Enter the heat load in BTU/h and select the supply-return temperature difference to calculate the required airflow in CFM.
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This page explains how to convert a heat rate in BTU per hour (BTU/h) into an approximate airflow in cubic feet per minute (CFM) for air distribution systems. By specifying the supply-return temperature difference, you can estimate how much air must move through a coil, furnace, or air handler to carry the required sensible load.
BTU/h to CFM Formula
BTU/h describes how quickly heat is being added or removed, while CFM measures the volume of air flowing each minute. For standard indoor air conditions, these values are related by the constant 1.08.
CFM = Q / (1.08 ร ฮT)
The equivalent form of the equation is:
Q = 1.08 ร CFM ร ฮT
Variables:
- CFM is the airflow in cubic feet per minute
- Q is the sensible heat load in BTU/h
- ฮT is the temperature difference between supply and return air in ยฐF
To calculate CFM from BTU/h, divide the heat load by 1.08 multiplied by the temperature difference.
What is BTU/h to CFM Conversion?
BTU/h to CFM conversion is used to estimate how much airflow is needed to move a specific amount of sensible heating or cooling through an HVAC system. A lower temperature difference requires more airflow to carry the same BTU/h, while a higher temperature difference allows the same heat transfer with less airflow.
This relationship is commonly used when sizing ducts, checking furnace airflow, estimating air handler requirements, or comparing cooling and heating performance. The formula provides a practical engineering estimate under standard indoor air conditions.
How to Calculate BTU/h to CFM?
The following steps outline how to convert BTU/h to CFM.
- First, determine the sensible heat load in BTU/h.
- Next, determine the temperature difference (ฮT) between the return and supply air in ยฐF.
- Then, calculate the airflow using the formula CFM = Q / (1.08 ร ฮT).
- Finally, compare your result with the calculator above.
BTU/h to CFM Conversion Table
The table below shows example airflow requirements for a 12,000 BTU/h sensible load at different temperature differences.
| Temperature Scenario | ฮT (ยฐF) | CFM for 12,000 BTU/h |
|---|---|---|
| Light Cooling | 15 | 741 CFM |
| Typical Cooling | 20 | 556 CFM |
| Moderate Heating | 25 | 444 CFM |
| Heating (Common) | 30 | 370 CFM |
| Warmer Supply Air | 35 | 317 CFM |
| High ฮT Heating | 40 | 278 CFM |
| Very High ฮT Heating | 50 | 222 CFM |
In practice, designers often target a particular airflow and then verify that the resulting temperature difference and equipment performance are acceptable. Understanding how BTU/h, CFM, and ฮT interact makes it easier to size ducts, choose equipment, and compare different operating conditions.
Example Problem:
Use the following variables as an example problem to test your knowledge.
Heat Load (Q) = 12,000 BTU/h
Temperature Difference (ฮT) = 20ยฐF
CFM = 12,000 / (1.08 ร 20) = 555.56 CFM