What Furnace Size Should I Get for an 1800 Square Foot House?

September 13, 2017

First off, furnaces are sized based on how much heat they create in an hour (measured in BTUs). The higher the BTU output of a furnace, they more heat it provides in an hour.

So how many BTUs would you need for an 1800 square foot home? Well, the best answer we can give you is 45,000 to 81,000 BTUs.

But here’s the BIG problem with that answer: The square footage of your home is just one of many factors that determine the correct size furnace a home needs. 

You see, sizing a furnace based on square footage alone is about as accurate as a tailor who doesn’t take measurements—you can make an educated guess on sizing but without specific calculations, you’re likely to end up paying a lot of money just to be uncomfortable.

So let’s take a closer look at what you should do instead to make sure you get the correct furnace size for your 1800 square foot house.

Need a professional to properly size your furnace?

How to properly size your furnace

The only way to ensure that your home’s furnace is correctly sized is to have a professional perform a Manual J calculation

A Manual J calculation determines exactly how much heating your home needs. Once a professional calculates this, they can determine exactly how many BTUs it will take to heat your home.

When a professional performs a Manual J calculation, they’ll look at:

  • Climate
  • Size, shape and orientation of your home
  • The rate that air leaks out/into your home
  • Type of lighting/major appliances in the home
  • Number of windows 
  • How shaded your roof, walls and home are
  • Layout (multiple levels, basement, etc.)
  • Where you have vaulted ceilings (if you do)
  • How much insulation your home has (and the quality of that insulation)
  • If your air ducts are sealed and insulated

The bad news is that:

  1. It can be tough to find a professional who is willing to take the extra time and effort to perform a Manual J calculation. In fact, some HVAC professionals only use rule-of-thumbs when sizing furnaces (i.e. “Your home is X square footage so you need this size furnace”).  And as a result, the majority of HVAC systems installed in US homes are incorrectly sized
  2. Even with a Manual J sizing report, you could end up with an incorrectly sized AC if your home was built with inadequate ductwork. For example, if the ductwork is undersized for your home, installers will be forced to provide an undersized unit to match the ductwork (unless the homeowner is willing to have the ductwork replaced/renovated, which is very expensive).

So what’s the big deal if your furnace isn’t sized correctly for your home? Well, let’s take a look…

Why correct furnace size matters

Making sure that your furnace is sized properly makes a huge different when it comes to your comfort and your wallet. 

Here’s what we mean:

If your furnace is oversized for your home, you’ll see:

  • Decreased comfort
  • Higher upfront cost than is really necessary
  • Shorter furnace lifespan
  • Frequent expensive repairs
  • Lowered furnace energy efficiency

If your furnace is undersized for your home, you’ll see:

  • Higher monthly energy bills
  • Shorter furnace lifespan
  • Cold/hot spots throughout the home

Want to avoid these problems? Have a Kansas City tech perform a Manual J calculation

Want to make sure you’re not wasting money when it comes to your furnace installation

Just contact us. We provide a comprehensive Manual J calculation as part of our free furnace estimate.

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