Ruud RP13, Installation Instructions Manual

37

High- and Low-Pressure Controls (HPC and LPC)

Standard on RP14: These controls keep the 

compressor from operating in pressure ranges 

which can cause damage to the compressor. Both 

controls are in the low-voltage control circuit.
The high-pressure control (HPC) is an automatic-

reset which opens near 610 PSIG and closes near 

420 PSIG.
The low-pressure control (LPC) is an automatic-

reset which opens near 15 PSIG and closes near 

40 PSIG.

CAUTION:

The compressor has 

an internal overload protector. Under some 

conditions, it can take up to 2 hours for this 

overload to reset. Make sure overload has had 

time to reset before condemning the compressor.

Components

Demand Defrost Control (cont.)

The demand defrost control monitors the outdoor 

ambient temperature, outdoor coil temperature 

and the compressor run time to determine when a 

defrost cycle is required.

Enhanced Feature Demand Defrost Control

This defrost control has high and low pressure 

control inputs with unique pressure switch 

logic built into the microprocessor to provide 

compressor and system protection without 

nuisance lockouts. The control cycles the 

compressor off for 30 seconds at the beginning 

and the end of the defrost cycle to eliminate the 

increased compressor noise caused by rapidly 

changing system pressures when the reversing 

valve switches. See next page for diagnostic flash 

codes and sensor resistance values at various 

temperatures.

Defrost Initiation

A defrost will be initiated when the three 

conditions below are satisfied:
1. The outdoor coil temperature is below 35°F as 

measured by a good coil sensor,

2. The compressor has operated for at least 34 

minutes with the outdoor coil temperature below 

35°F and

3. The measured difference between the ambient 

temperature and the outdoor coil temperature 

is greater than the calculated difference 

determined by the microprocessor.

Defrost Termination

Once a defrost is initiated, the defrost will continue 

until fourteen minutes has elapsed or the coil 

temperature has reached the selected termination 

temperature. The factory setting is 70°F but can 

be changed to 50°F, 60°F or 80°F by relocating the 

jumper on the control board.

Temperature Sensors

The coil sensor is located on the outdoor coil 

at the point fed by the distribution tubes from 

the expansion device (short 3/8” dia. Tube). The 

ambient air sensor is located outside the control 

box so it can sense outdoor temperatures.

If the ambient sensor fails, the defrost control will 

initiate a defrost every 34 minutes of compressor 

run time with the coil temperature below 35°F.
If the coil sensor fails, the defrost control will not 

initiate a defrost.

Test Mode

The test mode is initiated by shorting the TEST 

pins. The unit must have an active heat pump 

heating call to enter the test mode. In this mode 

of operation, the enable temperature is ignored 

and all timers are sped up. To initiate a manual 

defrost, short and hold the TEST pins. Remove the 

short when the system switches to defrost mode 

after the compressor noise abatement delay. The 

defrost will terminate on time (14 minutes) or when 

the termination temperature has been reached.

Test Sequence of Operation:

1) Provide a heating call to the heat pump.
2) Short test pins to bypass anti-short cycle timer. 

(If unit is running, this step is not necessary.)

3) Short test pins and hold them shorted to enter 

defrost mode.

4) Release test pins once control exits noise 

abatement delay.

5) Monitor coil temperature when control exits 

defrost.

6) Unit should return to heating mode. 

Trouble Shooting Demand Defrost

During the test mode the coil temperature should 

be monitored. If the system exits defrost at 

approximately the termination temperature, the 

control is operating normally.
If not, one item to check is the coil and ambient 

temperature sensor resistance, using the 

temperature/resistance chart.
Immerse the sensor in water and measure the 

resistance of the sensor. At 35°F the resistance of 

the sensor should be approximately 30,000 ohms.
Ensure that the coil sensor is properly installed 

that it is not loose or touching the cabinet.

COMPONENTS AND CONTROLS