EERE Consumer‘s Guide: Sizing Heating and Cooling Systems

U.S. Department of Energy - Energy Efficiency and Renewable Energy
A Consumer‘s Guide to Energy Efficiency and Renewable Energy
Sizing Heating and Cooling Systems
Olderspace conditioning systems (more than 10 years old) are oftenunreliable and much less efficient than a modern system. When it‘s timefor a new replacement, choosing one of the correct size (heating and/orcooling output) is critical to getting the best efficiency, comfort,and lowest maintenance and operating costs over the life of the newsystem. Some national surveys have determined that well over half ofall HVAC contractors do not size heating and cooling systems correctly.
The most common sizing mistake is in oversizing. This not only makesthe new system more expensive to install, but also forces it to operateinefficiently, break down more often, and cost more to operate.Oversized heating equipment also often creates uncomfortable and largetemperature swings in the house. Oversized air conditioners (and heatpumps) do not run long enough to dehumidify the air, which results inthe "clammy" feeling and unhealthy mold growth in many air-conditionedhouses (seedehumidifying heat pipes as one solution to this problem).
Incorrect Sizing Methods
It is the installer/contractor‘s job to perform the correct sizingcalculation for the building. However, many installers only check the"nameplate" (the label on the unit that has the Btu per hour outputamong other things) of the existing system and sell you one just likeit, or even worse, one that‘s larger. This is a not a correct sizing method and not in your best interests!Other methods include simple "rules of thumb" based on the size of yourhome or using a chart that accounts for a variety of factors. Whilethese methods might provide a first estimate, they should not be usedto size your system.
Why Most Older Systems are Oversized
Before the era of tightly constructed homes, it was not uncommon toinstall furnaces and air conditioners that had two to four times thenecessary capacity. Since many people have added new windows, caulking,weather-stripping, and insulation to their homes, going by thenameplate is likely to result in an oversized system. Makingimprovements such as these to reduce heat loss in the winter and heatgain in the summer should allow you to install a smaller systems whilestill being comfortable, as well as saving large amounts of energy.
Manual J and Manual D: The Correct Way to Size a System
Correct system sizing requires considering many factors other thansimply reading the nameplate of the existing unit. Key factors forcorrectly sizing a heating and cooling system include the following:
The local climate
Size, shape, and orientation of the house
Insulation levels
Window area, location, and type
Air infiltration rates
The number and ages of occupants
Occupant comfort preferences
The types and efficiencies of lights and major home appliances (which give off heat).
Homeowners should insist that contractors use a correct sizingcalculation before signing a contract. This service is often offered atlittle or no cost to homeowners by gas and electric utilities, majorheating equipment manufacturers, and conscientious heating and airconditioning contractors. Manual J, "Residential Load Calculation,"published by the Air Conditioning Contractors of America (ACCA), is therecommended method for use in the United States. There are also manyuser-friendly computer software packages or worksheets that cansimplify the calculation procedure. You should make sure that theprocedure used by the contractor follows Manual J.
If ducts are part of the installation, they should be sized using the ACCA‘s Manual D, "Residential Duct Design." The ACCA also offers a comprehensive guide for choosing home heating and cooling systems, called Manual S, "Residential Equipment Selection."
A Special Case: Sizing Steam Heating Systems
One exception to the above is in steam heating systems. For thesesystems, the boiler should be sized to match the radiators. However,there is still room for energy savings. First of all, the originalboiler may be oversized for the radiators, so the contractor shouldn‘tjust order the same capacity boiler, but instead should match theboiler to the radiators. Second, if you‘ve increased the energyefficiency of your home, it may have more radiators than it needs.
It may be possible to remove radiators in the core of the house andshift the others around, replacing larger radiators with smaller ones.Since radiators are modular, it is theoretically possible to downsize aradiator by removing sections; in practice, this is usually difficultto do without damaging them. In many parts of the country, usedradiators are available cheaply, so you can potentially buy smallradiators to replace large radiators; if you do so, be prepared toreplace the shutoff valves as well, since they often won‘t match. Newlymanufactured steam radiators are available as well.
In any case, you should work with a heating and cooling professionalwhen downsizing your system. Your house‘s heating needs should becalculated using Manual J, and your radiators should be downsizedappropriately. Match the new boiler to the remaining radiators. Notethat balancing steam heating systems is more an art than a science;ideally, you will find a heating professional with experience in steamheating systems.
Steps a Good Contractor Should Take to Size Your System
Many factors affect a home‘s heating or cooling requirement, or"load." A good estimator will measure walls, ceilings, floor space, andwindows to determine the room volumes, and will assess the R-value ofthe home‘s insulation, windows, and building materials. A closeestimate of the building‘s air leakage is also necessary. A blower doortest is the best way to measure air leakage.
A good estimate will also include an inspection of the size,condition of seals on joints and insulation, and location of thedistribution ducts in forced air systems. The placement of supply andreturn registers should be appropriate for the system type and size.
The orientation of the house also affects heat gain and heat lossthrough windows. Overhangs can reduce solar gain through windows. Makesure the contractor uses the correct design for the outdoor temperatureand humidity in your area. Using a higher summer design temperatureresults in oversizing air conditioners.
When the contractors are finished, get a copy of their calculations,assumptions, and the computer printout or finished worksheet. This isyour only proof that they did the job right. To summarize, whendesigning your new heating and air conditioning system, the contractoryou choose should do the following:
Use a computer program or written calculation procedure to size the system
Provide a written contract listing the main points of your installation and includes the results of the heating and cooling load calculation
Give you a written warranty on equipment and workmanship
Allow you to hold the final payment until you are satisfied with the new system.
Learn More
Codes & Standards
Manual J Residential Design Center
Air Conditioning Contractors of America
System Design - Residential
Air Conditioning Contractors of America
Professional Services
Find a Contractor
Plumbing-Heating-Cooling Contractors Association
Product & Service Directory
American Society of Heating, Refrigerating and Air Conditioning Engineers
Federal Government Resources
Energy-Efficient Rehab Advisor
U.S. Department of Housing and Urban Development
When is it Time to Replace?
ENERGY STAR® Heating and Cooling
Related Links
PATH Tech Set #3, HVAC: Forced Air System
Partnership for Advancing Technology in Housing
PATH Technology Inventory: HVAC Sizing Practice
Partnership for Advancing Technology in Housing
Reading List
"Consumer Guide to Home Energy Savings." (2003). American Council for an Energy Efficient Economy. 8th Edition.
"Right-Size Heating and Cooling Equipment" (PDF 184 KB). (January 2002). Technology Fact Sheet, DOE/GO-102002-1490. DOE Building Technology Program.
Cooling and Heating Load Calculation Principles (1998). American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE).
Gifford, H. "Downsizing Steam Systems" (November/December 1993). Home Energy Magazine Online. Posted on the Argonne National Laboratory Web site.
Peterson, G. (March/April 1987). "The Art and Science of Balancing Single-Pipe Steam Systems." Home Energy Magazine. p. 24.
Sherman, C.; Heldebrandt, E. (March/April 1998). "Efficient Cooling: Making It Happen." Home Energy Magazine.
Proctor, J.; Katsnelson, Z.; Wilson, B. (May/June 1995). "Bigger is Not Better: Sizing Air Conditioners Properly." Home Energy Magazine.