Properly Size a Gas Furnace for Your Home

Properly Size a Gas Furnace for Your Home Sizing gas furnaces use to be done with a pencil and paper. The HVAC consultant made note of your home’s square footage, as well as the climate. From these factors, the consultant could then estimate how many (BTUs) British Thermal Units were needed to heat your home, as well as the appropriate size of furnace to buy.

This is all pretty old school now. These days, most HVAC pros size furnaces with software. However, understanding the process will help you ask the right questions when your consultant provides an estimate.

Right Size

You don’t want want a furnace that’s too small or too large. If it’s too large, your furnace will short cycle and run inefficiently. If your furnace is too small, it will run continuously and never be able to heat your home sufficiently. Both instances waste money.

Manuals J, D and S

The HVAC industry developed Manual J software to properly size HVAC equipment. HVAC consultants will factor in your geographic location, your home’s indoor and outdoor design, climate data, air tightness, orientation of your home, window efficiency, duct location and how many people live in your home.

Once the consultant has all this information, he or she will enter it into the program and make the appropriate load calculation. Calculations are also made by Manual D, which determines design, layout and the size of your ductwork, while Manual S helps determine what size equipment is needed.


Modern furnaces have varying degrees of efficiency, from an AFUE (annual fuel utilization efficiency) rating of 78 to the most efficient at nearly 100 percent. Obviously, AFUE is an important factor in matching a furnace with the right BTU output for your home.

A high efficiency furnace producing 100,000 BTUs will produce more heat than a furnace of the same size with a lower efficiency rating. This means you may be able to buy a smaller, more efficient furnace to do the job.

For more information on how to size a gas furnace, contact Davis Air Conditioning in Southwest Oklahoma.

Our goal is to help educate our customers in the Lawton and Duncan, Oklahoma area about energy and home comfort issues (specific to HVAC systems).

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