Sizing Your System - Electric Furnaces
Can you help me size an Electric Furnace 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 the correct run time for maximum comfort. 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 electric furnaces you don't have to nail your requirement down to the very last BTU. Electric heating furnaces are typically sold in 10,000-15,000 Btu increments because the manufacturers know that with heating, you only need to be in the ballpark in terms of the your home's Btu requirements. If you are over by 20-25% there is no real harm.

Electric furnaces are actually an "air handler" with a heating element. These heating elements are sized based on their Kw, or kilowatt of power consumption. Each Kw produces 3400 Btu's of heat. If your determine your home requires 52,000 Btu's of heat then divide 52,000 by 3400 to arrive at the Kw figure required.

52,000 Btu's required ¸ 3400 Btu's produced per Kw = 15.29 Kw

Keep in mind that straight electric heat is the most expensive way to heat your home. Normally, electric heat is used where the heating demand is low or where no other good option exists. If your local utility rate is .10 cents per kilowatt-hour, then a 10 Kw heater will cost $1.00 per hour to operate and a 15 Kw will cost $1.50 per hour.

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.

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) 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.

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

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

3) If you are replacing an existing furnace within your home, simply locate the breaker that provides the over-current protection for your electric furnace. (Normally located in a breaker panel with other breakers) On this breaker pole there is a number. (The breaker pole is what you grasp to turn off the breaker) This number indicates the circuit size in Amps leading to your electric furnace. Following is how to convert this circuit amperage size into a Kw requirement.

30 Amps = 5Kw
40 Amps = 7Kw
50 Amps = 8Kw
60 Amps = 10Kw
80 Amps = 14-15Kw
100 Amps = 17-18Kw
120 Amps = 20Kw
150 Amps = 25Kw

If you are significantly increasing the size of an existing electric heater, the circuit may need to be re-wired along with a larger breaker. If you want to match to an existing circuit then use the chart above. Remember, each Kw generates about 3400 Btu's.

Measure the square footage of the heated area of your home and compare this with the heating capacity you are considering. In warmer climates it should be roughly 25-30 Btu's per square foot. In colder climates it should be around 35-40 Btu's/sq. ft. and in the coldest climates around 45 Btu's/sq. ft.

Warm climate

1000 square foot home requires roughly 25-30 Btu's per square foot, or about 25,000 to 30,000 Btu's.

Colder Climate

1000 square foot home requires roughly 35-40 Btu's per square foot, or about 35,000 to 40,000 Btu's

Coldest Climate

1000 square foot home requires roughly 45 Btu's per square foot, or about 45,000 Btu's

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.

4) 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 as scientific as the other approaches because it assumes all homes in each zone are approximately similar in construction type and insulation values.
You sometimes recommend an estimate by a local contractor prior to purchase. Why?
I've seen system sizing software on other sites for free. Why do you charge for yours?