fan

draws outdoor air in over the condenser coil. The use of

two-speed indoor fans can be advantageous in this type of

system because the cooling load often requires higher

airflows than the heating load. The lower speed can be

used for the heating season and for improved

dehumidification performance during the cooling season.

The condenser unit for a house air conditioner is shown

in Figure 12.21.

Circulation Fans

Air movement can make a person feel comfortable even

when dry-bulb temperatures are elevated. A circulation

fan (ceiling or portable) that creates an airspeed of 150 to

200 feet per minute can compensate for a 4°F (-16°C)

increase in temperature.

Ceiling circulation fans also can be beneficial in the

heating season by redistributing warm air that collects

along the ceiling, but they can be noisy. 

Evaporation Coolers

In dry climates, as in the southwestern United States, an

evaporative cooler or “swamp” cooler may provide

sufficient cooling. This system cools an airstream by

evaporating water into it; the airstream’s relative humidity

increases while the dry-bulb temperature decreases. A

95°F (35°C), 15% relative humidity airstream can be

conditioned to 75°F (24°C), 50% relative humidity. The

simplest direct systems are centrally located and use a

pump to supply water to a saturated pad over which the

supply air is blown. Indirect systems use a heat exchanger

between the airstream that is cooled by evaporating water

and the supply airstream. The moisture level of the supply

airstream is not affected as it is cooled.

Evaporation coolers have lower installation and operating

costs than electric air conditioning. No ozone-depleting

refrigerant is involved. They provide high levels of

ventilation because they typically condition and supply

100% outside air.

The disadvantages are that bacterial contamination can

result if not properly maintained and they are only

appropriate for dry, hot climates.

12-16 Heating, Air Conditioning, and Ventilating

Figure 12.20. Split-system Air Conditioner Figure 12.21. External Air-conditioning Condenser Unit 
Safety

Cooling homes with window air conditioners requires

attention to the maintenance requirements of the unit.

The filter must be cleaned or replaced as recommended

by the manufacturer, and the drip pan should be checked

to ensure that proper drainage from the unit is occurring.

The pans should be rinsed and disinfected as

recommended by the manufacturer. Both bacteria and

fungi can establish themselves in these areas and present

serious health hazards.

Condensation forms on the cooling coils of central air

units inside and outside the home. These units should

have a properly installed drip pan and should be drained

according to the manufacturer’s instructions. They also

should receive routine maintenance, flushing, and

disinfection. In the spring, before starting the air

conditioner, the unit should be checked by a professional

or someone familiar with the operation of the system.

This is a good time to check drip line(s) for conditions

such as plugs, cracks, or bacterial contamination because

many of these lines are plastic. The drip pan should be

cleaned thoroughly and disinfected if necessary or

replaced. A plugged drip line can cause water damage by

overflow from the drip pan. In the fall, the heat unit also

should be checked before starting the system. Care should

be taken with both window air conditioning units and

central air systems to use quality air filters that are

designed for the specific units and meet the specifications

required by the system’s manufacturer.

The housing inspector should be on the alert for unvented,

open flame heaters. Coil-type, wall-mounted water

heaters that do not have safety relief valves are not

permitted. Kerosene (portable) units for cooking or

heating should be prohibited. Generally, open-flame

portable units are not allowed under fire safety regulations.

In oil heating units, other than integral tank units, the oil

must be filled and vented outside the building. Filling oil

within buildings is prohibited. Cutoff switches should be

close to the entry but outside of a boiler room. 

Chimneys 

Chimneys (Figure 12.22) are often an integral part of a

building. Masonry chimneys must be tight and sound;

flues should be terra cotta-lined; and, where no linings are

installed, the brick should be tight to permit proper draft

and elimination of combustion gases.

Chimneys that act as flues for gas-fired equipment must

be lined with either B vent or terra cotta. When a portion

of the chimney above the roof either loses insulation or

the insulation peels back, it indicates potential poisonous

gas release or water leakage problems and a need for

rebuilding. Exterior deterioration of the chimney, if

neglected too long, will permit erosion from within the

flues and eventually block the flue opening.

Rusted flashing at the roof level will also contribute to the

chimney’s deterioration. Efflorescence on the inside wall

of the chimney below the roof and on the outside of the

chimney, if exposed, will show salt accumulations—a

telltale sign of water penetration and flue gas escape and a

sign of chimney deterioration. During rainy seasons, if

terra cotta chimneys leak, dark areas show the number of

flues inside the masonry chimney so they can actually be

counted. When this condition occurs, it usually requires

2 or 3 months to dry out. After drying out, the mortar

joints are discolored (brown). After a few years of this

type of deterioration, the joints can be distinguished

whether the chimney is wet or dry. These conditions

usually develop when coal is used and become more

pronounced 2 to 5 years after conversion to oil or gas.

An unlined chimney can be checked for deterioration

below the roofline by looking for residue deposited at the

base of the chimney, usually accessible through a cleanout

(door or plug) or breaching. Red granular or fine powder

showing through coal or oil soot will generally indicate, if

in quantity (a handful), that deterioration is excessive and

repairs are needed.

Unlined chimneys with attached gas units will be devoid

of soot, but will usually show similar telltale brick powder

and deterioration. Manufactured gas has a greater

tendency to dehydrate and decompose brick in chimney

flues than does natural gas. For gas installations in older

Healthy Housing Reference Manual 12-17

Figure 12.22. Chimney Plan [4]
homes, utility companies usually specify chimney

requirements before installlation; therefore, older

chimneys may require the installation of terra cotta liners,

nonlead-lined copper liners, stainless steel liners, or transit

pipe. Black carbon deposits around the top of the

chimney usually indicate an oil burner operation using a

low air ratio and high oil consumption. Prolonged

operation in this burner setting results in long carbon

water deposits down the chimney for 4 to 6 feet or more

and should indicate to the inspector a possibility of poor

burner maintenance. This will accent the need to be more

thorough on the next inspection. This type of condition

can result from other causes, such as improper chimney

height, or exterior obstructions, such as trees or buildings,

that will cause downdrafts or insufficient draft or

contribute to a faulty heating operation. Rust spots and sootmold usually occur on deteriorated galvanized smoke pipe.

Fireplaces

Careful attention should be given to construction of the

fireplace (Figure 12.23). Improperly built fireplaces are a

serious safety and fire hazard. The most common causes of

fireplace fires are thin walls, combustible materials such as

studding or trim against sides and back of the fireplace,

wood mantels, and unsafe hearths.

Fireplace walls should be no less than 8 inches thick; if

built of stone or hollow masonry units, they should be no

less than 12 inches thick. The faces of all walls exposed to

fire should be lined with firebrick or other suitable fireresistant material. When the lining consists of 4 inches of

firebrick, such lining thickness may be included in the

required minimum thickness of the wall.

The fireplace hearth should be constructed of brick,

stone, tile, or similar incombustible material and should

be supported on a fireproof slab or on a brick arch. The

hearth should extend at least 20 inches beyond the

chimney breast and no less than 12 inches beyond each side

of the fireplace opening along the chimney breast. The

combined thickness of the hearth and its supporting

construction should be no less than 6 inches at any point. 

It is important that all wooden beams, joists, and studs

are set off from the fireplace and chimney so that there is

no less than 2 inches of clearance between the wood

members and the sidewalls of the fireplace or chimney

and no less than 4 inches of clearance between wood

members and the back wall of the fireplace.

A gas-log set is primarily a decorative appliance. It

includes a grate holding ceramic logs, simulated embers, a

gas burner, and a variable flame controller. These sets can

be installed in most existing fireplaces. There are two

principal types: vented and unvented. Vented types

require a chimney flue for exhausting the gases. They are

only 20% to 30% efficient; and most codes require that

the flue be welded open, which results in an easy exit path

for heated room air. Unvented types operate like the

burner on a gas stove and the combustion products are

emitted into the room. They are more efficient because

no heat is lost up the flue and most are equipped with

oxygen-depletion sensors. However, unvented types are

banned in some states, including Massachusetts and

California. Gas fireplaces incorporate a gas-log set into a

complete firebox unit with a glass door. Some have builtin dampers, smoke shelves, and heat-circulating features

that allow them to provide both radiant and convective

heat. Units can have push-button ignition, remote

control, variable heat controls, and thermostats. Gas

fireplaces are more efficient than gas logs, with efficiencies

of 60% to 80%. Many draw combustion air in from the

outside and are direct vented, eliminating the need for a

chimney. Some of these units are wall-furnace rated.

There are also electric fireplaces that provide the

ambience of a fire and, if desired, a small amount of

resistance heat. These units have no venting requirements.

The advantages are that there are no ashes or flying sparks

that occur with wood-burning fireplaces. They are not

12-18 Heating, Air Conditioning, and Ventilating

Figure 12.23. Fireplace Construction [4]
affected by wood-burning bans imposed in some areas

when air quality standards are not met. Direct-vented gas

or electric models eliminate the need for a chimney.

The disadvantages are that the cost for equipment and

running the gas line can be high. 

References

1. US Census Bureau. Rooms, number of bedrooms,

and house heating fuel: 2000. Census 2000

Summary File 4 (SF4). Washington, DC: US

Census Bureau; 2000. Available from URL:

http://factfinder.census.gov [select Data Sets, then

select Census 2000 Summary File 4 (SF 4) -

Sample Data].

2. National Energy Efficiency Committee. Building:

heat pumps; application of heat pumps in

Singapore. Singapore: National Environment

Agency; 2003. Available from URL:

http://www.neec.gov.sg/building/

heat_pump.shtm.

3. National Energy Efficiency Committee.

Renewables: solar energy. Singapore: National

Environment Agency; no date. Available from

URL: http://www.neec.gov.sg/

renewables/solar.shtm.

4. Center for Disease Control. Basic housing

inspection. Atlanta: US Department of Health

and Human Services; 1976. 

5. Fairfax County. Fuel storage tanks. Fairfax, VA:

Fairfax County; 2004. Available from URL:

http://www.co.fairfax.va.us/dpwes/construction/

fuel_tanks.htm.

6. Wisconsin Department of Commerce.

Environmental services—residential fuel oil and

gasoline storage tanks. Madison, WI: Wisconsin

Department of Commerce; 2004. Available from

URL: http://www.commerce.state.wi.us/ER/ERBST-ResTk.html.

7. Michigan Department of Environmental Quality.

FAQ: Home heating oil tanks, Lansing, MI:

Michigan Department of Environmental Quality;

no date. Available from URL:

http://www.michigan.gov/deq/

0,1607,7-135-3311_4115_4238-9379--,00.html.

8. US Environmental Protection Agency. Overview

of the federal underground storage tank program.

Washington, DC: U.S. Environmental Protection

Agency; no date. Available from URL:

http://www.epa.gov/swerust1/overview.htm.

9. US Department of Housing and Urban

Development. Initiatives Programs. Washington,

DC: US Department of Housing and Urban

Development; 2004. Available from URL:

http://www.hud.gov/offices/cpd/energyenviron/

energy/initiatives.

10. US Environmental Protection Agency. Sources of

indoor air pollution—asbestos. Washington, DC:

U.S. Environmental Protection Agency; no date.

Available from URL: http://www.epa.gov/

iaq/asbestos.html.

11. The Commonwealth of Massachusetts Board of

Building Regulations and Standards (BBRS).

Hydronic system requirements. Boston: The

Commonwealth of Massachusetts Board of

Building Regulations and Standards (BBRS); no

date. Available from URL: http://www.mass.gov/

bbrs/commntry/cmpxhydr.htm.

Additional Sources of Information

McQuiston FC, Parker JD, Spitler JD. Heating,

ventilation, and air conditioning: analysis and design. 5th

ed. Hoboken, NJ: John Wiley and Sons, Inc.; 2000.

Kittle JL. Home heating and air conditioning systems.

New York: McGraw-Hill; 1990.

Pita EG. Air conditioning principles and systems: an

energy approach. 4th ed. New York: Prentice Hall; 2001. 

Healthy Housing Reference Manual 12-19
12-20 Heating, Air Conditioning, and Ventilating