On mild temperature heating days, the heat pump handles all heating needs. When the outdoor temperature reaches the “balance point” of the home (heat loss equals heat pump heating capacity), the two-stage indoor thermostat activates the furnace (secondary heat source). When the furnace fires, a heat relay deenergizes the heat pump.
When the second stage (furnace) need is satisfied and plenum temperature has cooled to 90 to 100°F (32.2 to 37.8°C), the heat pump delay turns the heat pump back on. It controls the conditioned space until the second stage (full heat) operation is required again.
When outdoor temperature drops below the setting of the low-temperature compressor monitor (field installed option) the control shuts out the heat pump. The furnace handles all of the heating need. The low temperature compressor monitor is standard on models dated 1974 and after.
During the cooling season the heat pump operates in its normal cooling mode. It uses the furnace blower as the primary air mover. See Fig. 17-21.