It’s a typical misconception that the cooling blower speed on a residential wait conditioning mechanism should always be collection to “high.” In many situations high speed for cooling will not provide you the best feasible performance and customer comfort.

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If the blower speed is collection too high for the application, the evaporator coil will certainly not be able to do one adequate job of dehumidification, and may an outcome in a client complaint. If the blower speed is set too low, you risk “freezing” the evaporator coil, which can lead to compressor failure due to liquid refrigerant floodback. Therefore, it is imperative that the proper blower rate for air conditioning is used.

To determine the ideal speed to use for waiting conditioning because that a details application, you should measure the total system exterior static push (or E.S.P.), and use the blower power chart listed with the heating system (see figure 1 because that sample chart) to determine the ideal speed come use. For optimum performance, most manufacturers recommend that 400 cfm per ton of air conditioning need to be used. for example, a 2-ton system should have actually 800 cfm the air moving through the evaporator coil.


Figure 1.
Blower chart.
External static pressure is a pressure that is pushing out in every directions on the duct system. Limitations in the duct system such together undersized duct, dirty filter or evaporator coil, or closeup of the door or blocked registers will cause the exterior static press to rise. As external static push rises, the heating system blower’s capability to move air declines.Most residential furnaces are designed to deliver their rated airflow as much as 0.5 inch wc complete external revolution pressure.

Taking measurements

A valuable tool because that measuring E.S.P. Is a Magnehelic gauge through a 0-2 inch wc scale. To determine the complete E.S.P., you must measure both the supply and return static pressures.

To measure the supply static pressure, attach the Magnehelic probe come the port significant “high” (Figure 2).


Figure 2.
affix probe to port marked “high.”
The probe should be inserted automatically off of the supply duct connection, under the evaporator coil if feasible (Figure 3).

Figure 3.
Insert probe turn off of the supply duct connection.
This will allow you to read the supply static and also the resistance to airflow imposed by the evaporator coil, supply duct, fittings, and registers. A usual supply static worth for a effectively designed it is provided duct system with a clean, dried evaporator coil is 0.3 to 0.35 customs wc.

To measure the return revolution pressure, affix the Magnehelic probe come the port marked “low.” The probe must be inserted in between the return air filter and also the furnace. This will permit you to check out the return static and also resistance to airflow implemented by the filter, return drop, return ductwork, fittings, and grilles. If access cannot be attained between the furnace and the filter, another an excellent place to measure return static is through the blower deck (Figure 4).


Figure 4.
Measuring v the blower deck.
The complete external revolution pressure deserve to be established either by taking the supply and also return statics individually and including them, or simply by using 2 probes and noting the reading on the Magnehelic gauge (Figure 5).

Figure 5.
Determining total external static pressure.
Once the total E.S.P. Has been determined, look at the blower performance chart (Figure 1) because that the heater or wait handler version you space working on. The cfm being ceded will be wherein the E.S.P. Analysis intersects through the blower speed being used. Remember that 400 cfm per ton that air conditioning is the target value. If required, advanced or lower cooling the blower speed. ~ a blower speed change, recheck the external static pressure and consult the blower power chart to verify cfm.

If Charts room Unavailable

If the manufacturer’s blower charts room not accessible for the heater you are including air air conditioning to (or for belt drive furnaces), use the complying with procedure to identify an approximate cfm.

Make sure the furnace is firing at 100 percent that its nameplate input.

set the blower rate to the rate tap you intended to usage for cooling.

~ 15 minutes of heater operation, measure the temperature rise across the furnace.

put the temperature rise into this formula: Btu Output/1.085 x Temperature Rise.

400 cfm/ton is the desired target value.

Taking a couple of extra minute to properly set up airflow for air air conditioning on every installation will ensure the best feasible customer comfort and also equipment longevity, when eliminating unnecessary callbacks.

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Nelmark is owner/multimedia developer of Axxess Media Interactive, Jackson, WI. He has 12 years of experience in the HVAC sector doing PC-based maintain of technicians top top HVAC equipment installations. For more information, contact 262-305-0545 or visit