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How Big Does My Furnace Need to Be? and What the Heck Is a BTU?

Furnaces are evaluated by the amount of heat they can put out or what we call British Thermal Unit or, you guessed it, a BTU.

A BTU, or known by its close friends as 1055 Joules, is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. Still confused? Here is a video to explain it in

How do you know how much you need?

According to, here is a very simple formula to get things started

Step 1.
Measure the square footage of your home. This is every room that you are trying to heat or cool and has ductwork going to it.
Step 2.
Multiply your total square footage by 25.
Step 3.
TADA, you now know how many BTU’s you need.
Step 1.
2,500 Square Feet house
Step 2.
2,500 x 25 =62,500 BTU Needed

In this example, you would need a furnace that produced 50,000 BTU’s to keep you comfortable. This is should give you a really rough idea however if you have a lot of windows in your home, poor insulation and basement these numbers will move around.

An HVAC professional will measure your home figuring these variables to give you a recommendation of what will be the best solution for you.

For more information, schedule an appointment with Legacy Heating and Air.

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