Minimum source temperature setting for the ASHP component

The minimum source temperature is the temperature below which the ASHP will switch off completely and allow the backup heat source to take over completely, rather than just supplement the ASHP output. When the backup heat source is electric resistance heating and both the ASHP heat and backup heat are delivered by a common fan, etc. (i.e., when there is no significant difference between these with respect to heating system or parasitic loads), it makes sense to switch the ASHP off completely only when low outdoor temperatures drive the COP to a value of less than 1.0. This is because both run on electricity and the electric resistance heating has an effective COP of 1.0. In such cases, the thermal and economic balance points for this hand-off should be essentially the same.

Because the PRM Baseline systems with ASHPs are required to use electric resistance as the backup heating source, we must assume this has an effective COP of 1.0. As the default ASHP curve has a COP of better than 1.0 down to 0°F, we have used 2°F as the default minimum source temperature as of VE 6.3. This is also consistent with industry best practice, which might set this between 0 and 10°F for a system with electric resistance backup, depending upon the equipment, climate, and so forth. Some recently offer ASHP technologies, however, have been developed with a greater emphasis on heating performance and are capable of operating efficiently well below 0°F.

When the backup heat source uses a different energy source, has relatively high efficiency, or has lesser associated system/parasitic loads, the economic balance point may be higher with respect to the heat pump COP. The economic balance point is the outdoor temperature below which it is cheaper to heat with the supplementary heat source rather than the heat pump. In such cases, it often makes sense to restrict ASHP operation, operating only the backup heat source below a specified outdoor temperature. An outdoor temperature sensor is used to shut off the heat pump when the temperature falls below the preset limit. Only the supplementary heat source operates below this temperature. This is the Minimum source temperature setting for the ASHP component in ApacheHVAC.

Note: While real-world ASHP applications do sometimes, as noted above, restrict operation of the ASHP below a specified temperature, it is very unusual to restrict the backup heat source operation in building for which the ASHRAE 90.1 PRM is applicable. While an individual homeowner may be given the option to prevent the use of the backup heat above a set outdoor temperature, the occupants of heated spaces in larger buildings generally expect the heating setpoint to be met by whatever means is available to do so, regardless of the particular outdoor temperature at the time.