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506318-01

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Issue 0946

Compressor Delay

The defrost board has a field-selectable function to

reduce occasional sounds that may occur while the unit is
cycling in and out of the defrost mode. The compressor will
be cycled “off” for 30 seconds going in and out of the defrost
mode when the compressor delay jumper is removed.

NOTE: The 30 second “off” cycle is not functional when
jumpering the TEST pins.

Time Delay

The timed-off delay is 5 minutes long. The delay help to

protect the compressor from short-cycling in case the power
to the unit is interrupted or a pressure switch opens. The
delay is bypassed by placing the timer select jumper across
the TEST pins for 0.5 seconds.

Pressure Switch Circuit

The defrost control incorporates 2 pressure switch

circuits. The optional high pressure switch connects to the
boards HI PS terminals. The low pressure switch is factory
installed.

During a single demand cycle, the defrost control will

lock out the unit after the fifth time that the circuit is
interrupted by any pressure switch wired to the control board.
In addition, the diagnostic LEDs will indicate a locked-out
pressure switch after the fifth occurrence of an open
pressure switch. The unit will remain locked out until power
to the board is interrupted, then re-established or until the
jumper is applied to the TEST pins for 0.5 seconds.

NOTE:

The defrost control board ignores input from the

low-pressure switch terminals as follows:

During the TEST mode,

During the defrost cycle,

During the 90 second start-up period,

and for the first 90 seconds each time the reversing
valve switches heat/cool modes.

If the TEST pins

are jumpered and the 5 minute delay is being by-
passed, the LO PS terminal signal is not ignored
during the 90 second start-up period.

DEMAND DEFROST SYSTEM DESCRIPTION

The demand defrost controller measures differential

temperatures to detect when the system is performing poorly
because of ice build-up on the outdoor coil. The controller
“self-calibrates” when the defrost system starts and after
each system defrost cycle. The defrost control board
components are shown below.

The control monitors ambient temperature, outdoor coil

temperature, and total run time to determine when a defrost
cycle is required. The coil temperature probe is designed
with a spring clip to allow mounting to the outside coil tubing.
The location of the coil sensor is important for proper defrost
operation.

NOTE - The demand defrost board accurately measures
the performance of the system as frost accumulates
on the outdoor coil. This typically will translate into
longer running time between defrost cycles as more
frost accumulates on the outdoor coil before the board
initiates defrost cycles.

DIAGNOSTIC LEDS

The state (Off, On, Flashing) of two LEDs on the defrost

board (DS1 [Red] and DS2 [Green]) indicate diagnostics
conditions that are described in table.

Mode

Green Led (DS2)

Red Led (DS1)

OFF

No power to control

Normal operation/

power to control
Anti-short cycle

lockout

Simultaneous Slow Flash