How to Determine What Size Furnace You Need

What size furnace do I need? That single question is likely to have a different answer for you, than it does for me… or possibly even your next door neighbor! In fact, if I had a nickel for every person who asked how do you determine what size furnace you need, I would be much better off today! While there are several “rule of thumb” type answers that will get you in the ballpark, most professionals will refer to what is called a Manual J calculation, which is a series of mathematical formulas that allow you to precisely determine the heat loss and gain on your home, and the exact size gas furnace for your own home.

In this gas furnace guide article, we want to look at the importance of properly sized equipment, as well as the various ways you can learn how to determine what size furnace you need.

Why is the Furnace Size Important?

A few of the reasons you need a properly sized gas furnace:

  • If the gas furnace is too small, your home will be cold and the furnace will run 100of the time. This results in huge heating bills, and maybe even having to supplement the furnace with space heaters!
  • If the gas furnace size is too large, the furnace will do what is called short-cycling, and you will be cold, then hot, then cold again (repeat endlessly) as the gas furnace turns on and off repeatedly, like a blast type oven.
  • If you buy a furnace that’s too large for your existing duct, it will sound like a leaf-blower in your home!
  • If the blower in your gas furnace is not sized properly, it will have a negative effect on central cooling systems also!

Different Ways to Size a Furnace

There are several ways you can determine what size furnace you need, and depending on who you ask, you are likely to get a different answer! If you decide to have a local company provide an estimate for a gas furnace, you should insist that the contractor provide you with a copy of the heat loss calculation as part of their estimate.

Buy Load Calculation Software

HVAC-Calc sells a single-use homeowner version of their Manual J Heat Loss and Gain software for $49. The software is normally priced at $389 and is the only true scientific method to get exact heat loss and gain figures for your home. Preparing a Manual J on your home is not a quick and easy 10 minute process and if you plan to run a manual J, you should take the time to do it correctly. Plan on 2-3 hours to prepare all measurements and put the needed information into the software.

Use Your Existing System Size

Many people, including a high percentage of HVAC Contractors use your existing system as a way to determine what size furnace you need. Since a load calculation was done on your home when it was built, in many cases, the same size will be used during the replacement of a furnace. Be aware that this is not a guaranteed method that it is correct!

Get a Contractor to Provide a Quote

The most popular way to figure out what size furnace you need, is to call out a local qualified contractor and ask them for a price estimate to replace your furnace. Many contractors are willing to provide a free estimate for replacing your system. You should insist that the contractor provide their heat loss and gain calculations as part of your estimate. Get 3 Free Estimates Now!

Hire an Energy Rater to Test Your Entire Home

In recent years with the various energy incentive programs like the furnace tax credits,  many homeowners are starting to look at the bigger picture of HVAC systems that includes a complete home energy audit, from a qualified energy rater. An energy rater will look not only at your heating system, but all other aspects of the how energy is used and how efficient your home is.

Most energy raters will check duct work, chimney systems, doors and windows for leakage, and just about every aspect of your home that is involved with the overall efficiency!

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4 Responses to “How to Determine What Size Furnace You Need”
  1. Joline says:

    One of the better articles I have read about how to figure out the size of a gas furnace! I appreciate how you don’t try to BS homeowners into thinking its an easy thing! I suggest they take the contractor route and get a free estimate. If nothing more, they at least get a pretty good idea of what someone in the business thinks.

    ~ Joline