On mild temperature heating days, the heat pump handles all heating needs. When the outdoor temperature reaches the balance point of the home, that is, when the heat loss is equal to the heat-pump heating capacity, the two-stage indoor thermostat activates the furnace (a secondary heat source, in most cases electric heating elements). As soon as the furnace is turned on, a heat relay deenergizes the heat pump. When the second-stage (furnace) need is satisfied and the plenum temperature has cooled to below 90 to 100°F (32 to 38°C), the heat-pump relay turns the heat pump back on and controls the conditioned space, until the second-stage operation is required again. Figure 2-12 shows the heat-pump unit. The optional electric heat unit shown in Fig. 2-13 is added in geographic locations where needed. This particular unit can provide 23,000 to 56,000 Btu/h and up to 112,700 Btu/h with the addition of electric heat.
If the outdoor temperature drops below the setting of the lowtemperature compressor monitor, the control shuts off the heat pump completely and the furnace handles all the heating needs.
During the defrost cycle, the heat pump switches from heating to cooling. To prevent cool air from being circulated in the house when heating is needed, the control automatically turns on the furnace to compensate for the heat-pump defrost cycle (see Fig. 2-14). When supply air temperature climbs above 110 to 120°F (43 to 49°C), the defrost limit control turns off one furnace and keeps indoor air from getting too warm.
If, after a defrost cycle, the air downstream of the coil gets above 115°F (65°C), the closing point of the heat-pump relay, the compressor will stop until the heat exchanger has cooled down to 90 to 100°F (32 to 38°C) as it does during normal cycling operation between furnace and heat pump.
During summer cooling, the heat pump works as a normal split system, using the furnace blower as the primary air mover (see Fig. 2-15).
In a straight heat pump/supplementary electric heater application, at least one outdoor thermostat is required to cycle the heaters as the outdoor temperature drops. In the system shown here, the indoor thermostat controls the supplemental heat source (furnace). The outdoor thermostat is not required.
Since the furnace is serving as the secondary heat source, the system does not require the home rewiring usually associated with supplemental electric strip heating.