Sizing a Gas Furnace and Central Air Conditioner for your Home

We want to be clear on sizing – we don’t care what size furnace you currently have because almost all gas furnaces are considerably oversized. Oversized furnaces use more fuel, reduce home comfort and wear out faster than properly sized furnaces. Why, because they cycle too often - they’re constantly going on and off. This cycling increases stress on the heat exchanger and simply reduces the life of the other components because of the frequency of their operation while at the same time wasting gas.

A properly sized furnace is critical to system longevity and personal comfort. How do you size a furnace for a house - there are a couple of ways. Some contractor’s use what is known as a manual J load calculation to determine the size furnace a home needs. While this is a good way to size a system for a house under construction where all the insulation values and the tightness of the home are known, it can be fairly inaccurate on existing structures. Other contractors rely on their years of experience in the business. This method is probably the most suspect and will usually result in an over sized system.

When determining the size of your furnace, compare three critical aspects of your home: the square footage, the age, and the configuration. The configuration is defined as ranch, two story, bi level, slab – with or without a basement, or contemporary. Using this data we can calculate the anticipated heat loss of your home and choose a furnace that will maximize your comfort and minimize your utility cost. One other factor that can help size your furnace, believe it or not, is your air conditioner. Our program can take the size of your central air conditioner and calculate the size of the furnace your home needs using some fairly simple mathematics.

Heating and cooling specialists often spend a tremendous amount of time trying to convince potential furnace buyers to purchase smaller furnaces. We do this because we know the buyers will be more comfortable and save money on their energy bills with a properly sized furnace. Please remember there is no gain for heating and air conditioning specialists to sell you a smaller furnace; we just want your furnace to work efficiently and comfortably. Also, a performance guarantee assures you that your furnace will not be undersized.

Sizing an air conditioner is a fairly simple process. Some HVAC service contractor’s use what is known as a manual J load calculation to determine the air conditioner size that a home requires. While this is a good way to size an air cooling system for a house under construction where all the insulation values and the tightness of the home are known, it can be fairly inaccurate when cooling existing structures. The best way to determine the correct air conditioning system size for your home is to access how well the old system worked. If your old system did the job in cooling, your new system will do the same job more efficiently.

If you are unsure of your central air system’s size, simply look at the nameplate on the outdoor AC unit. Almost all air conditioner manufactures put the Btu rating in the model number. One ton of cooling is equal to 12,000 Btu’s. Most units start at 2 ton, or 24,000 btu’s and increase in half ton increments, skipping 4-1/2 ton.

Sometimes you have to do a little detective work to find the Btu number because there is no set location within the AC model number and it varies by manufacturer. If you can’t figure it out, contact a heating and cooling specialist with the number and see if they can provide you with the system size.