29
Defrost —
The defrost control mode is a time/temperature sequence.
There are two time components: The continuous run
period and the test/defrost cycle period. The temperature
component is provided by the defrost thermostats (DFT1
and DFT2) mounted on the outdoor coil.
The continuous run period is a fixed time period between the
end of the last defrost cycle (or start of the current Heating
cycle) during which no defrost will be permitted. This period
can be set at 30, 60, 90 or 120 minutes by changing the
positions of DIP switches SW1 and SW2 (see Fig. 25 and
Table 9). The default run period is 60 minutes.
C10790
Fig. 25 -- DIP Switch Settings — Defrost Board
At the end of the continuous run period, the defrost control
will test for a need to defrost. DFT2 (located on the bottom
circuit of the outdoor coil) controls the start and termination
of the defrost cycle. If DFT2 is still open, the defrost test/run
window is closed and the control repeats the continuous run
period. If DFT2 is closed, the defrost cycle is initiated in
Circuit 2. The defrost period will end when DFT2 opens
(indicating the outdoor coil has been cleared of frost and ice)
or a 10 minute elapsed period expires, whichever comes
first.
Circuit 1’s defrost thermostat DFT1 (located on the upper
circuit of the outdoor coil) cannot initiate a unit defrost
cycle; only DFT2 may do this. But once Circuit 2 is in
defrost, the DFB will monitor the status of DFT1. If DFT1
closes during a Circuit 2 defrost cycle, Circuit 1 will also
enter a defrost cycle. Circuit 1’s defrost cycle will end
when DFT1 opens (indicating the upper portion of the
outdoor coil is cleared of frost and ice) or the Circuit 2
defrost cycle is terminated.
At the end of the unit defrost cycle, the unit will be
returned to Heating cycle for a full continuous run period.
If the space heating load is satisfied and compressor
operation is terminated, the defrost control will remember
where the run period was interrupted. On restart in Heating,
the defrost control will resume unit operation at the point in
the run period where it was last operating.
Defrost Thermostats —
These are temperature switches that monitor the surface
temperature of the outdoor coil circuits. These switches
are mounted on the liquid tube exiting the outdoor coil
heating circuits. These switches close on temperature drop
at 30
°
F (-1
°
C) and reset open on temperature rise at 80
°
F
(27
°
C).
Indoor Fan Off Delay —
The DFB can provide a 30 sec delay on Indoor Fan Off if
the thermostat’s fan selector switch is set on AUTO
control. DIP Switch SW3 on the DFB selects use of the
fan off time delay feature. Setting SW3 in the OPEN
position turns the Fan Off Delay feature on; setting SW3
in the CLOSED position disables this feature. The delay
period begins when Y1 demand or W1 demand by the
space thermostat is removed.
Defrost Speedup Functions —
The DFB permits the servicer to speed-up the defrost
cycle. There are two speed-up sequences: relative
speed-up and an immediate forced defrost. Speed-up
sequences are initiated by shorting jumper wires JMP17
and JMP18 together (see Fig. 23); use a straight-edge
screwdriver.
Shorting the jumpers for a period of 1 to 3 secs reduces
the defrost timer periods by a factor of 0.1 sec/minute.
(For example, the 90 min run period is reduced to 9 secs.)
The DFB will step the unit through a Heating cycle and a
Defrost cycle using these reduced time periods. This mode
ends after the Defrost cycle.
Shorting the jumpers for a period of 5 to 20 secs bypasses
the remaining continuous run period and places the unit in
a Forced Defrost mode. If the controlling DFT is closed
when this mode is initiated, the unit will complete a
normal defrost period that will terminate when the
controlling DFT opens or the 10 minute defrost cycle limit
is reached. If the controlling DFT is open when this mode
is initiated, the Defrost cycle will run for 30 secs. Both
modes end at the end of the Defrost cycle.
Table 9 – Dip Switch Position
Switch No.
1
2
1
2
1
2
1
2
3
1
J
1
J
1
1
J
J
1
On
0
J
0
J
0
J
J
0
0
J
Off
30 minutes
60 minutes
90 minutes
120 minutes
Fan Delay
38A
U
Q
Summary of Contents for 38AUQ 16 Series
Page 4: ...4 C10675 Fig 2 38AUQ 25 Unit Dimensions 38AUQ...
Page 19: ...19 C10784 Fig 16 38AUQ 16 Charging Chart 38AUQ...
Page 20: ...20 C10785 Fig 17 38AUQ 25 Charging Chart 38AUQ...
Page 21: ...21 C10786 Fig 18 38AUQ 16 Power Schematic 208 230 3 60 shown 38AUQ...
Page 22: ...22 C10787 Fig 19 38AUQ 25 Power Schematic 208 230 3 60 shown 38AUQ...
Page 23: ...23 C10788 Fig 20 38AUQ 16 25 Control Schematic 38AUQ...
Page 41: ...41 C10363 Fig 35 Wind Baffles Fabrication 38AUQ...
Page 42: ...42 C10366 Fig 36 15 Ton Wind Baffle Brackets Fabrication 38AUQ...
Page 44: ...44 RIGHT BACK LEFT FRONT C10368 Fig 38 Wind Baffle Installation 20 Ton Units 38AUQ...
Page 45: ...45 38AUQ...
