Short Cycling

By Ralph White
Danville, Calif.

On certain cooling applications there is a thermostat and liquid line solenoid in conjunction with a pressure control. The t-stat and solenoid are used to control fixture temperature. But serious compressor short cycling can occur during the start and end of each running cycle. I believe this may be the result of temporary imbalance between compressor capacity and a partially filled evaporator. I have been told that this problem can be overcome by a fairly simple variation in the electrical wiring of the control components, using one or two relays. Any thoughts?

From Gene Silberstein
Consulting Engineer

I do know of a relay configuration that will prevent the compressor from short cycling on systems that are equipped with thermostatically controlled liquid line solenoid valves.

I think it would provide a benefit to first briefly explain the basic operation of a standard pump-down system before taking a look at this somewhat more complicated control circuit configuration.

In a basic pump-down configuration, when the box reaches the desired temperature (in this case it is 5°F), the thermostat opens its contacts, de-energizing the liquid line solenoid. The compressor continues to operate, pumping the refrigerant from the evaporator and suction line into the discharge line of the system.

When the low-pressure drops to the proper pressure (in this case 2 psig), the contactor coil (C1) will be de-energized, opening contacts (CC1). This will de-energize the compressor. When the box temperature rises (to 10°), the liquid line solenoid will be energized and refrigerant will flow from the liquid line, through the expansion device and through the evaporator to the suction line.

The suction pressure will rise (the compressor is still not running). When the pressure in the suction line rises to 27 psig, the low-pressure switch will close, energizing C1. This will cause the CC1 to close, energizing the compressor.

If refrigerant is getting into the suction line for any reason, the compressor will begin to short cycle. The addition of two additional relays will prevent the compressor from being energized until the thermostat’s contacts close at the beginning of a new cycle.

Two additional relays with normally open contacts are used. The coils of these two relays are installed in parallel with the compressor and the liquid line solenoid. The normally open contacts of these relays are wired in parallel with each other, and both are wired in series with the low-pressure switch.

Under normal operating conditions (in the refrigeration mode), the thermostat and liquid line solenoid are both energized and contacts CC1, CC2, and CC3 are all closed. At the end of the refrigeration cycle, the thermostat opens, de-energizing the liquid line solenoid and holding coil C3. This causes the system to begin pumping down and also causes contacts CC3 to open.

Once the pressure in the suction line drops to 2 psig, the pressure switch opens its contacts, de-energizing holding coil C1, causing contacts CC1 to open. This in turn de-energizes the compressor and holding coil C2.

Therefore, in the off cycle, contacts CC1, CC2, and CC3 are all open. The compressor can no longer short cycle. If the pressure switch should close, holding coil C1 cannot be energized because both contacts CC2 and CC3 are open. The refrigeration cycle can only resume when the thermostat’s contacts close.

In operation, when the thermostat calls for refrigeration at the beginning of the cycle, the liquid line solenoid and holding coil C3 are energized. Since contacts CC3 are now closed, holding coil C1 will be energized when the low-pressure switch closes. Once C1 is energized, the compressor and holding coil C2 will also be energized, allowing the refrigeration cycle to start.

The basic operation is very similar to that of a standard pump-down system.

Tube Connection

By Dan Walker
Kimbell Walker Equipment Co.
Clinton, Ky.

I have a question about tube connections on a 58MVP-120 Series furnace. The unit is installed in the horizontal left configuration. What would be the net effect if, by chance, the relief port tube (green tag) and pressure switch tube (pink label) hoses were switched 180°? By this I mean that the green label relief port tube was attached to the place where the pink label pressure switch tube was originally attached. The pink label pressure switch tube was attached to the place on the condensate trap that the green label relief port tube was supposed to be attached.

From Carrier Corp.

In the horizontal left position the green tube (high location) acts as a relief tube for the collector box. The pink tube (low location) acts as a safety tube to shut down the furnace if the condensate trap would become clogged. The water would build up and act against the pressure switch/inducer pressure, which would eventually cause the furnace to trip on a pressure switch fault code.

If the hoses were switched, the safety provided by the lower hose being attached to the pressure switch would be eliminated. The furnace would eventually shut down from the inducer outside of rpm range. But this would take a longer amount of time, causing the furnace to possibly burn incorrectly.

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Publication date:03/26/2007