What Furnace Size Do I Need?

August 04, 2016

Are you replacing your current furnace or need one installed in a new home? Wondering what furnace size you need to keep yourself cozy all winter?

Finding the perfect furnace size you need is stupidly simple: Just call a furnace contractor for a free furnace installation estimate. The contractor needs to determine the furnace size you need as part of the estimate process.

The hard part? 

You have to find a reputable contractor who will determine the furnace size properly.

To put it bluntly, to determine the furnace size you need, there’s a wrong way and a right way. 

Let’s explore each so you know which companies to ignore, and who you should trust.

The wrong way: Rule of thumbs and laziness

Some inexperienced technicians will determine furnace size in two wrong ways:

  • Using a rule of thumb 
  • Using the current furnace’s size for reference

A rule of thumb
An example of overly simplistic rule of thumb is “Your home is # square feet, therefore you need this furnace size.”

But relying solely on the square footage of your home ignores other important factors like:

  • Number of windows 
  • How shaded your roof, walls and home are
  • Layout (multiple levels, basement, etc.)
  • Where you have vaulted ceilings (if you do)
  • The quality of your home’s insulation
  • If your air ducts are sealed and insulated

What if you have lots of windows? That means more heat loss during winter and your furnace may be undersized.

Using the current furnace size
Another lazy way to determine your furnace size: Just replace the current one with one of the same size. 

The problem with this method: What if the current furnace is oversized or undersized? Now you’re replacing an incorrectly sized furnace with ANOTHER incorrectly sized furnace!


Ignore metro Kansas City heating & air conditioning contractors who use the above methods. They’re just trying to get your money and are not looking out for your best interest. 

The right way: Manual J heat load calculation

Legit contractors will use a Manual J heat load calculation to determine your furnace size. The calculation can be done with a fancy calculator to help prevent human error.

However, furnaces only come in a few sizes, so some oversizing by a bit is necessary. But oversizing too much will cause the furnace to short cycle (turn on and off too often) causing it to run inefficiently, like a car that sputters down the road.  Bigger isn’t better, in this case.

Need a metro Kansas City HVAC contractor to find the right furnace size for you?

You found us! We have helped numerous homeowners in OlatheOverland ParkShawneeLenexaLeawood, Gardner and metro KC select the right furnace for their home. Our expert furnace technicianss will ensure that your new furnace is sized and installed properly. Contact us for a free furnace estimate.

Santa Fe Air Conditioning and Heating serves the Kansas City area. For more information, contact us online