“That mechanism needs 400 CFM per ton!“ Does this preeminence of ignorance sound familiar? return many contractors may have actually known about this ‘rule-of-thumb‘ for a lengthy time. Just how many contractors know that 400 CFM per ton is not always the case? 

Many factors can adjust the usual 400 CFM per ton come a different CFM value. For example, 

Relative humidity Dry-bulb temperatures Wet-bulb temperatures Air density Mass flow rate Elevation 

Furthermore, installers, start-up technicians, and also field technicians need to ensure the device has appropriate airflow, based on the OEM’s tables, or performance could drop and also delivered capacity can be reduced. Her clients room paying your firm a premium come install their HVAC tools correctly, and they have actually an expectation of performance. That is an essential that you deliver on her promise of performance by ditching thumbs’ rules and employing science to produce a properly operating and also maximum power system. 

Let’s take a look at at an instance using a 2.5-ton waiting conditioner in figure 1.0. If us stick come the 400 CFM per ton rule, then the would typical this 2.5-ton waiting conditioner would require 1,000 CFM that air.  

Using this example, we have actually a 75°F indoor dry-bulb temperature. However, three different airflow options can result in various operating capacities. Because that this example, we will usage the 875 CFM, v a 75°F outdoor approximately temperature and a 63°F entering at home wet-bulb temperature.  

Using this data, the mechanism will have a full capacity that 29,500 BTU’s (highlighted in orange) in ~ this stated condition. By the way, “M” is the Roman numeral for 1,000. The S/T method sensible temperature ratio. This way that 52% or 15,340 Btu/h is judicious capacity, which means that this system has 14,160 Btu/h of latent capacity. In hot, humid climates, reduced airflow favor 350 CFM every ton can ensure the customer is comfortable. 

If we store the exact same temperature measurements and adjust the airflow to 1,000, girlfriend will notice the capacity will vary. The complete capacity through 1,000 CFM of wait is 29,800 BTU’s (highlighted in green). The cautious temperature ratio is 58%, offering us a judicious temperature of 17,284 Btu’s. Due to the fact that this is tho the same 2.5-ton unit, this demonstrates just how a tiny change in airflow, 400 CFM every ton, tweaks the equipment’s capacity.  

Finally, utilizing the very same temperature measurements but adjusting the airflow to 1,125 CFM, we will have 30,300 BTU’s (highlighted in blue). However, the cautious temperature ratio is currently 61% providing us a sensible temperature that 18,483 BTUs. In warmer climates, that have actually less humidity, 450 CFM every ton lets the same system deliver more sensible capacity.


It is vital to recognize that airflow will impact the system operation and also other determinants such together indoor dry-bulb temperatures and entering wet-bulb temperatures. Expect technicians carry out not take into consideration these factors and also do no consult the OEM literature. In the case, it have the right to have one adverse impact on the system and possibly an outcome in repeated callbacks and unsatisfied clients.  

Since the OEM data is valuable to ar staff, take into consideration having the installers leave the OEM data with the indoor unit. By act this, your organization technicians will not need to spend extra time tracking down the literature. Throughout a maintain visit, this data deserve to be used to benchmark the mechanism on that offered day’s operation conditions. 

Because HVAC systems room spread throughout parts of the country and also used in different climate zones and elevations, it is critical to recognize the compelled CFM every ton when repairing or make airflow adjustments to one HVAC system.

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Don’t guess when you have actually the devices to test. For details on using a psychrometric chart, inspect out tasiilaq.net’s Manual p for diagrams and instructions on making this straightforward tool work for you.  

Reference charts : https://www.goodmanmfg.com/pdfviewer.aspx?pdfurl=docs/librariesprovider6/default-document-library/ss-gsx14.pdf?view=true