Enter the absorbed heat (J) and the input heat (J) into the calculator to determine the Furnace Efficiency. 

Furnace Efficiency Formula

The following formula is used to calculate the Furnace Efficiency. 

Efurnace = Qa / Qi * 100
  • Where Efurnace is the Furnace Efficiency (%)
  • Qa is the absorbed heat (J) 
  • Qi is the input heat (J) 

To calculate the furnace efficiency, divide the absorbed heat by the input heat, then multiply by 100.

How to Calculate Furnace Efficiency?

The following two example problems outline how to calculate the Furnace Efficiency.

Example Problem #1:

  1. First, determine the absorbed heat (J). In this example, the absorbed heat (J) is given as 40.
  2. Next, determine the input heat (J). For this problem, the input heat (J) is given as 50.
  3. Finally, calculate the Furnace Efficiency using the equation above: 

Efurnace = Qa / Qi * 100

Inserting the values from above and solving the equation: 

Efurnace = 40 / 50 * 100 = 80 (%)


What factors can affect furnace efficiency?

Furnace efficiency can be influenced by several factors, including the type of furnace, the quality of insulation in the building, the temperature setting, maintenance frequency, and the age of the furnace. Regular maintenance and proper insulation can help maintain or improve furnace efficiency over time.

How often should a furnace be serviced to maintain its efficiency?

It is generally recommended that furnaces be serviced at least once a year, typically before the heating season begins. This service can include cleaning, inspection, and any necessary repairs or adjustments to ensure the furnace operates efficiently and safely.

Can upgrading my furnace improve my home’s energy efficiency?

Yes, upgrading to a newer, more energy-efficient furnace can significantly improve your home’s energy efficiency. Modern furnaces are designed to use fuel more efficiently, converting a higher percentage of the fuel into usable heat. This can lead to lower energy bills and reduced environmental impact.