400 CFM Per Ton, Or Is It?
“That system needs 400 CFM per ton!“ Does this rule of thumb sound familiar? Although many contractors may have known about this ‘rule-of-thumb‘ for a long time, how many contractors know that 400 CFM per ton is not always the case?
Many factors can change the typical 400 CFM per ton to a different CFM value. For example,
- Relative humidity
- Dry-bulb temperatures
- Wet-bulb temperatures
- Air density
- Mass flow rate
Furthermore, installers, start-up technicians, and field technicians must ensure the system has proper airflow, based on the OEM’s tables, or performance could drop and delivered capacity could be reduced. Your clients are paying your company a premium to install their HVAC equipment correctly, and they have an expectation of performance. It is critical that you deliver on your promise of performance by ditching thumbs’ rules and employing science to produce a correctly operating and maximum performance system.
Let’s take a look at an example using a 2.5-ton air conditioner in figure 1.0. If we stick to the 400 CFM per ton rule, then that would mean this 2.5-ton air conditioner would need 1,000 CFM of air.
Using this example, we have a 75°F indoor dry-bulb temperature. However, three different airflow options can result in various operating capacities. For this example, we will use the 875 CFM, with a 75°F outdoor ambient temperature and a 63°F entering indoor wet-bulb temperature.
Using this data, the system will have a total capacity of 29,500 BTU’s (highlighted in orange) at this specified condition. By the way, “M” is the Roman Numeral for 1,000. The S/T means sensible temperature ratio. This means that 52% or 15,340 Btu/h is sensible capacity, which means that this system has 14,160 Btu/h of latent capacity. In hot, humid climates, lower airflow like 350 CFM per ton can ensure the customer is comfortable.
If we keep the same temperature measurements and change the airflow to 1,000, you will notice the capacity will vary. The total capacity with 1,000 CFM of air is 29,800 BTU’s (highlighted in green). The sensible temperature ratio is 58%, giving us a sensible temperature of 17,284 Btu’s. Since this is still the same 2.5-ton unit, this demonstrates how a small change in airflow, 400 CFM per ton, tweaks the equipment’s capacity.
Finally, using the same temperature measurements but adjusting the airflow to 1,125 CFM, we will have 30,300 BTU’s (highlighted in blue). However, the sensible temperature ratio is now 61% giving us a sensible temperature of 18,483 BTUs. In warmer climates, that have less humidity, 450 CFM per ton lets the same system deliver more sensible capacity.
It is critical to understand that airflow will affect the system operation and other factors such as indoor dry-bulb temperatures and entering wet-bulb temperatures. Suppose technicians do not consider these factors and do not consult the OEM literature. In that case, it can have an adverse effect on the system and possibly result in repeated callbacks and unsatisfied clients.
Since the OEM data is valuable to field staff, consider having the installers leave the OEM data with the indoor unit. By doing this, your service technicians will not have to spend extra time tracking down the literature. During a maintenance visit, this data can be used to benchmark the system on that given day’s operating conditions.
Because HVAC systems are spread throughout parts of the country and used in different climate zones and elevations, it is crucial to know the required CFM per ton when repairing or making airflow modifications to an HVAC system.
Don’t guess when you have the tools to test. For information on using a psychrometric chart, check out ACCA’s Manual P for diagrams and instructions on making this simple tool work for you.
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