How to Tell if Your Furnace Is Oversized

Old and dirty gas furnace in a Framingham, MA homeHaving a furnace that’s too big for your space is extremely inefficient. There are two simple signs that indicate that your furnace is most likely oversized:

  1. Your furnace runs for a short period of time before shutting off.
  2. Your home has uncomfortable hot and cold rooms.

Let’s go into more detail about each of these.

Furnace Turns Off and On

An oversized furnace has short run times (or “short cycles”) because it’s quickly heating your home to the thermostat’s temperature setting. By short, we’re talking 10-15 minutes. Now, that seems like a good thing (especially on frigid nights), but it’s not for a few reasons:

1) It’s not very energy efficient

The AFUE rating (the efficiency rating) you see on your furnace assumes your furnace is running at a “steady-state operation” (basically like a car’s mileage on the highway).

But an oversized furnace will come on and cycle off before it can reach that steady-state operation level of efficiency. So you’ve got several short, inefficient bursts wreaking havoc on your energy bill. Think of this as the furnace’s version of “city miles.”

If your furnace is properly sized, it will stay on longer, running almost constantly on the coldest days.

2) It dramatically reduces your furnace’s lifespan

Short cycling wears down the mechanical parts of your furnace. It’s like if you constantly turned your car off and on every time you came to a stop light.

Uncomfortable Hot and Cold Rooms

Do you feel a massive heat blast coming from vents in certain rooms?

Once again, the oversized furnace strikes! Its heat output is far more than your home actually needs, so you’re left with a heat hurricane in certain rooms. Ironically, an oversized furnace can cause rooms farther away from the thermostat to be too cool for comfort.

Why?

Because when the furnace cycles off your home cools down from the outside in. By the time your thermostat, which is located on an interior room wall, feels a temperature difference and turns the furnace back on, the rooms farther away have already cooled down a good 5-7 degrees.

If your furnace is properly sized, it should have longer run times, distributing heated air more evenly throughout your home.

What Can You Do If You Have an Oversized Furnace?

  • To even out the heated air in your home, you can run your fan setting to “ON.” But this wastes even more electricity.
  • If you’re thinking of getting a new furnace installed, ask several HVAC contractors how they plan to size your new furnace.
  • If they respond with, a rule of thumb (i.e., a certain number of BTUs per sq. ft.) or just say they’ll replace your furnace with one of the same size or larger, look elsewhere.
  • A good HVAC contractor should do a Manual J heat load calculation or use the manufacturer’s sizing software.

Framingham Furnace Experts

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