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
shortens the defrost initiation time periods above 2°C (35°F)
and extends the defrost initiation time periods below 2°C (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 10 shows how defrost initiation times are
automatically compensated for changes in outdoor
temperature.
EXAMPLE: If the timer is factory set on pin T3 (110 minutes at
2°C (35°F) outdoor) and the outdoor temperature climbs to 7°C
(45°F), the time initiation cycle decreases to 100 minutes.
If the outdoor temperature drops to -12°C (10°F) where ice is
less likely to form, the 110 minute interval increses to 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 -2°C (28°F).
Defrost terminates when the liquid line sensor reaches 13°C
(55°F) or after 10 minutes. If one or both circuits defrosts,
electric heat is energized.
The defrost time cycle restarts 10 minutes after the start of the
defrost cycle even though the liquid sensor terminated defrost
after three minutes.
During troubleshooting, the defrost time can be reduced to 20
seconds by shorting out the SW1 test pegs on the module. The
pegs are 13mm (
1
2
in.) long, 5mm (
3
16
in.) apart and are
mounted on a white base. See Figure 11.
LOCKOUT CONTROL
Any one of four conditions put the system into a lock-out
condition during the heating or cooling mode:
1. The discharge line temperature reaches 124°C (255°F)
(102°C reset [215°F]) or,
2. The discharge pressure reaches 2,786 kPa (398 psig)
(2,170 kPa reset [310 psig]) or,
3. The suction line freezestat equals -3°C (26°F) (3°C reset
[38°F]), or
4. The low-pressure cut-out equals 49 kPa (7 psig) (154 kPa
[22 psig]).
A lock-out energizes 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 service man.
NOTE: Models with an anti-cycle accessory have a five
minute delay before restarting.
035-18895-000-A-0902
14
Unitary Products Group
FIG. 10 - DEFROST INITIATION TIMES
-23
(-10)
-17
(0)
-12
(10)
-7
(20)
-1
(30)
5
(40)
10
(50)
16
(60)
°
C (
°
F)
