Is it better to oversize or undersize a furnace?

Author: Shanie Weissnat  |  Last update: Monday, December 15, 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.

Should you oversize your furnace?

it is best to have the right size furnace as oversize will cause too many on-off cycles. too small causes extended periods of running and the possibility that it won't handle extreme cold.

What is the rule of thumb for furnace sizing?

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.

Is it better to undersize or oversize HVAC?

A undersized system will run more and remove more humidity in cooling, but you give up being able to keep your home at, or near, setpoint on the really hot days. A oversized system in a humid area will not run enough to remove enough moisture from the home and it could cause issues with mold and mildew.

What is the difference between undersize and oversize furnace?

An undersized furnace will run constantly (running up your energy bills) without ever properly heating your home. It won't ever meet your heating needs. An oversized furnace will heat your home quickly and then shut off.

What happens when your furnace is oversized or undersized?

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.

How many square feet will a 100,000 BTU furnace heat?

For example, in zone 3 to 4, which typically requires 40-45 BTU per square feet, you can determine that a 2,500 square foot home would need a 100,000 to 112,000 BTU furnace.

What happens if your furnace is undersized?

An undersized HVAC system has to work harder and run longer to maintain the desired temperature, which leads to higher energy bills. Frequent cycling and extended run times can also significantly increase energy usage. Consider upgrading to a properly sized HVAC system.

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 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.

How big of a furnace for 2000 square feet?

A mid-sized home of 2,000 square feet would need approximately 50,000 to 60,000 Btu to heat it properly. With a less efficient furnace operating at 80 percent efficiency this would require a 60,000- to 72,000-Btu furnace.

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.

What is the rule of thumb for furnace size?

A general rule of thumb is that you'll need about 30 to 60 BTUs per square foot of your home, depending on your climate zone. For example, if you have a 2,000-square-foot home, you might need a furnace rated between 60,000 and 120,000 BTUs. BTUs, or British Thermal Units, measure a furnace's heating capacity.

What happens if an HVAC unit is oversized?

Again, because oversized HVAC units blast your home with cool air instead of cooling it down gradually, the temperature swings prompt your HVAC system to start up and shut down in rapid succession-a condition commonly known as “short cycling.” Short cycling can wreak havoc on your HVAC system if left unchecked.

Can a furnace have too many BTUs?

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 the difference between oversize and normal size?

Oversized T-shirts are designed to have a relaxed, loose fit that intentionally appears larger than the standard size. Unlike traditional T-shirts, which follow the contours of the body, oversized T-shirts drape comfortably, providing ample room for movement and a laid-back aesthetic.

Do I need oversize or undersize bearings?

Undersized bearings are used when a crankshaft has been machined or ground undersize. Oversize bearings have more material on the outside facing the bearing housing bore. Oversized bearings are used when the block has been machined or line bored oversized.

What happens if you oversize a valve?

Again, an oversized valve will result in a high stroke count, which over time will wear down your valve trim and packing much faster than it should under normal operation. This can lead to deterioration of the trim, resulting in poor performance and potential environmental issues.

Is it bad to oversize a furnace?

Your home comfort will suffer and you will have issues with your indoor air quality. Furthermore, it is expensive to operate a furnace that is too large. Your monthly energy costs will increase. Also, a system that is too large will require more repairs than one that is properly sized.

How do I increase the airflow in my furnace?

Here are a few ways to increase airflow through your air ducts and improve the comfort of your home.
  1. Clean the Air Ducts. ...
  2. Add Ductwork Extensions or Registers. ...
  3. Install Automatic Dampers on Your Vents. ...
  4. Replace Furnace Filters. ...
  5. Install a Whole House Fan. ...
  6. Add More Insulation to Your Walls. ...
  7. Seal Up Drafty Windows and Doors.

Why is my furnace not keeping my house warm enough?

Check the air filter first

If the filter is heavily clogged with dust and debris, it cuts down on airflow, which in turn means less heated air is getting around the house. Change the filter for a clean one if it's clogged, and continue to change it every 1 to 3 months while the furnace is running.

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.

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.

How big is a house with an 80,000 BTU furnace?

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.

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