Buildings for

the 21st Century

Buildings that are more

energy efficient, comfortable,

and affordable...that’s the

goal of DOE’s Office of Building

Technology, State and

Community Programs (BTS).

To accelerate the development

and wide application of energy

efficiency measures, BTS:

• 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

• Provides support and grants

to states and communities

for deployment of energyefficient technologies and

practices

RIGHT-SIZE

HEATING AND

COOLING EQUIPMENT

The correct size improves comfort and reduces

costs, maintenance, and energy use

BIGGER IS NOT ALWAYS BETTER

WITH HVAC SYSTEMS

Determining the correct size of residential

heating and cooling equipment is key to

achieving comfortable interior conditions —

temperature and humidity — and saving on

initial and operating costs. Size of cooling

systems is particularly critical for optimal

energy efficiency and comfort. When equipment

is oversized, initial costs are higher, efficiency

is reduced, energy costs increase, and comfort

may be compromised. One Florida study

showed a typical 9 percent increase in annual

space cooling electricity usage for units that

were oversized by 50 percent or more.

Correctly sized equipment is especially important in humid climates, where short-cycling

of air conditioning equipment can lead to poor

humidity control. Similar to an automobile in

stop-and-go traffic, the overall efficiency of

an air conditioner, furnace, or heat pump is

reduced by excessive cycling. More starts and

stops accelerate wear and tear on the equipment, which can lead to premature failure.

Oversized systems also use more fan power for

the blower and often exhibit more duct leakage

due to higher operating duct pressures. Finally,

oversized air conditioners and heat pumps

greatly aggravate the summer utility peak

demand on hot days. In the Florida study cited

above, a 13 percent higher summer peak electrical demand was correlated to oversized units.

USE THE BEST GUIDE

Beware of casual sizing estimates based on

home floor area or contractor “experience.”

The residential equipment sizing procedures

recommended for use in the United States are

found in Manual J and Manual S, produced by

the Air Conditioning Contractor’s Association of

America (ACCA). Good computer programs are

available to simplify the load calculations

specified in Manual J. Once the heating and

cooling loads are known, Manual S can be

used to select compatible equipment.

Unfortunately, ACCA-approved sizing methods

are often not used and over-sizing is very

common. In one large-scale survey, nearly

40 percent of contractors indicated that they

purposefully over-sized equipment, citing reasons such as “to reduce call backs,” “to allow

for future expansion, ” or “customers demanded it.” Those who claimed to size in accordance with Manuals J and S were heavily outnumbered by the “bigger is better” school.

SIZING AND EFFICIENCY GO HAND IN HAND

Manual J procedures determine the design

heating and cooling loads based on the amount

of wall, ceiling, window, and floor area, their

insulation value, and building envelope and

duct air leakage. Building orientation, roof surface

color, and occupancy can also make a difference. The sizing calculations for new construction should take into account the various

efficiency measures incorporated in the home.

As a home’s efficiency is improved, the recommended size of the HVAC system should drop.

OFFICE OF BUILDING TECHNOLOGY, STATE AND COMMUNITY PROGRAMS

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

Technology Fact Sheet
HOW TO DETERMINE “JUST THE RIGHT SIZE”

• Insist that documented sizing calculations be performed on

your home using the ACCA Manual J procedure. Manual S

should be used to select equipment. ACCA methods have

sufficient built-in safety factors to accommodate most conditioning needs. Therefore, it is important to follow all instructions in Manuals J and S, using precise area measurements

and other specific data.

• The most recent version of Manual J should be used, since

it incorporates large impacts on efficiency, such as duct

leakage and duct heat gains and losses.

• Insect screening can have a significant impact on peak

cooling heat gain and should be considered with the new

procedures in the latest version of Manual J.

• If drapery or blind positions are not known, they should be

assumed to be in place. Otherwise sensible loads will often

be overestimated.

• Infiltration assumptions should be approached with caution.

In lieu of other information, an infiltration rate of 0.35 ACH is

recommended.

• Don’t alter the suggested design temperatures for your

location. Using the annual extreme temperatures will lead to

over-sizing. For Manual J, the 971

/2 percent design temperatures should be used for cooling and the 21

/2 percent design

temperature should be used for heating.

• Be aware that exaggerating temperatures for indoor heating

and cooling set points can have large impacts on required

equipment size. Generally, Manual J recommends 75°F for

cooling and 70°F for heating. Use judgment based on

homeowner comfort preferences before altering these

recommendations.

An hourly computer simulation shows

that efficiency improvements along with

re-sizing would cut heating energy use by

63 percent and cooling energy use by 53

percent. Without re-sizing, the efficiency

measures would save 54 percent for

heating and 47 percent for cooling.

In this case, right-sizing reduces costs

for the air conditioner, heat pump,

furnace, and ductwork, and these savings

partly offset the cost of the efficiency

improvements. Better yet, right-sizing

provides a 10 percent bonus energy

savings otherwise not available.

COMPARE THE DIFFERENCE

Consider HVAC sizing factors for a hypothetical

2,000-square-foot home in Raleigh, North Carolina

Conventional Construction With Energy Efficiency Upgrades

Wall insulation level R-11 R-19

Ceiling insulation level R-19 R-38

Window glass single pane double-glazed, low-e solar

Window overhangs one foot two foot

Duct leakage average, with ducts in none, with ducts inside the

unconditioned attic conditioned space

House air leakage* 8 air changes per hour at 6 air changes per hour at

50 pascals pressure 50 pascals pressure

Manual J design heating load 46,100 Btu/hr 21,300 Btu/hr

Manual J design cooling load 52,100 Btu/hr 23,300 Btu/hr

Electric heat pump size 4.0 to 4.5 ton 2 ton (24,000 Btu/hr)

(48 to 54,000 Btu/hr)

Annual heat energy usage 12,641 kWh 4,677 kWh Savings $717**

Annual cooling energy usage 3,808 kWh 1,790 kWh Savings $182**

○○○○○

○○○○○○○○○○○○○○○○○○○○○○○○○

* Measured by blower door test

** Savings at $0.09/kWh
RIGHT-SIZE HEATING AND

COOLING EQUIPMENT

• Select a programmable thermostat so that your home can be

conditioned before you arrive, thus avoiding the temptation to

over-size equipment for rapid cool-downs or warm-ups. This

will improve sizing-related performance and potentially save

energy associated with more reliable thermostat set-back and

set-ups.

• Select variable speed air handlers to gain important advantages for both hot humid and hot dry climates. In hot dry

climates, the air handlers can be set to provide high air flow

rates (for instance 450 to 500 cfm/ton) so that the air conditioner provides the sensible air cooling that is needed. For

humid climates, many variable speed systems “ramp up”

blowers slowly, providing energy savings and improved

dehumidification.

• Selecting variable speed air handlers also allows for proper

adjustment of air conditioner and heat pump air flow. Maintaining proper air flow is essential for heat pumps, which use

supplemental strip heat if the achieved heating capacity is

deficient. Inadequate air flow can rob systems of 5 percent or

more of their potential performance.

A NOTE ABOUT HEAT PUMPS

For climates without extended periods of sub-freezing temperatures, heat pumps are an excellent energy-efficient alternative to

furnaces and air conditioners. Similar to a refrigerator, a heat

pump uses electricity (or natural gas) to move heat from a cool

space into warm space. During the heating season, heat pumps

move heat from the cool outdoors into your warm house; during

the cooling season, heat pumps move heat from your cool house

into the warmer outdoors.

Because they move heat rather than generate heat, heat pumps

can provide up to four times the amount of energy they consume.

If you heat with electricity, a heat pump can trim usage by as

much as 30 to 40 percent. High efficiency heat pumps with

variable speed air handlers can also dehumidify better than

standard single capacity units, resulting in less energy usage

and more cooling comfort in summer.

• Heat pumps require a ductwork system, generally with larger

duct sizes than other central heating systems. For proper heat

pump operation, air flow should be 50 to 60 liters per second

per kilowatt-hour (kWh), or 400 to 500 cubic feet per minute

per ton of cooling capacity.

• Heat pumps also require the proper thermostat type to

function correctly. When choosing a set-back thermostat

for a heat pump, choose a model that locks out resistance

heating during temperature ramp-up.

• Size the electric resistance heat (heat strips) normally

installed on heat pumps using procedures outlined in

Manual S. Often, when the system is turned on or the

thermostat setting is changed, the heat strips are activated.

Since they are much less efficient, they cause a large increase

in energy use (and an even bigger spike in the utility’s peak

demand). With excess strip heat capacity on a cold morning,

costly resistance heat will do most of the duty while the more

efficient heat pump does less than its share. In sunbelt

climates such as southern California and Florida, where there

are less than 2,000 heating degree days, consider installing

the smallest amount of electric resistance heat (4-5 kW).

In any climate, consider installing an outdoor thermostat

set at 35°F or 40°F, which prevents the heat strips from

activating unless outdoor temperatures fall lower.

• Manual S advises that, in colder climates, electric heat pumps

be sized up to 25 percent larger than the required cooling size

to reduce the use of supplemental resistance heat. Units that

adjust the indoor blower speed according to humidity level

can help ensure good indoor cooling comfort in these cases.

There are also two-capacity heat pumps that can be installed

to run only at the lower capacity when cooling and at low or

high capacity when heating. These heat pump units can

provide good indoor comfort while minimizing energy use

and peak electric demand in both operating modes.
RIGHT-SIZE HEATING AND COOLING EQUIPMENT

DON’T FORGET THE DUCTS

Air leakage and heat losses from ducts can rob

your space conditioning system of its capacity

under peak conditions. In many residential air

conditioning systems, the duct system passes

through unconditioned zones, which may have

thermally disadvantageous consequences. The

insulation level of ducted systems tends to be

between R-4 and R-6, with very high temperature differences across the insulation in many

cases.

The latest edition of Manual J shows how important certain variables can be. For instance,

in the Sunbelt states where slab-on-grade construction dominates, the ducts are often in the

unconditioned attic. This space can reach 135°F

To ensure efficient operation, an air conditioning

technician examines the refrigerant charge during

installation of a two-ton air conditioner.

EFFICIENT OPERATION

For more information,

contact:

Energy Efficiency and

Renewable Energy

Clearinghouse (EREC)

1-800-DOE-3732

www.eren.doe.gov

Or visit the BTS Web site at:

www.eren.doe.gov/buildings

Written and prepared for

the U.S. Department of

Energy by:

Florida Solar Energy Center

321-638-1000

www.fsec.ucf.edu

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.

January 2002 DOE/GO-102002-1490

under peak summer conditions, while the air

in the ducts may be 55°F to 60°F. Simulation

studies show that cooling systems can easily

lose 30 percent of their peak sensible capacity

from this heat gain alone. If a radiant barrier or

colored tile roof is present, peak attic temperatures might drop to 115°F. Furthermore, a white

tile or white metal roof might drop the peak

temperature to 100°F. Very different conditions

affect duct systems located in crawlspaces or

basements.

Duct leakage can produce even larger impacts.

Supply leaks can lose highly conditioned air and

depressurize your home, adding more infiltration. Return leaks can add to the air conditioner

or furnace loads and draw air from unintended

locations.

All these issues add up to a strong case for

having the duct system properly sealed and

placed within the conditioned space. Once these

decisions are made:

• Make sure that duct sealing and testing are

part of your plan for sizing and installing a

new air conditioning system. Sealed ducts

are particularly critical for systems with variable speed air handlers, which may operate

with higher air flows and duct pressures.

• In new construction, seriously consider installing the duct system inside the insulated

building envelope. This will both reduce

space conditioning energy use and improve

comfort by reducing the time it takes the

indoor temperature to reach the thermostat

set-point.

Photo credit: Florida Solar Energy Center