How to Calculate CFM in a Residential Furnace

As of 2009, the volatility of heating prices has many people concerned with the efficiency of their residential furnaces. Because the furnace's CFM plays such a vital role in the overall performance of the unit, it is useful to know exactly what CFM is and how to calculate it. CFM stands for "cubic feet per minute" and refers to the amount of air being circulated through the device at any given time. Knowing your furnace's CFM will give you a better idea of how well your unit is functioning.

Things You'll Need

• Gas furnace

Instructions

1. Find your furnace's BTU rating. This can be found on the manufacturer's nameplate attached to the furnace.

2. Divide the BTU rating number by 10,000. For example, if you find that your furnace's BTU rating is 80,000, you would divide this number by 10,000 for a factor of 8.

3. 

   Determine the type of gas furnace that you have. There are three basic types, which are classified as natural draft furnace, induced draft furnace and condensing furnace. To determine which type of furnace you have, you must examine the structure of the furnace itself. If the unit has a fan located below the flue, then you have an induced draft furnace. If you find that the flue is made out of PVC pipe, then you have a condensing furnace. If you have neither a fan nor a PVC pipe flue, then you are dealing with a natural draft furnace. Refer to the figure for the basic structure of each of these three types of gas furnaces.

4. Multiply the factor obtained in Step 2 by the appropriate multiplier for your particular furnace. If you have a natural draft furnace, the appropriate multiplier is 100. If you have an induced draft furnace, then the appropriate multiplier is 130. Finally, if you have a condensing furnace, the appropriate multiplier is 150. So, for example, let's assume that you determine your furnace to be a natural draft furnace. Continuing with the example from Step 2 where the factor obtained was 8, you would simply multiply 8 by 100, for a product of 800, which would be the CFM of your natural draft furnace.