DEFROST SEQUENCE OF OPERATION
These heat pumps have a unique “ambient modified”
time-temperature defrost control that automatically adjusts to
changes in the outdoor temperature. The defrost control will
shorten the defrost initiation time periods above 35
°
F and will
extend the defrost initiation time periods below 35
°
F. The
control is factory set to defrost at 110 minutes (T3), but it can
be field adjusted to defrost at 80 minutes (T2) or 50 minutes
(T1) in areas with high humidity.
The curve in Figure 7 shows how defrost initiation times are
automatically compensated for changes in outdoor temperature.
EXAMPLE: If the timer is factory set on pin T-3 (110 minutes at
35
°
F outdoor) and the outdoor temperature climbs to 45
°
, the
time initiation cycle will decrease to every 100 minutes.
If the outdoor temperature drops to 10
°
F where ice is less likely
to form, the 110 minute interval will extend to every 150
minutes.
Two requirements must be met before a defrost cycle can be
initiated.
1. The defrost time cycle must be complete.
2. The liquid line temperature must be less than 28
°
F.
Defrost will terminate when the liquid line sensor reaches 55
°
F
or after 10 minutes. If both or just one circuit defrosts, electric
heat is energized.
The defrost time cycle will restart 10 minutes after the start of
the defrost cycle even though the liquid sensor terminated
defrost after 3 minutes.
During troubleshooting, the defrost time can be reduced to 20
seconds by shorting out the SW1 test pins on the module. The
pins are 1/2" long, 3/16" apart and are mounted on a white
base. See Figure 10.
LOCKOUT CONTROL
Any one of four conditions will put the system into a lock-out
condition during the heating or cooling mode:
1. The discharge line temperature reaches 255
°
F (215
°
F
reset) or,
2. The discharge pressure reaches 398 PSIG (310 PSIG
reset) or,
3. The suction line freezestat equals 26
°
F (38
°
F reset) or,
4. The low-pressure cut-out equals 7 PSIG (22 PSIG reset).
A lock-out will energize the emergency heat light on the
thermostat and the red LED light on the unit relay board.
Turning the thermostat switch to “Off” then back to “On”, will
reset the system.
NOTICE TO OWNER:
If a lockout occurs, check for the following problems before
calling a serviceman:
1. Dirty filters.
2. Snow accumulation.
3. Leaf or debris blockage.
After eliminating the problem, attempt to restart the system as
follows:
•
turn the system switch on the thermostat to its “OFF” position
for 10 seconds.
•
turn it back to its original position.
If the unit doesn’t start, call a serviceman.
NOTE: Models with an anti-recycle accessory will have a
5-minute delay before starting.
FIG. 7 - DEFROST INITIATION TIMES
511.06-N2W
12
Unitary Products Group
