What Size HVAC System Do I Need?
Learn how many BTUs to look for when heating and cooling your home
By Brad Painting
An oversized HVAC system can shorten equipment life span, increase utility bills, and produce uneven temperature gradients. An undersized model will leave the building too hot or cold on certain days. HVAC systems come in many shapes and sizes. Furnaces, boilers, heat pumps, and radiant systems can handle heating, while various types of air conditioners handle cooling. However, the prime factor in a decision to replace or install an HVAC system is the required size.
Definition of Size
The size of an HVAC system refers to the amount of heat it can either release into a house or absorb away from a house. In the United States these ratings are provided in British Thermal Units, or BTUs. One BTU is the amount of heat required to raise 1 pound of liquid water by 1 degree Fahrenheit. A BTU rating for an air conditioner, furnace or heat pump signifies how many BTUs it can add to or extract from a house per hour.
Do not confuse a BTU output rating with a BTU input rating. Due to widely varying efficiencies of HVAC systems, a BTU input rating can be much less or much more than the output rating. You should always size an HVAC system based on its BTU output capability, because that is the factor that affects the temperature of a house.
Two homes of identical shape and size can require completely different HVAC systems if they have different energy-efficient measures. The factors most pertinent to the size of an HVAC system are the qualities of a home's insulation and air sealing. The term insulation refers to any building material, especially foam, cellulose and fiberglass fills that impede the flow of heat across walls, floors and roofs. Air sealing refers to how air-tight the connections are at wall and floor joints, window and door frames, and various plumbing and electrical penetrations.
Climate and Environment
What goes on around a house has as much to do with heating and cooling requirements as the intrinsic properties of the house's design. Aside from the obvious factor of external temperature, different climates may have different wind speeds, intensities of sunlight, and quantities of precipitation. All factors of a climate affect heating and cooling requirements. For example, wind causes air to infiltrate buildings, and solar rays sink into house roofs and pour through windows, heating their interiors. For these reasons, trees, shrubs or other landscape features that block wind and sunlight will affect the required size of an HVAC system.
When you consider all factors inherent in construction and climate, calculating a required HVAC size produces a dizzying amount of variables. The accepted practice for bringing these all together is delineated in the Manual J "Residential Load Calculation," published by the ACCA (Air Conditioning Contractors of America). The Manual J protocol addresses all factors sensitive to heating and cooling needs, from window glazing to roof color. The Manual J calculation is often provided by HVAC contractors, or may be provided by your system manufacturer or gas and electric utilities.
Do not choose a differently sized HVAC system than what is indicated by Manual J calculations; you should choose the smallest available size that meets the calculated BTU requirements. A properly sized HVAC system will yield the best thermal comfort, functionality and energy efficiency.
About the Author
Brad Painting is a regular contributor to DexKnows and specializes in green building design.
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