It’s a typical misconception that the cooling blower rate on a residential waiting conditioning device should constantly be collection to “high.” In many situations high rate for cooling will certainly not provide you the best feasible performance and customer comfort.

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If the blower rate is collection too high because that the application, the evaporator coil will not have the ability to do an adequate job of dehumidification, and also may an outcome in a customer complaint. If the blower rate is set too low, you threat “freezing” the evaporator coil, which could lead come compressor failure as result of liquid refrigerant floodback. Therefore, the is imperative that the proper blower speed for air conditioning is used.

To identify the finest speed to use for waiting conditioning for a details application, you must measure the complete system external static pressure (or E.S.P.), and also use the blower power chart listed with the heating system (see figure 1 for sample chart) to recognize the best speed to use. For optimum performance, many manufacturers recommend the 400 cfm per ton of waiting conditioning should be used. because that example, a 2-ton mechanism should have actually 800 cfm the air relocating through the evaporator coil.


Figure 1.
Blower chart.
External static press is a pressure that is pushing out in every directions ~ above the duct system. Restrictions in the duct mechanism such as undersized duct, dirty filter or evaporator coil, or closeup of the door or clogged registers will reason the external static push to rise. As exterior static push rises, the furnace blower’s capability to relocate air declines.Most residential heaters are designed to provide their rated airflow approximately 0.5 customs wc full external static pressure.

Taking measurements

A valuable tool because that measuring E.S.P. Is a Magnehelic gauge v a 0-2 customs wc scale. To identify the full E.S.P., you need to measure both the supply and also return revolution pressures.

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


Figure 2.
affix probe to port significant “high.”
The probe have to be inserted automatically off that the supply duct connection, underneath the evaporator coil if possible (Figure 3).

Figure 3.
Insert probe off of the it is provided duct connection.
This will enable you to check out the it is provided static and also the resistance to airflow enforced by the evaporator coil, supply duct, fittings, and registers. A typical supply static worth for a properly designed supply duct system with a clean, dried evaporator coil is 0.3 come 0.35 inch wc.

To measure the return revolution pressure, attach the Magnehelic probe to the port significant “low.” The probe must be inserted between the return wait filter and the furnace. This will permit you to read the return static and also resistance to airflow imposed by the filter, return drop, return ductwork, fittings, and also grilles. If accessibility cannot it is in attained in between the furnace and also the filter, another good place to measure up return static is v the blower deck (Figure 4).


Figure 4.
Measuring with the blower deck.
The complete external revolution pressure deserve to be figured out either by acquisition the supply and return statics individually and including them, or merely by using two probes and noting the analysis on the Magnehelic gauge (Figure 5).

Figure 5.
Determining total external revolution pressure.
Once the total E.S.P. Has actually been determined, look in ~ the blower power chart (Figure 1) because that the furnace or air handler model you room working on. The cfm being delivered will be wherein the E.S.P. Reading intersects with the blower rate being used. Remember the 400 cfm every ton of air conditioning is the target value. If required, progressive or lower cooling the blower speed. After a blower speed change, recheck the external static pressure and also consult the blower power chart to verify cfm.

If Charts are Unavailable

If the manufacturer’s blower charts room not obtainable for the heating system you are adding air air conditioning to (or for belt journey furnaces), use the following procedure to identify an approximate cfm.

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

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

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

placed 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 minutes to properly set up air flow for air conditioning on every installation will certainly ensure the best possible 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 actually 12 years of suffer in the HVAC market doing PC-based maintain of technicians ~ above HVAC devices installations. For more information, contact 262-305-0545 or visit