Is it better to undersize or oversize a furnace?

Author: Ms. Blanca Wiza DDS  |  Last update: Friday, September 19, 2025

Even a slightly undersized furnace will accomplish the job the majority of the time. Taking things down a few sizes will more than likely solve some of your common heating problems like too high of a utility bill while half of your house is blazing and the other half is an icebox.

What is the correct size furnace for a 1500 square foot house?

A 1,500-square-foot home will require between 45,000 to 90,000 BTUs.

Is it better to oversize or undersize HVAC?

Larger unit may be more efficient (larger condensers generally are), but make sure you don't oversize it. If the unit is oversized it'll cycle more frequently, which is hard on it. Oversized can also cause some issues with humidity if you have any outside air connections.

What happens if a furnace is undersized?

Having a furnace that is too small for your home means the system won't be able to keep up with the dropping temperatures outside. They simply can't make up the difference to heat your home properly.

What will happen if a furnace is oversized?

Furnaces are equipped with a safety feature that prevents them from overheating. When a furnace is oversized, it will run hotter than it should, which puts it closer to the temperature at which it will shut itself off. Throw in a dirty furnace filter, and it will probably be enough to push the furnace over the edge.

What happens when your furnace is oversized or undersized?

Is it better to oversize or undersize a furnace?

Even a slightly undersized furnace will accomplish the job the majority of the time. Taking things down a few sizes will more than likely solve some of your common heating problems like too high of a utility bill while half of your house is blazing and the other half is an icebox.

How do you tell if your furnace is too small?

Before you go shopping, find out if your furnace is too small.
  1. The Furnace Runs Continuously. ...
  2. Your Furnace Struggles to Reach the Desired Temperature. ...
  3. There Are Cold Rooms in a Mostly Warm House. ...
  4. The Furnace Has a Premature Lifespan. ...
  5. There's Humidity in the Home. ...
  6. Ductwork Size Doesn't Match the Furnace.

Is it better to have a higher BTU furnace?

The more BTUs a furnace has, the larger the area it can heat. However, bigger isn't always better. If your furnace has insufficient BTUs, it'll work non-stop and never keep your home at a comfortable temperature. Alternatively, a furnace with too many BTUs will frequently cycle and be inefficient.

What is considered a high efficient furnace?

A high efficiency gas furnace has AFUE ratings of 90% or higher, up to a maximum of 98.5% AFUE. AFUE stands for Annual Fuel Utilization Efficiency, and is a measure of how much heat the furnace can pull from the fuel it consumes, ultimately determining how energy efficient a heating and cooling system is.

What is the difference between undersize and oversize?

Undersize bearings therefore have a smaller inside diameter than standard, and oversize bearings would have a larger inside diameter than standard! Amazing how these terms get interchanged once in awhile!

Is oversizing ductwork bad?

Each section of ducting that gets installed in your home needs to be appropriately sized for your HVAC unit to perform efficiently. If your ducting is oversized or undersized, it could lead to premature wear out of your heating and cooling units and an uncomfortable living environment.

How do I know if my HVAC is undersized?

Symptoms of an undersized AC unit include poor cooling, high energy bills, uneven temperatures and excess humidity.

What is the rule of thumb for furnace size?

A basic rule of thumb for furnace size calculation is that it takes 30 BTUs for every square foot of house. So, if you have a 1,000 square foot house, you need a furnace that has a 30,000 BTU output.

What is the most reliable gas furnace brand?

The 5 Best Furnace Brands
  • Carrier Furnace: Best Warranty.
  • Lennox Furnace: Best Company Reputation.
  • Bryant Furnace: Best for Durability.
  • Trane Furnace: Most Years of Experience.
  • Amana Furnace: Best for Efficiency.

How many square feet will a 80,000 BTU furnace heat?

How many square feet will an 80000 BTU furnace heat? A furnace with 80,000 BTU capacity can effectively warm up a typical home measuring 2,000 square feet.

What happens if I oversize my furnace?

An oversized furnace causes excess wear and tear, higher energy bills, and more problems (and expenses!) that make it tough to stay warm in the winter. This article goes over how to tell if the furnace installed in your home is too big.

What happens if BTU is too high?

An air conditioner with BTUs that exceed the recommended square footage in your home may also take on more wear and tear over time because it will initiate its on/off cycle more frequently. It could also create a subpar cooling experience because your AC's compressor may turn off much sooner than it should.

What happens if your furnace is undersized?

An undersized furnace is incapable of keeping up with the heating demands of a big house on the coldest days. The result is that your furnace won't shut off automatically. It will instead run constantly, which means your heating bills will skyrocket and your furnace will break down often as well.

What is one disadvantage of over sizing a furnace?

An oversized furnace can cause a number of problems, including increased energy costs, shortened furnace life, and uneven heating. If you're not sure what size furnace you need, talk to a heating and cooling professional.

Do you count the basement when sizing a furnace?

When sizing your furnace, you need to count all the square footage of rooms you want to heat. If you want to heat your basement, count it when sizing your furnace.

What size furnace for a 1200 sq ft house?

1,200-square-foot home will require between 35,000 and 75,000 BTUs. 1,800-square-foot home will require between 45,000 to 90,000 BTUs.

How many BTUs to heat a 40x60 shop?

To heat a 40×60 shop, which is 2400 square feet, you'll typically need between 108,000 and 132,000 BTUs.

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