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Reverse Cycle Defrosting Heat Pump Mode
Reverse Cycle Defrost shall be controlled as a
function of compressor run time and outdoor coil
temperature.
a)
Defrost Enable
The reverse cycle defrost shall begin when
either:
1. The outdoor coil temperature, as measured
by the S4 thermistor during compressor
operation is less than or equal to 14°F for
at least 3 consecutive minutes.
-OR-
2. The outdoor temperature as measured by
the S3 Thermistor, is less than or equal
to 42°F OR the outdoor coil temperature
is less than or equal to 32°F for at least 3
hours of accumulated compressor run time.
NOTE
: As described at step 1, if the outdoor
coil temperature, as measured by the S4
thermistor, rises above 14°F before 3 minutes
has elapsed; the accumulated time register is
reset to zero. A defrost sequence will not be
initiated, as a result.
NOTE
: As described in step 2, if the outdoor
temperature, as measured by the S3
thermistor, rises above 42°F AND the outdoor
coil temperature as measured by the S4
thermistor, rises above 42°F before 3 hours of
compressor run time has been accumulated;
the accumulated time register is reset to zero.
A defrost sequence will not be initiated, as a
result.
b)
Defrost Operating Sequence
When either of these conditions are detected,
the Zoneline will initiate a reverse cycle defrost.
The following steps comprise the reverse cycle
defrost:
1. Immediately stop heat pump operation.
2. Run stage 3 heating until room
temperature is satisfied.
3. Wait for compressor off time (3 minutes) to
be satisfied.
4. Run reverse cycle defrost (cooling mode
without fans) until either:
• the outdoor coil temperature rises to 68°F
• or the total elapsed time of compressor run
in reverse cycle defrost reaches 9 minutes
5. Run a stage 3 heating cycle to satisfy room
temperature.
6. If reverse cycle defrost is exited due to
• the outdoor coil rising above 68°F, resume
normal heat pump operation
• the total elapsed time exceeding 9 minutes
and total number of defrost cycles is less
than 5, go back to step 4 and run another
defrost cycle. If the total number of defrost
cycles equals 5, see
Abnormal Defrost
Fault
below
c)
Abnormal Defrost Fault
If five consecutive defrost cycles have been
completed and the outdoor coil has not
reached 68°F, an Abnormal Defrost Fault will
be initiated. Heat Pump operation is disabled
while this fault is active. The Zoneline will wait
for the following conditions to be met.
1. Elapsed time > 4.5 days
and
2. Outdoor Coil Thermistor > 42°F
and
3. Outdoor Ambient > 42°F
At this time, up to five more defrosts will be
attempted. If the outdoor coil thermistor stays
below 68°F, it will reenter abnormal defrost
fault. If it increases above 68°F, it will resume
normal heat pump operation.
Reverse cycle defrost has the highest priority.
Once the defrost starts the defrost operation
continues regardless of any temperature
conditions and any kind of manual operation,
except for changing the operation switch.
Reverse cycle defrost is used in Class 2 mode.
During reverse cycle defrost, all Class 2 signal
modes are overridden except when "Cooling"
is called from the thermostat.
