Heat Loss Heat Gain H.V.A.C. Calculations - The Correct Way to Size an H.V.A.C System!

Heat Loss Heat Gain H.V.A.C. Calculations - The Correct Way to Size an H.V.A.C System!

How an Energy Survey is done.

Heat Loss and heat gain in a building is figured by either manual calculations (manual J) or by using a computer software program. We have done hundreds of these calculations for new building construction. Read on to find out why it is difficult to do one on an existing home or building. It involves finding all the insulation values of the building, measuring the length, width and volume of all rooms, measuring all the windows and glass and recording their composition ( ie: single pane, double pane, low "E" value and other parameters ).

The amount of glass, their orientation, and the number of sky lights will have a big effect, especially on the cooling side of the calculations. Over hangs over windows will also affect this calculation. The tightness of the building construction has to be included as a loose building will allow more out door air to enter the building and let the heated or cooled indoor air escape the same way (called infiltration). There are many other factors involved in doing a COMPLETE Heat Loss Heat Gain calculation, down to and including how many bathroom fans exhaust air from the building. This full calculation is most of the time called an H.V.A.C. Energy Survey. It is very time consuming to do correctly. Beware of a company just measuring the floor space and saying they have done a " Survey ".

A true survey will require help from the building owner to answer questions. For instance, just one example is how do most people who own a home with vaulted ceilings that have been finished with sheet rock actually know what insulation is behind the sheet rock? We know if it were our own home and we hadn't been there when it was built, we would have to " guess ". This makes the survey inaccurate and really not much better than using a general rule of thumb that most contractors use such as 600 square feet per ton of air conditioning or 30 B.T.U.H. per square feet of actual out put capacity for heating.

On existing homes or buildings that already have a heating system, the blower size of the furnace and the ductwork installed in the building will dictate what size system can be installed. It is of little value to do an Energy Evaluation to find that even though a 5 ton air conditioner is needed, the building only has a 3 ton blower in the furnace or the duct work can only handle 3 tons of air conditioning. To put a 5 ton ac in under these circumstances will result in major equipment failures and it still won't cool any better than a 3 ton would have in the first place! The same things happen when we are talking about installing a " bigger " furnace.

If however, you want a true survey and are willing to take the time it requires to find all the answers needed, any GOOD Heating and Cooling Contractor can do a Energy Survey for you at a reasonable cost. This survey should spell out the exact heating and cooling needs of your building based on your requirements for the building. You will sometimes receive a printed report of all aspects of the building. One of these surveys should be done especially on new construction while the building is being built. Some of our local clients ask us to do one to determine if the system in an existing building was ever sized correctly to begin with.

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