Heat Pump Temperature Rise

One of the best ways to check the refrigerant charge of a heat pump during the heating season is to measure the temperature rise across the indoor coil and compare it to data published for that unit. The two main issues with this charging method are getting the required data and measuring the system airflow. Not all manufacturers publish the temperature rise for their equipment. Another potential problem is system airflow. The amount of temperature rise achieved is directly related to the amount of air moving across the coil. If the ACTUAL airflow is different from the manufacturer's stated conditions, the temperature rise will also vary. Increased airflow will reduce the temperature rise and decreased airflow will increase the temperature rise. For the temperature rise method to work, the system must be operating with the correct airflow.

Heat pumps are rated by AHRI for heat output at two outdoor temperatures, 47°F and 17°F. The indoor return air temperature rating point is 70°F. Most heat pumps produce a heat output of close to their "tonnage rating" at the 47°F AHRI rating point and approximately half of their "tonnage rating" at 17°F. Most units operate correctly with an airflow somewhere in the range of 400 CFM per ton of capacity. Of course these are generalizations.

However, a general idea of temperature rise parameters can prove useful. Using these generalized assumptions, let's develop some generalized temperature rise performance parameters. The basic formula to use is BTU = 1.08 x CFM x TEMP RISE. This can be rewritten as TEMP RISE = BTU / (1.08 x CFM). At 47°F the capacity per ton will be 12,000 BTU. Plugging this into the formula we get TEMP RISE = 12,000 / (1.08 x 400). This yields a projected temperature rise of 28°F at a 47°F ambient. On the other end of things, the assumed capacity per ton at 17°F ambient temperature is 6000 BTU. Plugging this in we get TEMP RISE = 6000 / (1.08 x 400). This yields a projected temperature rise of 14°F. You can readily see that the result of a 50% reduction in capacity yields a 50% reduction in temperature rise. Now let's test our mathematical extrapolations against some REAL data taken from a manufacturer’s performance specifications.

Model	ΔT  @ 47°F	ΔT  @ 17°F	BTUh  @ 47°F	BTUh  @ 17°F	CFM
ERHQ18	26°F	13°F	18,500	11,000	700
ERHQ24	25°F	12°F	24,400	12,500	800
ERHQ30	27°F	16°F	32,200	18,800	1055
ERHQ36	26°F	13°F	36,000	19,500	1310

The manufacturer states that a properly operating unit should be + or – 3°F of these typical values. You will note that our extrapolated values are all within the 3°F margin of error.

But what if the temperature is neither 47°F nor 17°F, what temperature rise do you look for then? Another assumption is in order here; we will assume that the capacity reduction due to ambient temperature drop is even. Since the temperature rise changed from 28°F to 14°F over a 30°F ambient change, this would represent approximately a 1°F change in temperature rise for every 2°F change in ambient temperature. In the above table, we see a temperature rise change of 13°F in 3 out of the 4 units listed. This gives similar results. In our assumed system, 28°F rise at 47°F ambient and a 14°F rise at 17°F ambient, a temperature rise of 23°F would be expected at 37°F ambient based on the relationship of 1°F rise per 2°F ambient temperature. Clearly, manufacturer's data for a specific unit is more accurate than extrapolated benchmarks; however, extrapolated temperature rise calculations are considerably more accurate than blocking the outdoor coil and pretending it's summer. Remember: the best method for charging any unit is the method recommended by the people who made it!