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A Heating Rule of Thumb.
Estimating the Size of a Furnace.

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A Heating Rule of Thumb . . .

         Larry writes, "How does one figure the size of a gas heater or furnace? Do you know of a formula I could use? "

            Choosing the correct heating and cooling output for your building is fundamental to achieve the highest efficiency and comfort. In previous decades, it was a common construction practice to install furnaces and air conditioners that had as much as double the capacity actually needed. Back then, people didn't bother with making tight buildings. Energy was cheap so excess capacity was not considered a problem.
            Today, tightly constructed buildings allow for smaller systems to handle the heat load while saving energy. Correctly sizing the heating and cooling system can save energy and money for the life of the structure. However, the Department of Energy estimates that more than half of all HVAC contractors do not size heating and cooling systems correctly. Understandably, most contractors slightly oversize the equipment that they specify. Better to install too much capacity, they reason, than not enough.
            Determining a building's heat load accurately is a very difficult and involved calculation. The size of all walls, ceilings and windows must be measured. The R-value of all the materials used to construct the building must be known or assumed. The amount of air leakage in and out of the building must be assessed. Other factors considered are the type and construction of the ductwork, the orientation of the dwelling with regard to the sun, the size and shape of overhangs and even the amount of shade provided by trees or other buildings nearby!
            The accepted standard formula for accurately determining a building's heat load is available as 'Manual J' which is published by the Air Conditioning Contractors of America (ACCA). The ACCA conducts classes to teach contractors how to use the 'Manual J' to accurately assess heat loads. More and more mortgage institutions are requiring contractors to submit a copy of their 'Manual J' calculations as part of the construction financing process. Contractors building structures which qualify for federal funding are increasingly being asked to provide verified 'Manual J' calculations.
            But time is money. For a house with one furnace and a central AC, a heating and air conditioning contractor could easily consume all profit in the time required to perform a complete 'Manual J' calculation. Computer software simplifies the calculation procedure but software costs money too. Most heating and air conditioning contractors will not bother with 'Manual J' calculations unless absolutely required to do so. They will say, by experience, they know what size equipment will work where. And, who can argue with that?
            So, what can a do-it-yourselfer do to get a handle on what size furnace to install? First option would be to pay an engineer to assess your heat-load. Or, why not ask a few heating and air conditioning contractors for bids? You might inquire at your local gas or electric company if they offer a heat-load calculation service. This service is often offered free or at minimal cost to homeowners by gas and electric utilities. Or, you could check for University Engineering Extension Programs in your area that would assist in performing heat load calculations.

            That being said, here is one "Heating Rule of Thumb": It is a simplified way to approximate the amount of furnace needed in a building. First, choose a construction type. Then multiply the given factors by the amount of floor space. Choose a furnace within the Btu/Hour range according to the relative climate in your area. Of course, the results are little more than an educated guess but will get you "in the ballpark" . . . After all, it is a rule of thumb!

A Heating Rule of Thumb

            First, pick the best description of the construction for the building you intend to heat. Jot down the two multiplier factors associated with that particular construction type.
  1. No insulation in walls, ceilings, or floors. No storm windows installed. Air leakage around loose fitting windows and doors. Construction typically found in garages, sheds, recreational dwellings or storage buildings. For this type of building, use multiplier factors 90 to 110.

  2. Construction typically found in older homes and better quality garages, sheds, recreational dwellings or storage buildings. Walls and ceilings are insulated to R-11 but there is no insulation in the floors. Doors are of standard construction; not insulated. Windows are ordinary single pane; not thermal double pane nor storm windows. However, the fit of the doors and windows is good enough that they do not allow noticeable air leakage. For this type of building, use multiplier factors 50 to 85.

  3. Newer standard construction for homes, offices and stores. Walls insulated to R-19. Ceilings insulated to R-30. Weather stripping on all doors and windows. Thermal double pane windows or storm windows fit tightly. For this type of building, use multiplier factors 29 to 35

  4. Buildings that claim to be super-insulated or Energy-Star. Walls insulated to R-24. Ceilings insulated to R-40. Weather stripping on all doors and windows. Thermal double pane or storm windows fit tightly. Tyvek or similar vapor barrier was taped and sealed carefully during construction. For this type of building, use multiplier factors 20 to 25.

           Next, multiply your building's square footage by each of the multiplier factors. (NOTE: use the floor space of the building in square feet NOT the total area of the building.) That will give you two numbers which signify the Btu-per-hour (Btu/Hr)output range of a furnace for that size and type of house. Pick a furnace with a Btu/Hr output rating in the lower part of the range if your building is in a warm climate. Choosing a furnace with a Btu/Hr output rating in the higher part of the range would be appropriate if your building is in a colder climate.
            As an example, a 1750 home in Minneapolis was built in 1950. Over the years, some insulation has been done but the best description for the construction would probably be Type 'B'. So, we will use the multiplier factors 50 to 85.

            1750 X 50 = 87,500
            1750 X 70 = 148,750

            So, the Btu-per-hour range of a furnace that would be suitable for this size and construction of house is 87500 to 148750 Btu/Hour output. If this house was in Phoenix, an 87500 Btu/Hour furnace would probably be sufficient, The example house, however, is located in Minneapolis so we will choose a furnace around 148750 Btu/Hour output. If the example house was located in Kansas City, using a Btu/Hour figure from the middle of the range would be appropriate; say 118000 Btu/Hour.

            Nobody will recommend that you size your furnace according to any Rule of Thumb (wink). In truth, it is probably used all the time - but nobody will admit it.  And, you should be aware that you might not be able to get financed unless you provide correct 'Manual J' calculations. But, if none of that applies, you wouldn't be the first to use the Heating Rule of Thumb . . . And you certainly won't be the last.

Good luck



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