1 TR = 400 CFM - Explained: Formula, HVAC and More

First, let's understand what each of these acronyms stands for and how they relate to each other in the context of heating, ventilation, and air conditioning (HVAC):

  1. TR: This stands for Tons of Refrigeration. In the HVAC world, this is a unit used to measure the amount of heat a cooling system can remove in a certain period. A "ton" doesn't refer to the weight of the system but is rather an energy measure. One ton of refrigeration is approximately equal to the cooling power of one ton (2000 pounds) of ice melting in a 24-hour period. It's also equivalent to 12,000 BTUs per hour (British Thermal Units, another measure of heat).

  2. CFM: This stands for Cubic Feet per Minute. It's a measurement of the volume of air an HVAC system can move or process in one minute.

The connection between the two is important for designing or sizing HVAC systems. In general, an HVAC system needs to be able to move a certain amount of air (CFM) to remove a specific amount of heat (TR) to maintain a comfortable environment.

Now, let's discuss the '1 TR = 400 CFM' rule.

This rule is a generalized rule of thumb in the HVAC industry. It suggests that to effectively remove one ton of heat (1 TR), an HVAC system should be able to move approximately 400 cubic feet of air per minute (400 CFM).

Here's a simple way to think about it: Imagine you're trying to cool down a big, hot room. If you blow a small, gentle breeze through the room, it's not going to have much of an effect. But if you blow a large amount of air quickly, it's much more likely to cool the room down. That's effectively what an HVAC system is doing when it moves 400 CFM to remove 1 TR of heat.

But please remember that this is a general rule and may not apply exactly in all situations because the actual CFM required can vary based on factors like the specific cooling needs of a space, the efficiency of the HVAC system, and local climate conditions. Therefore, in practice, HVAC system designers would often perform a more detailed calculation to determine the most appropriate system size and air flow rate.

Hopefully, this helps clarify the relationship between TR and CFM. Let me know if you have any further questions!


  • Ashish Sharma
    Ashish Sharma
    Actually one ton of refrigeration = 12,000 BTU/hr.

    When it comes to relating this to air quantities, the basic equation is:

    Q (btu/hr) = 1.08 (constant for air at standard conditions)*density correction factor*cfm*delta T

    400 cfm per ton is a manufacturers nominal standard, or rule of thumb. It should get you in the ballpark, but is not exact.
  • Harshad Katre
    Harshad Katre
    Thank You for the reply.
    But I want to know whether there are any theorotical calculations involved to arrive at 1 TR = 400 CFM.
  • Diego Páez
    Diego Páez
    The concept, as I understand it, is that the supply air is kind of a constant (at 54°F). Being the formula Q = m cp dT, if cp is a constant at a particular condition, then for 54°F you would have about 400cfm to get 1 TR
  • Swapnil Chavan
    Swapnil Chavan
    hi try to work back words with 400 CFM and 12000 Btu/hr as Total Heat gain. with enthaply correction factor as 4.45 and considering no bypass we will end up with a delta enthalpy of 6.3 when subtracted with enthalpy of room air i.e 24 @ 24 deg C it will be 18.33 enthalpy which is cooling coil adp = 53.6 deg F approx equal to 12 deg. This is all theory and calculation however; 400 cfm/Tr shll be used only for thumb rule application in actual while selecting unit you shall analyse what supplier offers and proceed further.
  • Abdul Riyaz
    Abdul Riyaz
    1 TR = 400 cfm is a thumb rule used in this industry.
    However, it is not a constant and varies with different manufacturers.
    As said by Ashish sharma,
    Q (btu/hr) = 1.08 (constant for air at standard conditions)*density correction factor*cfm*delta T
    This formula also holds good to calculate the cfm from the TR and delta T
  • Bala Venkat
    Bala Venkat

    CFM is cubic feet per minute. TR is tonnage of refrigeration.
    From what we have been taught,
    For a centralized air conditioning system, 1 TR = 400 CFM
    For the non-centralized conditioning system, 1 TR =300CFM.

    Formula to convert is : CFM = (Volts x Amps x 3.413) / (ΔT x 1.08)‎

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