Armstrong Air Conditioning 4HP14 Installation & Start-Up Instructions - Page 11

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OPERATIONAL DESCRIPTION

The defrost control board has three basic operational

modes: normal, calibration, and defrost.

Normal Mode—The demand defrost board monitors the O

line, to determine the system operating mode (heat/cool),

outdoor ambient temperature, coil temperature (outdoor coil)

and compressor run time to determine when a defrost cycle

is required.

Calibration Mode—The board is considered uncalibrated

when power is applied to the board, after cool mode

operation, or if the coil temperature exceeds the termination

temperature when it is in heat mode.

Calibration of the board occurs after a defrost cycle to

ensure that there is no ice on the coil. During calibration,

the temperature of both the coil and the ambient sensor are

measured to establish the temperature differential which is

required to allow a defrost cycle.

Defrost Mode—The following paragraphs provide a detailed

description of the defrost system operation.

DETAILED DEFROST SYSTEM OPERATION

Defrost Cycles—The demand defrost control board initiates

a defrost cycle based on either frost detection or time.

Frost Detection—If the compressor runs longer than

30 minutes and the actual difference between the clear

coil and frosted coil temperatures exceeds the

maximum difference allowed by the control, a defrost

cycle will be initiated.

IMPORTANT - The demand defrost control board

will allow a greater accumulation of frost and will

initiate fewer defrost cycles than a time/temperature

defrost system.

Time—If 6 hours of heating mode compressor run time

has elapsed since the last defrost cycle while the coil

temperature remains below 35°F (2°C), the demand

defrost control will initiate a defrost cycle.

Actuation—When the reversing valve is de-energized, the

Y1 circuit is energized, and the coil temperature is below

35°F (2°C), the board logs the compressor run time. If the

board is not calibrated, a defrost cycle will be initiated after

30 minutes of heating mode compressor run time. The

control will attempt to self-calibrate after this (and all other)

defrost cycle(s).

Calibration success depends on stable system temperatures

during the 20-minute calibration period. If the board fails to

calibrate, another defrost cycle will be initiated after 45

minutes of heating mode compressor run time. Once the

defrost board is calibrated, it initiates a demand def rost

cycle when the difference between the clear coil and frosted

coil temperatures exceeds the maximum difference allowed

by the control OR after 6 hours of heating mode compressor

run time has been logged since the last defrost cycle.

506318-01

NOTE - If ambient or coil fault is detected, the board

will not execute the "TEST" mode.

Termination—The defrost cycle ends when the coil

temperature exceeds the termination temperature or after

14 minutes of defrost operation. If the defrost is terminated

by the 14-minute timer, another defrost cycle will be initiated

after 30 minutes of run time.

Test Mode—When Y1 is energized and 24V power is being

applied to the board, a test cycle can be initiated by placing

the termination temperature jumper across the "Test" pins

for 2 to 5 seconds. If the jumper remains across the "Test"

pins longer than 5 seconds, the control will ignore the test

pins and revert to normal operation. The jumper will initiate

one cycle per test.

Enter the "TEST" mode by placing a shunt (jumper)

across the "TEST" pins on the board after power-up. (The

"TEST" pins are ignored and the test function is locked out

if the shunt is applied on the "TEST" pins before power-up).

Board timings are reduced, the low-pressure switch is

ignored and the board will clear any active lockout condition.

Each test pin shorting will result in one test event.

For each "TEST" the shunt (jumper) must be removed for

at least 1 second and reapplied. Refer to flow chart for

"TEST" operation.

Note: The Y1 input must be active (ON) and the "O"

room thermostat terminal into board must be inactive.

DEFROST BOARD DIAGNOSTICS

See defrost control board diagnostic LED table on next

page to determine defrost board operational conditions and

to diagnose cause and solution to problems.

Issue 0946

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