Airflow (CFM) Determination of an Air-Source Heat Pump (ASHP)

If you are a HVAC/R technician, you should always check any air-source heat pump (ASHP) for proper airflow across the coils when troubleshooting or during preventive maintenance.

Correct airflow is of crucial importance to the operation of any ASHP. Part of the heat transfer rate is determined by the airflow across the indoor and outdoor coils. If the airflow is incorrect, then the heat transfer rate is incorrect and can drastically affect the equipment's performance.

Accurate airflow measurement is essential in troubleshooting any heat pump system. In fact, no refrigeration test is valid if the air flow volume is not correct!

ASHP manufacturers follow the tenants of the Air Conditioning, Heating, and Refrigeration Institute (AHRI) which has a "Test Stand Value" requiring the measured air volume rate, when divided by the measured indoor air-side total capacity, must not exceed 37.5 SCFM per 1,000 Btu/h [this is a maximum of 450 cubic feet per minute (CFM) of airflow across an indoor coil per 12,000 Btu/h of capacity]. Most manufacturers use an acceptable range of 350 to 450 CFM per 12,000 Btu/h of capacity, and over 750 CFM per 12,000 Btu/h of capacity across outdoor coils (most outdoor fans move approximately 1,000 CFM, up to 1,500 CFM, per 12,000 Btu/h of capacity). In the HVAC/R industry, 12,000 Btu/h of capacity is referred to as a "Ton" of refrigeration. Typically, most manufacturers focus on around 400 CFM per "Ton" when rating their equipment.

Before performing any airflow determination, always make sure that all registers and grilles are open filters and coils are clean, and that blowers and fans are running properly delivering airflow across the indoor coil and the outdoor coil.

There are various methods that help determine the airflow amount across an indoor coil. The indoor coil is typically checked as the airflow must cross this coil to allow the refrigerant to either absorb (ASHP cooling) or reject (ASHP heating) the heat to the appropriate "sink." In summer, the "sink" is the outdoors, and during winter, the "sink" is indoors.

One of the most widely utilized methods when checking ASHP airflow has been the "temperature rise" method across the auxiliary or emergency (back up) heater(s). This method can be performed regardless of outdoor ambient temperature.

When performing this test, you should remember that typically, most air source heat pumps operate with the same airflow regardless of mode of operation. In cooling or heating, or during emergency heat or defrost, the heat pump simply delivers the "same" airflow per ton across the indoor coil. The only change is possible during the cooling mode, as water condensing on the indoor coil increases resistance slightly, lowering airflow amounts somewhat. Of course, you can also change blower speeds and CFM amounts for either cooling or heating on some ASHPs.

To check airflow (CFM) of an ASHP, you have to perform several measurements and use some math. The common formula for calculating CFM is:

Emergency Heat Output(Btu/h)

CFM= -----------------------------

Temperature Rise x 1.08

You must find the CFM per Ton traveling through the indoor coil during cooling or heating modes. To accomplish this, you should place the system in the emergency heat mode and place thermometers in the supply air and return air paths as close to the air handler as possible without being affected by the radiant effect of the heaters.

In the previous formula, CFM equals the emergency heat output in Btu/h. Since you will be checking heaters, you will be calculating electrical data. When finding Btu/h output of the heater, you will simply measure the voltage and the amperage at the disconnect for the heater(s) and record the values. This will require use of a voltmeter and an ammeter.

Supply voltage multiplied by amperage equals wattage. Wattage multiplied by 3.413 (Btus per Watt) equals Btu/h and you will then have the value for the CFM formula numerator.

In the CFM formula denominator, temperature rise comes from the difference in the return air and supply air temperature after the heater(s) has/have stabilized and a difference has occurred. This difference (sometimes called TR or ΔT) is then multiplied by a constant of 1.08 to find the adjusted temperature difference. Always pay close attention to temperature change when finding the difference in temperature.

This adjusted temperature difference is the denominator for the CFM formula.

When the formula is completed with the necessary inputs, the answer is the total delivered airflow (CFM) traveling across the heater in emergency heat mode. You then simply divide this value by the tonnage of the installed outdoor coil to find the CFM per ton. The cooling airflow (CFM) will be near to this value or slightly lower due to increased pressure drop from water condensing on the indoor coil during cooling. The ASHP compressor heating airflow (CFM) per ton will be a similar value to that found from emergency heat. The CFM per ton should be in the AHRI range to be acceptable.

If inaccurate air flow (CFM volume) across the indoor coil (evaporator in cooling, condenser in heating) is determined, this situation must be corrected prior to further analysis of the refrigeration cycle.

Determining ASHP indoor coil airflow (CFM) during emergency heat example:

Two (2) Ton Unit

240 Volts at disconnect

20 Amps at disconnect

Return air = 70ºF

Supply air = 88ºF
CFM = (240 x 20 x 3.413) ÷ (1.08 x 18ºF) = 843
843 CFM ÷ 2 Tons = 421.5 CFM per Ton

(This amount is acceptable per AHRI)

You should also check the outdoor coil for minimal clearance per manufacturer around the unit from shrubs, trees, and decks to allow for proper heat rejection in the summer and absorption in the winter. You can also determine the airflow across the outdoor coil by using the product specifications from the manufacturer as shown below. Most ASHP outdoor coils will typically deliver close to this amount if the coil is installed correctly and unobstructed.

Phillip A. Rains