Carrier 38AUQ User Manual

Page 23

Fig. 21 — Defrost Control Board (DFB) Location

Fig. 22 — Defrost Control Board (DFB) Arrangement

Reversing valve control

The DFB has two outputs for unit reversing valve control. Op-

eration of the reversing valves is based on internal logic; this

application does not use an “O” or “B” signal to determine re-

versing valve position. Reversing valves are energized during

the Cooling stages and de-energized during Heating cycles.

Once energized at the start of a Cooling stage, the reversing

valve will remain energized until the next Heating cycle de-

mand is received. Once de-energized at the start of a Heating

cycle, the reversing valves will remain de-energized until the

next Cooling stage is initiated.

Compressor control

The DFB receives inputs indicating Stage 1 Cooling and Stage

1 Heating from the space thermostat or unit control system

(PremierLink); it generates commands to start compressors

with or without reversing valve operation to produce Stage 1

Cooling (one compressor), or Stage 1 Heating (both compres-

sors run).

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 thermostat (DFT1) 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 cy-

cle) 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. 23 and Table 13). The

default run period is 60 minutes.

Fig. 23 — DIP Switch Settings — Defrost Board

At the end of the continuous run period, the defrost control will

test for a need to defrost. On unit sizes 04-07 (single compres-

sor designs), DFT1 controls the start and termination of the de-

frost cycle. If DFT1 is still open, the defrost test/run window is

closed and the control repeats the continuous run period. If

DFT1 is closed, the defrost cycle is initiated. The defrost period

will end when DFT1 opens (indicating the outdoor coil has

been cleared of frost and ice) or a 10 minute elapsed period ex-

pires, whichever comes first.
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 con-

trol will resume unit operation at the point in the run period

where it was last operating.

Table 13 —Dip Switch Position

DIP
Switches

Speed-Up
Jumpers

Switch No.

■

Off

30 minutes

60 minutes

90 minutes

120 minutes

Fan Delay