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Defrost
When defrost occurs in a Hybrid Heat system, the furnace will operate during the
defrost cycle. At the end of the defrost cycle, the furnace and heat pump will be
de−energized while the fan is energized for 2 minutes allowing the heat
exchanger to cool down. If Option 34, Hybrid Heat Furnace Latch is set to Off,
at the end of the 2 minute time, the heat pump will be re−energized if a call for
heat still exists. If Option 34 is set to On (default), the furnace will remain on
until the end of the heating cycle.
Defrost detection is not available if the installer has configured the O/W2/B
output to function as a B output. During heat pump heating, a defrost signal shall
be considered valid if the compressor output is energized and the defrost signal
has been active for less than 15 consecutive minutes. Any defrost signal present
for longer than 15 minutes shall be considered invalid.
Heat pump/fan coil and Hybrid Heat systems shall use this input to:
S Detect that defrost is in progress and energize the auxiliary heat to
provide homeowner comfort during the defrost cycle
S Allow a defrost cycle to run to completion regardless of the system
demand
Three−Minute Minimum on Time
In normal operation, when a stage turns on, it will not turn off for a minimum of
3 minutes. In Hybrid Heat systems, the minimum on time for the furnace is 5
minutes. If the setpoint is changed, this timer is canceled, allowing the equipment
to turn off immediately when the demand is removed. The 3 minute minimum
on timer applies to all stages of heating and cooling, except Hybrid Heat. A stage
will run for a minimum of 3 minutes before the thermostat will be allowed to
stage to a lower stage.