CENTRAL HEAT PUMP

AND AIR CONDITIONER

INSTALLATION

BUILDING TECHNOLOGIES PROGRAM

OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY

Buildings for

the 21st Century

Buildings that are more

energy-efficient, comfortable,

and affordable...that’s the

goal of DOE’s Office of Building

Technologies Program.

To accelerate the development

and wide application of energy

efficiency measures, the

Building Technologies Program:

• Conducts R&D on technologies

and concepts for energy efficiency, working closely with

the building industry and with

manufacturers of materials,

equipment, and

appliances

• Promotes energy/money

saving opportunities to both

builders and buyers of homes

and commercial buildings

• Works with State and local

regulatory groups to improve

building codes, appliance

standards, and guidelines for

efficient energy use

Technology Fact Sheet

INTRODUCTION

A typical family may spend 40 to 60 percent or

more of its annual utility budget on heating

and cooling. Careful installation of heating and

cooling equipment can increase operating

efficiency, increase occupant comfort, and

reduce utility costs.

WHY IS INSTALLATION IMPORTANT?

Incorrect installation of central heat pumps

and air conditioners leads to high electric bills,

poor indoor comfort, and maintenance

problems. Many studies have shown that heat

pumps and air conditioners — even new units

— are often improperly charged, which can

reduce system capacity and efficiency by

20 percent or more.

Incorrect air flow across the indoor coil is

another common problem in air conditioner

and heat pump installations, with a

subsequent impact on indoor comfort and

operating costs. Low air flow affects the

cooling and heating capacity and efficiency of

the unit. Excess air flow is a less common

problem, but it can cause poor dehumidification during cooling, duct noise, and drafts.

Reduced air flow across the outdoor coil or

inadequate air circulation around the outside

unit also lowers capacity and operating

efficiency. This often occurs when bushes are

planted too close to the unit, the unit is

installed under a porch, or the outdoor coil

becomes clogged with leaves, pollen, or lint

from a dryer.

Improperly installed controls can cause poor

humidity control during cooling operation and

inefficient operation of a heat pump during the

heating season. Other installation problems

that lower capacity and operating efficiency

and cause maintenance problems include

refrigerant lines that are too long, incorrectly

pitched, or crimped.

DESIGN AND INSTALLATION ELEMENTS

Proper heat pump and air conditioner

installation is just one of four elements needed

to provide an economical, efficient, and

comfortable heating and cooling system. The

design and installation of central heat pump

and air conditioner systems must also address

the following elements before or during

equipment installation:

• Equipment must be properly selected and

sized to meet house heating and cooling

loads and to be as efficient as possible.

• The duct system must be correctly designed

to work with the air conditioning and heat

pump equipment and to distribute

conditioned air properly throughout the

house.

• The duct system must be properly installed

and sealed to ensure its design performance.

Lighting, 

cooking 

and other 

appliances

Refrigerator

Heating 

& cooling

Hot water 

heating

HOME ENERGY COSTS
INSTALLATION GUIDELINES

Heat pumps and air conditioners should be installed by

knowledgeable technicians according to the manufacturer’s

installation instructions and all national and local code

requirements. General installation guidelines are applicable to

most units and should be followed insofar as they do not

conflict with these other requirements.

Proper air conditioner and heat pump installation can be

achieved by addressing four critical areas:

• indoor air handler, especially the air flow over the fan-coil unit

and through the forced-air duct system

• refrigerant system, especially the refrigerant charge

• outdoor unit, especially its air supply

• control system, especially the thermostat that turns the

equipment on and off.

INDOOR AIR HANDLER

INDOOR AIR FLOW

For most heat pump and air conditioning equipment, air flow

over the indoor coil should be 400 cubic feet per minute (cfm)

per ton of air conditioning capacity (plus or minus 50 cfm/ton)

when the air conditioner has been operating long enough to

wet the indoor coil (usually about 15 minutes). If wet coil

conditions cannot be obtained (e.g., installation during the

winter), then air flow should be 425 to 450 cfm/ton (plus or

minus 50 cfm/ton). In humid climates where removing

humidity from the interior air is an important function, the best

system efficiency may occur where the air flow is between 350

and 400 cfm/ton. In dry climates where little dehumidification

is needed, the best system efficiency may occur when the air

flow is between 400 and 450 cfm/ton.

Before indoor coil air flow measurements are performed, the

duct system should be inspected and tested to ensure that

leaks are minimized. Although no technique for air flow

measurement currently has universal acceptance, the best flow

measurement method available should be performed to verify

and document that the air flow is within acceptable ranges.

There are several air flow measurement approaches:

• The most common technique involves measuring the static

pressure across the air handler unit and using the

manufacturer’s static pressure vs. flow rate curves for the air

handler to estimate air flow. The manufacturer’s curves will

STATIC PRESSURES

To measure air flow

usually be based on the pressure between the return plenum

and the area either before or after the indoor coil. Measuring

the pressure before the coil is difficult, so it may be easier to

calculate it by measuring the pressure after the coil and

compensating for the pressure drop across the indoor coil.

This method can give good results if accurate pressure

measurements can be made and flow curves are available.

However, pressure measurements are often not accurate

because pressures fluctuate significantly around the air

handler unit fan and coil as a result of flow turbulence induced

by the fan, abrupt changes in flow direction near the air

handler, etc. Flow curves are often not available to technicians

in the field, and curves developed for new fans and coils are

often not applicable to fans and coils that have become dirty

while operating in the field.

A variation of this method is to measure the static pressure

across the indoor coil and use a flow curve supplied by the

coil manufacturer to infer flow rate. Again, measuring the

pressure before the coil is difficult and leads to errors.

• A second method is applicable just to heat pump systems

equipped with supplementary strip heaters. The unit is

operated with just the strip heaters on. The air flow rate can be

calculated by measuring the temperature rise of air across the

strip heaters and the energy consumption of the strip heaters
and indoor fan [air flow rate (cfm) = 3.16 x energy use

(watts)/∆T (°F)]. The accuracy of this method depends on the

accuracy of the temperature and energy consumption

measurements. In making the temperature measurements, it is

important that the air be well mixed where the temperature is

being measured and that the temperature sensor be out of the

line of sight of the strip-heat elements to avoid an inaccurately

high temperature reading due to radiative effects. The air flow

measured using this technique must be corrected to wet coil

conditions. Also, in some equipment, the fan speed during

operation in the strip-heat mode is different from that during

operation in the air-conditioning mode unless the thermostat

and/or fan is properly configured by the technician for the test.

• Another method involves the use of flow hoods to measure air

flow rates. In this approach, air flow rates are measured at the

return registers or supply registers and added to obtain the

total flow rate for the system. Researchers have found that

many flow hoods used in residential applications in this

manner are relatively inaccurate because of low air flow rates,

non-uniform air flow through the flow hood, and other

reasons. In addition, flow hoods underestimate the actual flow

across the indoor coil when duct leaks are present.

• A new method that shows much promise for simplifying the

measurement process while providing sufficiently accurate

results is a specially designed orifice plate. The air filter in the

system is removed and the orifice plate is inserted in its place.

The pressure drop across the orifice plate is measured and

used to estimate the system air flow rate using a flow curve for

the orifice plate and a small correction factor for the different

pressure drop induced by the orifice plate compared with the

air filter. Non-uniform air flow entering the orifice plate (due to

bends in the return duct before the orifice plate) reduces the

accuracy of this method. As with flow hoods, duct leaks make

this method underestimate the actual air flow across the

indoor coil if the air filter is not located at the air handler.

• A final, more costly and complicated method involves use of a

duct blower to measure air flow. A duct blower is a device

used to measure duct leakage in an air distribution system. It

comes equipped with its own fan and a flow metering system.

First, the pressures in the supply and return ducts are

measured under normal air-conditioning operation. The duct

blower is then connected to the return register or the air

handler directly (blocking off the existing return) so that all

return air flows through the duct blower. The equipment is

again operated in its normal air-conditioning mode, with the

duct blower fan operated so that the same pressures are

achieved in the supply and return ducts. The flow measured

through the duct blower is the same as the flow rate through

the equipment during normal operation.

AI R FILTERS

Air filters should be located as close to the indoor fan and coil

as possible and where they are easily accessible by the

occupants. Forced-air heat pump and air conditioning systems

use air filters to minimize the accumulation of dust on the

indoor coil and fan because dust accumulation can reduce

system efficiency. An air filter that is left in place too long will

become plugged and will reduce air flow across the coil. Most

manufacturers recommend replacing filters monthly, which

requires frequent and ready access by the occupants.

REFRIGERANT SYSTEM

CHARGE

Proper refrigerant charge is critical to efficient and reliable

operation of air conditioners and heat pumps. Before new

systems or lines that have been exposed to the atmosphere are

charged, they should be evacuated by the deep vacuum

method. This method of system evacuation requires use of a

gauge that can accurately measure pressures at least as low as

500 microns of mercury and a pump that can achieve those

levels of evacuation. The low pressures are needed to eliminate

moisture from the refrigerant loop.

After the system is assembled and before refrigerant is added,

the system should be evacuated to a pressure no higher than

500 microns. The pump should be valved off and the system

pressure monitored and recorded once a minute for a minimum

of five minutes. Subsequent action depends on the

measurements obtained.

• In a tight, dry system, the pressure will quickly rise to about

1000 microns and then remain stable. Such a system is ready

to receive an initial charge.

• In a tight system that contains too much moisture, the

pressure will rise above 2000 microns before stabilizing.

Continued pumping is needed to remove the moisture.

• A system that has a leak will show pressure that does not

stabilize, slowly rising to atmospheric pressure. A leaky system

must be sealed and re-evacuated.
CENTRAL HEAT PUMP AND AIR CONDITIONER INSTALLATION

Once the system has been properly evacuated, refrigerant can

be weighed into the system per specifications or a pre-charge

stored in the outdoor unit can be released (package systems

are most often shipped fully charged). In both cases, account

for any additional charge needed for refrigerant line lengths.

After the initial charge has been introduced and the correct air

flow rate across the indoor coil had been verified, the charge

should be checked after the system has operated for 15

minutes before the system can be considered accurately

charged. There are three preferred methods of testing the

refrigerant charge of heat pumps and air conditioners:

superheat method, subcooling method, and approach method.

• The superheat method is suitable for systems that use fixed

metering systems (e.g., a capillary tube or a fixed orifice). In

general, the following measurements are made: the evaporator

(indoor coil) pressure at the suction line service port, the

suction line temperature within five feet of the compressor

(e.g., at the suction line service port for split units, just before

the accumulator of packaged units), the dry bulb temperature

of the air entering the condenser (outdoor coil), and the wet

bulb temperature of the indoor air entering the evaporator. The

actual superheat is calculated by subtracting the saturation

temperature based on the evaporator pressure from the

suction line temperature. The desired superheat is determined

from manufacturer specifications based on the entering

evaporator and condenser air temperatures. The actual

superheat is compared with the desired superheat, and the

refrigerant charge is adjusted (refrigerant is added if the actual

superheat is too high and removed if it is too low) until the

actual superheat is within 2°F of the desired value. The system

should operate for at least 10 minutes between adjustments.

• The subcooling method is used for systems with thermostatic

expansion valves (TXVs). In general, the following

measurements are made: the condenser pressure at the

discharge line service port, the discharge (liquid) line

temperature (e.g., entering the expansion valve, the condenser

SUPERHEAT AND SUBCOOL METHODS
CENTRAL HEAT PUMP AND AIR CONDITIONER INSTALLATION

outlet), and the dry bulb temperature of the air entering the

condenser (outdoor coil). The actual subcooling is calculated

by subtracting the discharge (liquid) line temperature from the

saturation temperature based on the condenser pressure. The

desired subcooling is determined from manufacturer

specifications, possibly based on the entering condenser air

temperature. The actual subcooling is compared with the

desired subcooling, and the refrigerant charge is adjusted

(refrigerant is added if the actual subcooling is too low and

added if it is too high) until the actual subcooling is within 2°F

of the desired value. The system should operate for at least 10

minutes between adjustments.

• The approach method is a less widely used method

recommended by some manufacturers. Two measurements

are made: the discharge (liquid) line temperature (between the

condenser outlet and expansion device) and the dry bulb

temperature of the air entering the condenser (outdoor coil).

The difference between these two values is the approach

temperature. The charge should be adjusted until the

recommended approach temperature is achieved. The system

should operate for at least 10 minutes between adjustments.

If ambient temperatures are below 80°F and the manufacturer

does not provide superheat or subcooling data for outdoor

temperatures below this value, then the tests must be modified

to simulate higher outdoor temperatures, usually by partially

blocking air flow to the outdoor coil.

The correct charge generally ensures that the evaporator and

condenser operate at their design pressure and temperature. As

a final check on system operation, the temperature difference

across the indoor coil should be between 10 and 20°F (usually

about 15°F).

REFRIGERANT LINES

In split system designs:

• Refrigerant lines should be as short and straight as possible

to minimize heat loss/gain and pressure drop. Normally,

refrigerant lines should be less than 50 feet long, with a

vertical length of less than 20 feet. Where longer runs are

necessary, lines must comply with special manufacturer

requirements.

• Refrigerant lines should be pitched and trapped according to

manufacturer specifications. Bends and elbows should have a

large radius (greater than twice the pipe diameter) to minimize

pressure loss. Lines should be supported with hangers that

avoid cutting the line or insulation.

• Suction lines should be insulated with a minimum 3/8-inchthick closed-cell elastomeric foam pipe insulation to prevent

condensation and heat gain. Refrigeration line insulation

located outside should have a waterproof covering and should

be protected from ultraviolet radiation (i.e., sunlight). Liquid

lines normally do not need to be insulated.

• The hole in the outside wall through which the refrigerant

lines go should be caulked. Similarly, the entry hole into the

indoor unit should be sealed.

• Unless otherwise specified by the manufacturer, a liquid-line

filter/dryer should be installed.

OUTDOOR UNIT

The outdoor unit of heat pumps and air conditioners

exchanges heat with outdoor air. To ensure effective heat

dissipation, the outdoor unit should be located where air can

circulate around it freely. Most outdoor units have a coil

surrounding a centrally-located fan that pulls air through the

coil. There should be no obstruction within 36 inches on the

side with the service panel to allow for maintenance access,

and there should be no obstructions within 12 inches on any

of the remaining sides to allow for free air circulation. There

should be no obstructions (e.g., porches) within 48 inches of

the vertical air discharge from the unit that would force

discharged air to recirculate directly back to the fan intakes.

Landscaping should be kept at least 3 feet away from the

outdoor unit so that leaves, seeds, and pollen do not plug the unit.

To prevent maintenance and other problems, the outdoor unit

should be placed:

• on a level concrete (or equivalent) pad, elevated (if necessary)

to prevent problems from snow or water accumulation, and

leveled

• away from where collected rainwater, snow, or ice are likely to

fall on it (e.g., under roof eaves)

• away from dryer vents (lint can plug the outdoor coil and

chlorine in the dryer exhaust can cause corrosion)

• away from decks and windows to prevent the noise from

disturbing occupants.
For more information, contact:

Energy Efficiency and

Renewable Energy

Clearinghouse (EREC)

1-800-DOE-3732

www.eere.doe.gov

Or visit the Building

Technologies Program

Web site at

www.buildings.gov

Or refer to the Specification of

Energy-Efficient Installation

and Maintenance Practices for

Residential HVAC Systems,

Consortium for Energy

Efficiency

617-589-3949

www.cee1.org/resid/rs-ac/

hvac.php3

Written and prepared for

the U.S. Department of

Energy by:

Southface Energy Institute

404-872-3549

www.southface.org

U.S. Department of Energy’s

Oak Ridge National

Laboratory

Buildings Technology Center

865-574-5206

www.ornl.gov/btc

CENTRAL HEAT PUMP AND AIR CONDITIONER INSTALLATION

NOTICE: Neither the United

States government nor any

agency thereof, nor any of their

employees, makes any warranty,

express or implied, or assumes

any legal liability or responsibility

for the accuracy, completeness,

or usefulness of any information,

apparatus, product, or process

disclosed. The views and opinions of authors expressed herein

do not necessarily state or reflect

those of the United States government or any agency thereof.

Printed with a renewable-source ink on paper containing at

least 50% wastepaper, including 20% postconsumer waste.

February 2003 DOE/GO102002-0781

CONTROL SYSTEM

THERMOSTAT

The heart of the control system is the

thermostat. The thermostat should be placed

on an inside wall, in a central area of the

house, in a location protected from direct

sunshine, and out of drafts induced by air

supply registers. The interior wall cavity on

which the thermostat is installed should be

sealed at the top and bottom to prevent

circulation of cool air in winter and warm air in

summer, which would cause the thermostat to

operate ineffectively. Make sure the control

voltage of the thermostat is matched to the

voltage specified by the thermostat

manufacturer. Verify the correct operation of

the thermostat by checking the anticipator

setting and the cut-on and cut-off

temperatures relative to the set point.

Programmable thermostats automatically allow

different temperature settings during the day

and week to save energy, although not all

users are comfortable with their complexity.

Setback can be performed manually using

standard thermostat models, but a consistent

pattern is usually not achieved in practice. If a

programmable thermostat is installed, it

should be one with an Energy Star® label.

Energy Star® thermostats allow better control

of heating and cooling equipment by ensuring

essential programming features and accuracy.

HEAT PUMP CONSIDERATIONS

For heat pump systems, a thermostat compatible

with such systems must be installed.

Changeover between heating and cooling should

be performed manually rather than automatically.

The thermostat should be equipped with an

emergency heat switch that permits all resistance

heaters to be energized when the refrigerant

system is inoperative and activates an indoor

indicator light. Programmable models for heat

pumps that meet Energy Star® requirements

ensure that costly back-up heat is not engaged

under routine conditions or in using setback

practices.

If the unit is not installed with an outdoorlockout thermostat, such a device should be

installed to prevent the supplemental heat

from operating when the ambient temperature

is above the heat pump balance point (that is,

the temperature at which the heat pump has

just enough capacity to keep the house warm,

typically 25 to 40°F), especially when a nonintelligent or non-ramping thermostat is used.

The outdoor-lockout should not prevent

emergency heat operation or strip heat

operation during defrost.

Check the operation of the resistance heaters

to ensure that they are wired correctly to their

control circuits (that they cycle correctly, are

stagged properly, and are not on all the time).

The heat pump should preferably have a

microprocessor defrost control to better

defrost the system just when needed.

Conventional defrost controls use temperature

or pressure difference across the outdoor coil

or time to initiate the defrost cycle; these

should be set to provide the highest operating

efficiency for the local weather conditions.

DOCUMENTATION

Proper installation of heat pump and air

conditioning equipment should be

documented by providing the following

information:

• results of the deep evacuation test that notes

the initial evacuation pressure and time, and

the system pressure each minute for at least

five minutes after the initial pressure rise

• air flow rate across the indoor coil and the

method used to measure this rate

• initial and final measurements and

calculations relating to the charge check, as

well as the manufacturer specifications

• temperature difference across the indoor coil.