Lennox International Inc 06/11 50672801 System Operation, Defrost System, Defrost Control CMC1

System Operation

IMPORTANT

Some scroll compressor have internal vacuum protector that will unload scrolls when suction pressure goes below 20 psig. A hissing sound will be heard when the compressor is running unloaded. Protector will reset when low pressure in system is raised above 40 psig. DO NOT REPLACE COMPRESSOR.

High Pressure Switch (S4)

This unit is equipped with a auto-reset high pressure switch (single−pole, single−throw) which is located on the liquid line. The switch shuts off the compressor when discharge pressure rises above the factory setting. High Pressure (auto reset) − trip at 590 psig, reset at 418 psig.

Low Pressure Switch (S87)

This unit is equipped an auto−reset low pressure switch which is located on the vapor line. The switch shuts off the compressor when the vapor pressure falls below the factory setting. Low Pressure (auto reset) − trip at 25 psig, reset at 40 psig.

Low Pressure Switch Bypass (S41) (Optional)

For use in applications where the heat pump is operated in outdoor ambient temperatures below 15°F.

SPrevents nuisance trips form the low pressure switch

SWired in parallel with the low pressure switch

Thermal Protection Switch (S173)

Mounted

Some units are equipped with a compressor mounted normally closed temperature switch that prevents compressor damage due to overheating caused by internal friction. The switch is located on top of the compressor casing (see figure 1). This switch senses the compressor casing temperature and opens at 239−257°F (115°C−125°C) to shut off compressor operation. The auto−reset switch closes when the compressor casing temperature falls to 151−187°F (66°C−86°C), and the compressor is re−energized. This single−pole, single−throw (SPST) bi−metallic switch is wired in series with the 24V Y input signal to control compressor operation.

Crankcase Thermostat (S40) (−042, −048 and −060

Units Only)

The reference models are equipped with a 70 watt, belly band type crankcase heater. HR1 prevents liquid from accumulating in the compressor. HR1 is controlled by a thermostat located on the liquid line. When liquid line temperature drops below 50° F the thermostat closes energizing HR1. The thermostat will open, de−energizing HR1 once liquid line temperature reaches 70° F .

Defrost Thermostat (S6)

The defrost thermostat is located on the liquid line between the check/expansion valve and the distributor. When defrost thermostat senses 42°F (5.5°C) or cooler, the thermostat contacts close and send a signal to the defrost control to start the defrost timing. It also terminates defrost when the liquid line warms up to 70°F (21°C).

Bi−Flow Liquid Line Filter Drier

The unit is equipped with a large−capacity biflow filter drier which keeps the system clean and dry. If replacement is necessary, order another of the same design and capacity. The replacement filter drier must be suitable for use with HFC−410A refrigerant.

Defrost System

The XP13 defrost system includes two components: a defrost thermostat (S6) and a defrost control (CMC1) (figure 14).

FIELD SELECT
TIMING PINS
TEST
PINS
	DIAGNOSTIC
	LEDS
COMPRESSOR
DELAY PINS
REVERSING
VALVE
	24V TERMINAL
	STRIP
S87	CONNECTIONS
LOW PRESSURE
SWITCH
DEFROST
THERMOSTAT
S4
HIGH PRESSURE
SWITCH

Figure 22. Defrost Control (CMC1)

DEFROST CONTROL (CMC1)

The defrost control includes the combined functions of a time/temperature defrost control, defrost relay, diagnostic LEDs and terminal strip for field wiring connections.

The defrost control provides automatic switching from normal heating operation to defrost mode and back. When the defrost thermostat is closed, the control accumulates compressor run time at 30, 60 or 90 minute field adjustable intervals. When the selected compressor run time interval is reached, the defrost relay is energized and defrost begins.

Defrost Control Timing Pins (P1)

Each timing pin selection provides a different accumulated compressor run time period for one defrost cycle. This time period must occur before a defrost cycle is initiated. The defrost interval can be adjusted to 30 (T1), 60 (T2), or 90 (T3) minutes (see figure 22). The maximum defrost period is 14 minutes and cannot be adjusted.

NOTE part number is listed near the P1 timing pins.

SUnits with defrost control 100269−02: Factory default is 60 minutes

SUnits with defrost control 100269−04: Factory default is 90 minutes

If the timing selector jumper is missing, the defrost control defaults to a 90−minute defrost interval.

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