035-15713-003 REV. A (0500)
14
Unitary Products Group
DEMAND DEFROST:
The defrost control is a demand defrost control. See Figure
13. Due to the arrangement of the refrigerant circuit within the
outdoor coil of these units, frost may accumulate unevenly in
different sections of the coil. However, a normal defrost may
occur even though the coil is not completely covered with
frost. The defrost cycle will start only under the following con-
ditions:
1.
Contactor is energized.
2.
A minimum of 39 minutes have elapsed since the last
defrost.
3.
The liquid temperature measured by defrost coil sensor
must be less than 40° F.
4.
The liquid temperature measured by the defrost coil sen-
sor must be below the defrost initiation temperature for
4.5 minutes continuously.
The defrost mode is identical to the cooling mode except that
the outdoor fan motor stops and the first stage of heat is
turned on through “66" to continue warming the conditioned
space.
The defrost cycle will terminate when the defrost coil sensor
reaches 75° F or after 14 minutes defrost time.
NOTE:
Under certain low ambient conditions normal initia-
tion of a normal defrost cycle may not occur. There-
fore as a precautionary measure a forced “defrost”
is initiated every six hours to ensure compressor
longevity by returning oil back to the sump of the
compressor.
TEST PINS ON DEMAND DEFROST:
The test pins labeled "speed-up" and "defrost" are provided
to aid in field servicing of the heat pump. See Figure 14. The
pins are designed to be temporarily shorted together either
by a screwdriver or with a 1/4" female spade connector.
When the "speed-up" pins are shorted together. ALL timings
are speeded up by a factor of 64. This reduces the anti-recy-
cle compressor timer from 5 minutes to about 5 seconds and
the defrost terminate time from 14 minutes to 13 seconds:
additionally, the 39 minute minimum run time would be
shorted to about 40 seconds or less if the system has oper-
ated for some period of time since last defrost.
When the "defrost" pins are shorted together, the control will
allow the unit to go into a defrost cycle if the compressor is
operating. When the short is removed, the heat pump will
remain in defrost until the coil temperature reaches 75° F or
14 minutes expires.
If the liquid (coil) temperature happens to be above 75° F, the
short must be maintained to keep the unit in a defrost cycle. It
should be used since the unit WILL REMAIN in defrost until
the short is removed regardless of coil temperature. This
could cause a lockout due to high discharge pressure!
NOTE:
Anytime the “defrost” shorting pins are jumpered, it
automatically resets all timing cycles. After a power
failure, all timing cycles would also reset except one
that is the 5 minute delay for compressor restart. It
can be reduced, however by jumpering the “speed-
up” pins on the control board.
FIGURE 17 :
FLOW DIAGRAM - 018 THRU 048 (10 & 12 SEER)
