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AC 4 Life Knowledge Base - Frequently Asked Questions

Gas Furnace System Sizing

How do I determine the correct gas furnace size for my home?

One of the most important aspects in your selection of a heating system is proper sizing. A properly sized heating system will provide a high level of comfort, while maintaining maximum efficiency. An oversized system will "short-cycle", meaning that it will cycle on and off more frequently.

Short cycling will limit your comfort due to the frequent on/off cycling and reduce the system operating life. Having said all of this keep in mind that with gas furnaces you don't have to nail your requirement down to the very last BTU. Furnaces are typically sold in 20,000-25,000 BTU increments because the manufacturers know that with heating, you just need to be in ballpark in terms of how many Btu's your home needs.

If you are off by 20-25% there is no real harm. If your heating requirement is about 10% or more above a given furnace's capacity go up to the next size furnace. So how do you select the proper system size for your home? Here are your options in descending order of most accurate to least accurate.

Here are Your Sizing Options :

1) Download a system sizing program, available here, and input all the construction data about your home. This is by far the most effective as all of the guesswork, opinion, and "rules of thumb" are removed. Your system will perform as the manufacturer intended, your power bills will be as low as possible, and your comfort is assured. (Our sizing program is free with any equipment purchase.)

Software sizing calculations are required by many municipalities across the country when permitting a new construction home. No "best guesses" are allowed when it comes to a professional, code-correct installation.

2) Use our free sizing calculator to determine the proper heating and air conditioning capacities for your home. This is a detailed program so be prepared to spend about 30 minutes preparing dimensional and construction data about your house. Once you're prepared, the program will require approximately 20 - 30 minutes for you to complete.

This program is not as accurate as the program above but is also an excellent way to size your systems for free.

3) Have a contractor visit your home and provide an evaluation of the sizing needed. An experienced contractor in your area may be familiar with your home's floor plan and already know the size needed based on previous evaluations and installations. Unfortunately, contractors will not always agree on the proper size for your home. If not, ask them what criteria they used in their evaluation.

If they are using only square footage in the evaluation there is absolutely no assurance that the recommended system will function properly. Studies conducted by heating industry trade groups have repeatedly shown that over 50% of the furnaces sold in this country are dramatically oversized. By "dramatically oversized" we mean furnaces that produce up to twice the Btu's actually required. Our industry is trying to correct this problem through training programs that emphasize proper sizing procedures.

4) If you are replacing an existing furnace within your home, locate the Btu capacity of your existing furnace by removing the top front service panel from the furnace. Usually, the "Btu input rating" and "Btu output rating" are listed on the nomenclature plate on the furnace behind the service door.

Most older furnaces are 80% efficient so the output rating will be 80% of the input rating. For example if the rated input is 100,000 BTU, the output will be 80% of this figure, or 80,000 Btu's. Next, measure the square footage of the heated area of your home. In warmer climates it should be roughly 25-30 BTU's per square foot. In colder climates it should be around 40 BTU's/sq. ft. and in the coldest climates around 45 BTU's/sq. ft.

If used properly, this method will provide the existing furnace sizing. Assuming it was sized properly to begin with the same size replacement system is a fairly safe bet. This method is more reliable if you have lived in your home for at least one or two heating seasons and have experienced the system performance first-hand.

5) Use our system size selector located here. This selector uses square footage and your climate zone to estimate the approximate sizing for your home. It is not scientific as it assumes all homes in each zone are approximately similar in construction type and insulation values.

Don't be afraid to ask questions and expect well thought out answers. Use your judgement when determining who really seems the most interested in providing you with straight-forward, valuable information.


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