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Heat Gain/Loss Calculations


July 10, 2009
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What is a heat gain/loss calculation?

It's a way of determining the amount of heat each room in a home or structure gives up in the heating season, and picks up in the cooling season. The rate of heat transfer, either into or out of each room in a structure, depends on the construction of the envelope, or outer shell of the building, among other factors. These include the direction that the structure faces, the number and types of windows used, the number and types of doors, as well as the type of insulation and the geographic location of the building itself. By evaluating the size of each room and the other things I mentioned, we can come up with an accurate determination of the British thermal units (Btus) a room gains or loses. We have to reintroduce the amount of heat lost during the winter months to the structure, just as we have to remove the amount of heat introduced to the structure during the summer months. An accurate heat grain/loss calculation will closely match the actual rate of heat gain and heat loss at design conditions. The amount of heat in Btus per hour that the system must add or remove from a room determines the required duct sizes.

Can I do a heat gain/loss calculation on my home by myself?

Sure,but the accuracy of the information you input will directly affect the accuracy of your results. You'll need to know such things as the insulation type and its R-value, the building construction methods and materials used, the types and classifications of fenestrations (windows, etc.), the structure's orientation as well as its geographic design temperatures, foundation information and other goodies. Guessing at these values or characteristics will greatly affect the results. And since we base the size of heating and cooling equipment on these calculations, improper results will result in the improper sizing of the air-conditioning and heating equipment.
Improperly sized heating and air conditioning equipment can lead to humidity problems, as well as your general discomfort. Excessive humidity can stimulate mold growth, while excessively low relative humidity can dryout your skin, furniture and the building materials used to construct the dwelling.
HVAC professionals perform heat gain/loss calculations every day. They have the expertise to determine what is behind or inside your walls without actually seeing into them.They are experts at what they do and will help ensure that the heating and cooling equipment is sized properly and that the air distribution system is adequately designed and installed.
But be prepared to pay for this service (it's worth it). Most of the time,you will have to pay for the heat gain/loss information before you receive the report, and most companies will credit you with the cost of the report if you decide to hire them to do the installation.
Finally,you should be concerned about companies that offer free heat gain/loss calculations for the asking. The accuracy of these “free” estimates is often suspect. Remember, you get what you pay for.

How can I get a professional heat gain/loss calculation performed on my home?

The best way is to contact a reliable, local contractor. And realize that accurate heat /gain loss calculations take time to complete and have costs connected to them. The result, though,will be a set of numbers that will help provide the information that is vital to the proper selection of equipment and the layout of the air-distribution system.

Many companies will not provide a job estimate until they see an accurate heat-gain/loss calculation. This is a separate job and you should have it done before you get estimates. The company providing the calculations will charge for their work, and will often issue a job credit in one form or another if you choose to have them to do the work.