T
he chiller/heat pump when operating in the heating mode utilizes the outdoor coil as an evaporator. The evaporator
(outdoor coil) in heating must be 20° or more below the outdoor ambient temperature. This means that on a 30° day
the coil temperature will be at a 10° or below operating temperature. Operating in this temperature range causes frost
to form on the outdoor coil surfaces, which cut down on the heating efficiency and capacity (BTUH). To rid the outdoor
coil of this frost build up and restore heating capacity and efficiency a Ranco DLD
TM
Demand Defrost Control is used.
Demand Defrost Control
The demand defrost control (Ranco DDL
TM
) is a printed circuit board assembly consisting of solid-state control devices
with electro-mechanical outputs. The demand defrost control monitors the outdoor ambient temperature, outdoor coil
temperature, and compressor run-time to determine when a defrost cycle is required.
Defrost Initiation
A defrost will be initiated when the three conditions below are satisfied:
1)
The outdoor coil temperature is below 35°F.
2)
The compressor has operated for at least 3 minutes with the outdoor coil temperature below 35°F. 3) The
measured difference between the ambient temperature and the outdoor coil temperature is greater than the calculated
delta T (temperature difference).
Additionally, a defrost will be initiated if six hours of accumulated compressor run-time has elapsed without defrost,
and the outdoor coil temperature below 35°F.
Defrost Termination
Once defrost is initiated, the defrost will continue until fourteen minutes has elapsed or the coil temperature has reached
the terminate temperature. The terminate temperature is factory set at 70°F although the temperature can be changed to
50°F, 60°F, 70°, or 80° by relocating a jumper on the board (jumper and pin location center/right of board).
Temperature Sensors
The coil sensor is clipped to the midsection of the outdoor coil at a point fed by the distribution tubes from the TXV
(Thermostatic Expansion Valve). The air sensor is located on the defrost control board, or on a separate wire whip
similar to the coil sensor, and is left hanging in the outdoor air stream.
if the ambient sensor fails the defrost control will initiate defrost every 34 minutes with the coil temperature below
35°.
If the coil sensor fails the defrost control will not initiate defrost.
Test Mode
The test mode is initiated by shorting the TEST pins. In this mode of operation, the enable temperature (35°F coil) is
ignored and all timers are sped up by a factor of 240. To initiate a manual defrost, short the TEST pins. Remove the short
when the system switches to defrost mode. The defrost will terminate on time (14 minutes) or when the termination
temperature has been achieved. Short TEST pins again to immediately terminate defrost.
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