Correctly Sizing Your Heating & Cooling Equipment
According to the American Council for Energy-Efficient Economy (ACEEE), energy consumption for home air conditioning accounts for almost 5% of all the electricity producing in the U.S., a cost of $15 billion to homeowners. As homeowners are looking to replace their older inefficient heating and cooling systems with new energy efficient ones, they need to understand the critical importance of sizing the system correctly. The correct size of heating and cooling equipment in your home will improve comfort and reduce costs, maintenance and energy use.
In the past homes were not as tightly constructed as they are today. It was not uncommon to install furnaces and air conditioners that had two to four times the necessary capacity. Now most people have added new windows, caulking, weather-stripping, and insulation to their homes. Making improvements such as these to reduce heat loss in the winter and heat gain in the summer should allow you to install a smaller system while still being comfortable and saving money in your utility bills.
The most common sizing mistake is in oversizing the system. This will make the new system not only more expensive to install, but also forces it to operate inefficiently, break down more often and cost more to operate. Oversized heating equipment creates uncomfortable and large temperature swings. Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the air feeling “clammy” and the potential for unhealthy mold growth in homes. The frequent starts and stops (known as “short cycling”) will accelerate the 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.
Beware of a contractor/installer who only checks the “nameplate” (the label listing BTU’s) of the existing unit and tries to sell you one just like it or even worse, one that’s larger. This is not a correct way to size the equipment and is not in your best interest. Another method includes using simple “rule of thumb”. They may base the size of your home on similar homes like it, using a chart that takes into account a variety of factors. This method can be used for a first estimate, but should not be used to size the actual system.
The best way and only method a homeowner should use is the Manual J and Manual D method of sizing. The Manual J method, “Residential Load Calculation,” is published by the Air Conditioning Contractors of America (ACCA) and is the recommended method of use for the United States. Manual D “Residential Duct Design,” should be used when ducts are part of the installation.
Using these manuals (J & D) some of the key factors that are looked at in a home are: the local climate, size, shape and orientation of the house, roof surface color, insulation levels, amount of wall, ceiling floor and window area, window location and type, air infiltration rates, duct air leakage, the number and ages of occupants, occupant comfort preferences, and the types and efficiencies of lights and major home appliances (which give off heat).
A home’s heating or cooling requirement, also called a “load”, is affected by all of the above factors. A good estimator will measure walls, floor space and windows to determine room volumes, assess the R-value of the home’s insulation, window and building materials. A close estimate of the building’s air leakage is also necessary. This estimate should also include an inspection of the size, condition of seals on joints and insulation and location of distribution ducts in forced air systems. The placement of supply and return ducts should be appropriate for the system type and size.
When designing a new system make sure to get the following from your installer/contractor: a written calculation procedure to size the system, a written contract listing the main points of your installation, including the results of the heating and cooling load calculation, and a written warranty on equipment and workmanship.
Sources: www.aceee.org
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