What Size Furnace Do I Need?
The honest answer is that nobody can tell you from square footage alone, and anyone who does is guessing.
Furnace size depends on your home's insulation, windows, ceiling height, air leakage, ductwork and local climate. Two houses the same size can need very different furnaces.
Why the online rules of thumb are unreliable
Most guides tell you to multiply square footage by a number of BTUs. The trouble is that the number varies enormously depending on the house.
A well insulated new build with modern windows might need half what a 1950s house of the same size needs. Vaulted ceilings change it. A basement changes it. Leaky ductwork changes it.
The rule of thumb gets you a rough bracket. It does not get you a system that works properly.
The altitude problem, which matters here
This one catches people out.
Furnaces lose heating capacity at altitude, because there is less oxygen for combustion. The Salt Lake Valley sits above 4,200 feet, and some benches are considerably higher.
A furnace rated at 80,000 BTUs at sea level delivers noticeably less here. If someone sizes your furnace from a national chart without accounting for elevation, you can end up undersized without anyone realizing.
It is one of the reasons a local load calculation matters more here than in most places.
What actually determines the size
A proper assessment looks at:
The size and layout of the house, including ceiling heights.
Insulation levels in the walls, attic and floors.
Window type, size and orientation.
How airtight the house is.
The ductwork, which is often the real limitation. A bigger furnace cannot push more air through ducts that are already at capacity.
Local design temperature, meaning how cold it actually gets here rather than a national average.
Elevation.
This is called a load calculation, and it is the difference between a system that works and one you live with.
Why bigger is not better
An oversized furnace heats the house quickly, shuts off, then starts again a few minutes later. That is called short cycling, and it causes real problems.
The equipment wears out faster, because starting and stopping is harder on it than running.
Rooms end up uneven, because the system never runs long enough to move air properly through the house.
It costs more to run than a correctly sized unit.
And it costs more to buy in the first place.
Oversizing is the most common mistake in residential heating, and it usually happens because sizing from square footage is quicker than doing the calculation.
What about undersized?
Less common, but it happens. The symptoms are a furnace that runs almost constantly in cold weather and still struggles to reach the thermostat setting.
If your current furnace does that, it is worth finding out whether it is genuinely undersized or whether something else is wrong, like airflow or a failing component. Replacing a furnace that was never the problem is an expensive way to find out.
Before you call
Find the model number on your current furnace, usually on a plate inside the front access panel. The BTU rating is often in the model number itself.
Note how the house actually behaves in winter. Which rooms are cold. Whether the furnace seems to run constantly or in short bursts. Whether it was ever right, or whether it has always struggled.
That tells an installer more than the square footage does.
How we approach it
Every installation we arrange starts with a load calculation and a look at your existing ductwork, so the equipment matches your home rather than a round number.
We install Goodman, Rheem and Carrier furnaces. And if your current furnace is repairable and worth repairing, we will tell you that first.