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AC 4 Life Knowledge Base - Frequently Asked Questions

Electric Furnace System Sizing


Heating Square Footage Range by Climate Zone
ZONE 1  ZONE 2 ZONE 3 ZONE 4 ZONE 5
20 - 25 Btu's per square foot 30 - 35 Btu's per square foot 35 - 40 Btu's per square foot 40 - 45 Btu's per square foot 45 - 50 Btu's per square foot

Use the lower of the two numbers if your home is well insulated and the higher number if it is older or poorly insulated. (Hint: Use the larger of the two numbers above if you're unsure of your home's insulation)

Simply multiply the appropriate factor above by your home's total heated square footage to arrive at your approximate required heating capacity. For example, if you live in the orange zone, your home is not well insulated, and you have 1500 heated square feet, the equation will look like this:

1500 square feet
X 35 heating factor (from the chart above)
52,500 Btu's required to heat your home

Then, to determine the heating output on a given electric furnace, simply view the heating element Btu options that most closely match your requirement. The electric furnace heating element options are displayed within the electric furnace pricing. Each Kw produces about 3400 Btu's of heat, so the heating output of the various heat strip options are as follows.

Element size Heating output

5kw 17,000 Btu's
7Kw 24,000 Btu's
8Kw 27,000 Btu's
10Kw 34,000 Btu's
12Kw* 41,000 Btu's*
15Kw 51,000 Btu's
17kw* 58,000 Btu's*
20Kw 68,000 Btu's

*12Kw and 17kw options offered only with mobile home electric furnaces

If you need more than 68,000 Btu's we suggest either two electric furnaces with separate duct systems for each, or a gas furnace. Remember that the larger the heating element, the more power it will consume.

Most electric furnaces are offered in 7,000-10,000 btu increments so you just need to get close in terms of sizing. If the furnace you selected is more than 10% below your heating requirement, we suggest you go up to the next size. A little under sizing or over sizing is fine, just don't over size by more than about 20% of your heating requirement, or short cycling can occur which wastes energy and reduces your comfor.

Selecting the proper CFM

You will see various CFM (cubic feet per minute) options within the electric furnace prices. This refers to the volume of air delivered and is normally used for air conditioning considerations. The following is the airflow (CFM) requirements for various air conditioning needs:

800 CFM 1.5 - 2.0 tons
1200 CFM 2.5 - 3.0 tons
1600 CFM 3.0 - 4.0 tons
2000 CFM 4.0 - 5.0 tons


If you intend to add air conditioning at some later point, use these CFM considerations based on a professional estimate of your future air conditioning requirements. Otherwise, if you are considering heating only, simply select the Btu requirement of your home and choose the least expensive or middle range CFM option offered within your heating range (they will overlap somewhat).

If you're still unsure which system size is right for you, please e-mail us or call our toll free number at 1-866-862-8922. An experienced design technician will be happy to assist you. Your home's construction quality and insulation is unique and can greatly affect furnace sizing, so this information is intended to provide a general guideline but should not be the only consideration when selecting a furnace for your home.

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