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506212−01 11/10
TERMINOLOGY
The following terminology is used throughout the
remainder of this section.
1.
Power Defrost
A defrost cycle during which the
compressor operates to melt any accumulated frost
from the outdoor coil. A power defrost is needed in
order to calibrate the demand defrost control. During
power defrost, the following occurs:
S
Reversing valve is energized (as in a cooling
mode)
S
Compressor is energized.
S
Outdoor fan is de−energized
S
Indoor blower is de−energized
2.
Natural Defrost
A defrost cycle where the
compressor does not operate. Because the outdoor
ambient is above freezing, just allowing the unit to sit
idle will melt any frost from the outdoor coil. During
natural defrost, the following occurs:
S
Reversing valve is energized (as in a cooling
mode)
S
Compressor is de−energized
S
Outdoor fan is de−energized
S
Indoor blower is de−energized
3.
Gas Heat Inactive Mode
When the outdoor
temperature is above 65°F (18.3°C), the gas heat is
not allowed to respond to a first−stage room
thermostat heat call
Y1
. The furnace is allowed to
respond to a second−stage thermostat heat call
W1
.
4.
Heat Pump Inactive Mode
When the outdoor
temperature is below 32°F (0°C), the heat pump is not
allowed to respond to a first−stage room thermostat
heat call
Y1
. The demand defrost control redirects this
Y1
heat call to the furnace.
HEAT PUMP INACTIVE MODE
Demand defrost control will enter heat pump inactive mode
when any of the following conditions occurs:
S
Outdoor temperature is less than 32
F (0
C).
S
First Heat Pump Heating Call (after demand defrost
control calibration) Compressor run time in Heat
Pump heating mode was l
ess than
30 minutes before
demand defrost control requested a natural defrost.
S
Second Heat Pump Call Compressor run time in
Heat Pump heating mode was l
ess than
70% of the
first heating call before demand defrost control
requested a natural defrost.
S
Third Heat Pump Call Heat Pump will become
inactive at next call for natural defrost
.
RETURN FROM HEAT PUMP INACTIVE MODE
When demand defrost control determines outdoor
temperature has been above 35
F (1.6
C) for calculated
predetermined time, upon the next heat pump heating
demand from the room thermostat, the demand defrost
control will immediately begin a sacrificial power defrost
cycle as per figure 25.
NOTE Placing a jumper on the demand defrost control’s
TEST pins will not activate the heat pump out of inactive
mode. Power (24VAC) must be cycled to the demand
defrost control to activate the heat pump out of inactive
mode.
CALIBRATION MODE
The demand defrost control is considered uncalibrated
when:
S
24VAC power is applied to demand defrost control
during initial start up, or any other time that power is
reapplied to the demand defrost control.
S
After being in cooling mode
The demand defrost control will perform a sacrificial
power defrost after thirty (30) minutes of accumulated
compressor runtime in the first heat pump heating
demand with the coil temperature below 35ºF (1.6ºC).
This will ensure a clear outdoor coil before an attempts
to calibrate the demand defrost control.
S
When the heat pump comes out of inactive mode.
The demand defrost control will perform a sacrificial
power defrost. This will ensure a clear outdoor coil
before an attempts to calibrate the demand defrost
control
NOTE If he heat pump is running in a gas furnace
heating demand when the heat pump comes out of inactive
mode, the demand defrost control will cycle the gas
furnace OFF and cycle the heat pump ON. On this first call
for heat pump heating the demand defrost control will initial
a sacrificial defrost to ensure a clear outdoor coil before an
attempts to calibrate the demand defrost control.
Calibration of the demand defrost control occurs after a
power defrost cycle to ensure that there is no ice on the
coil. During calibration, the temperature of both the coil and
ambient sensors are measured to establish a clear coil
condition.
When the controller is in an uncalibrated state, the
controller should initiate a sacrificial defrost after 30
minutes of accumulated compressor runtime with coil
temperature below 35°F (1.6°C). The defrost cycle will
terminate if the coil sensor reaches the selected
termination temperature or after a 14−minute defrost. If the
above defrost was concluded due to termination
temperature, or if terminated by time with a coil
temperature of greater than 35°F (1.6°C) for more than a
defined time, a clear coil (non−iced condition) can be
assumed. Otherwise, the demand defrost control remains
in an uncalibrated state.
If the coil temperature is not considered stable, the
demand defrost control will operate in time−temperature
mode for 45 minutes. Upon the completion of the
45−minute operation the demand defrost control will initiate
a sacrificial defrost to calibrate. At that point a clear coil
temperature at the particular outdoor ambient temperature
can be determined.
