Cooling Heat Loss Energy Calculators     
Topics & Tools

Section: Heating  
Featured Calculators

Heat Loss Online™ Calculator Version 2.0

Calculate Energy Savings

Size Your Humidifier

Calculate Fuel Savings Thermostat Set-back

Save Energy with Attic Insulation

Heating Systems

Heating Systems
  Q & A
Maintenance Tips

Gas Furnace Size BTU

A furnace is sized according to the amount of heat it can produce.

This is known as a BTU rating. A furnace will have two BTU ratings. One for INPUT and one for OUTPUT. The BTU input rating is the gross amount of BTU produced.

The BTU output rating is the amount of usable BTU. In other words, Input is how much heat produced and Output is how much of that heat can be delivered to your house.

The difference between Input and output is the efficiency rating.
A 100,000 BTU input furnace with an 80 percent efficiency rating will make 80,000 BTU of heat available for your house. (Eighty percent of 100,000 = 80,000)

A 100,000 BTU input furnace with a 95 percent efficiency rating will make 95,000 BTU available for your house.

Both furnaces will use the same amount of energy, one will simply waste more.

To properly size a furnace, a heat loss calculation is performed on the building. This determines how many BTU per hour is needed to condition the space. A heat loss calculation is specific to your area and building. Common sense tells you that the same house in Maine will require a different amount of BTU’s than a similar one in Florida.

  Back to Gas Furnace Troubleshooting




Heating   Cooling   Air Quality   Home