Most residential heat pumps are of the air-source type whereby heat is transferred from the outdoor air to a refrigerant in the outdoor section and then transferred from the refrigerant to the indoor air in the air handler (fan/coil). When the outdoor air temperature decreases the COP and heating capacity of the heat pump decrease while the heat loss from the heated structure increases. The outdoor temperature at which the capacity of the heat pump matches the structure heat loss is known as the 'balance point'. When the outdoor temperature drops below the balance point a supplemental electric resistance heater operates in an ON/OFF manner with a duty cycle that maintains the desired indoor temperature. (The heat pump operates continuously during this time.) When the outdoor air rises above the balance point the electric heater is shut off and the heat pump operates in an ON/OFF manner at its required duty cycle to maintain the desired indoor temperature.

The balance point can be used to maximize the trade-off between comfort and operating cost especially when setback is used at night and when the residents are away. An increase in the balance point over time can be used as a diagnostic to detect chronic heat pump problems, e.g., slow refrigerant leakage, and structure problems (like increased infiltration losses). The magnitude of these problems can be determined by comparing duty cycles from subsequent tests.

When the outdoor temperature drops below 45°F frost may form on the surface of the outdoor coil thereby acting as an insulator and also impeding air flow across the coil. To maintain energy efficiency and heating capacity heat pumps have an automatic defrost mode. This mode is identical to the cooling mode of the unit except that the outdoor fan motor stops and supplemental heat is turned on to continue warming the conditioned space. This heat must be included in determining the balance point.

The design outdoor temperature for estimating heat losses of structures in the State College , PA area is 5°F. The effects of supplemental heat at this temperature can be determined by comparing a supplemental COP with a conventional one.

As implemented from the above the heat loss of the structure (residence) is determined in finding the balance point. The heat loss is assumed to be a linear function of the outside temperature being zero where the outside temperature is equal to the inside design temperature (usually 70°F). Also, as implemented above the outside design temperature depends on location.

A test was conducted on Feb 08, 2003 to determine the parameters discussed above to serve as 'baselines' for operating and monitoring a new state-of-the-art heat pump and a structure that recently had thick carpeting installed and a patio enclosed thereby reducing heat loss.

The following input data was used for the test of a system consisting of a Carrier Heat Pump model no. 38YRA018 and the residence that it heats and cools located in south-west State College, PA:

1. Carrier heat capacity and power data (Data Sheet 1).
2. Inside and outside temperature data (Data Sheet 2).
3. Time and time interval data (Data Sheet 3).
4. Weather data (Data Sheet 4).

The carrier data includes the ARI certified points at 47°F and 17°F with 10°F intervals. It should be noted that the units (MBtu h) are standard in the HVAC industry and mean (100 Btu/hr).

The temperature data was taken with a calibrated wireless thermometer consisting of a main unit and two remote sensors. The main unit was located in the northern part of the residence, remote sensor #1 was located in the air stream entering the outdoor section of the heat pump and remote sensor #2 was located near the thermostat in the southern part of the residence. A temperature difference existed between the two inside locations because of the sun on the south side. (An average of the two was used in the calculations). Temperatures were taken on-the 8's to coincide with the weather to coincide with the weather data on TV channel 32.

The 'Time Data' of events occurring during the test was recorded with an audio tape recorder placed under the outdoor section of the heat pump. The data was transcribed using the stop watch function of an HP-41CX calculator to get accurate event timer. The recorder and stop watch were started simultaneously with the start of the compressor outdoor fan and indoor blower.

The TV (Ch 32) weather data consisted of the outside temperature (the constant value of 21°F checks with the test average of 21.33°F), dew point temperature (constant at 5°F) and wind direction and speed. All of these were taken to serve as baseline for the heat loss calculations. The dew point temperature was also taken because it is expected that the duration of defrost cycle is principally a function of humidity ratio (The higher this ratio, the thicker the frost and the longer it takes to defrost).

1. Plot the heating capacity, qs vs to from the carrier data or derive a linear equation for each of the 4 segments that can connect the 7 data points.
2. Derive an expression for the total heat, qt transferred to the indoor air (MBtu) during each of the 3 heating cycles.
3. Calculate qt for each of the cycles.
4. Calculate the total heat transferred during the whole test, qtt (MBtu/test).
5. Assuming steady state conditions, find the heat loss, qt, from the structure (MBtu h).
6. Plot ql vs to on the same plot as qs or derive a linear equation for it. (Note: ql=0 at to=ti=70°F).
7. Find the balance point (ql=qs)
8. Determine the duty cycle of the heat pump during the test.
9. Predict the heat loss, q5, at 5°F.
10. Determine the supplemental heat, qs5, required at 5°F and the duty cycle of the heater.
11. Calculate COP at 5°F.
12. Calculate a supplemental SCOP5 at 5°F that accounts for the supplemental heat.
13. Compare SCOP with COP using the ratio (COP/SCOP) to determine the effect of supplemental heat on performance.
14. Estimate the kilowatt-hours used each hour to maintain 70°F.
15. Humidity ratio and average Defrost time.