Walk-In Cooler Pressure Controls: Low and High Pressure Trips

Pressure controls are the walk-in compressor's safety net. When one trips, the system is telling you that something upstream is wrong, and the trip is what's keeping that problem from destroying the compressor.

What pressure controls do

A pressure control is a switch connected to the refrigerant system through a small tube. When the pressure it senses crosses its setting, its contacts open and stop the compressor. Walk-in condensing units usually have two, sometimes combined in one "dual" control.

Settings depend on the refrigerant, the application and the manufacturer's specification. There is no universal number, so your technician sets them from the equipment data.

The low pressure control

On a pump-down system, the low pressure control is part of normal operation. When the thermostat closes the liquid line solenoid valve, the compressor pumps refrigerant out of the evaporator until suction pressure falls to the control's cut-out, and the compressor stops. When the solenoid opens again, pressure rises to the cut-in and the compressor restarts.

It also acts as a safety. If suction pressure falls when it shouldn't, the control stops the compressor. That can happen with:

• low refrigerant charge from a leak;
• a starved evaporator from a stuck expansion valve, a restricted filter-drier or a solenoid that won't open;
• an iced or airflow-starved evaporator coil;
• very cold outdoor conditions pulling head pressure down on an outdoor unit.

Repeated low-pressure trips often look like short cycling: the compressor runs briefly, stops, and restarts a few minutes later.

The high pressure control

The high pressure control is a pure safety. Head pressure climbs when the condensing unit can't get rid of heat or the system has problems such as:

• a dirty condenser coil;
• a failed condenser fan motor;
• hot air recirculating around a boxed-in or sun-baked unit;
• too much refrigerant, or air and other non-condensable gases in the system;
• a restriction on the high side.

High pressure controls come in two styles:

• Automatic reset closes again by itself when pressure falls. The compressor restarts, and on a hot day may trip again and again.
• Manual reset stays open until someone presses the reset button. This forces a person to look at the problem before the compressor runs again.

Which style your unit uses is on the control and in the wiring diagram. Either way, a trip is information, not the problem.

What a trip looks like

Controllers often report pressure trips. From the manuals:

• Heatcraft Beacon II: E7 means compressor shutdown because the high or low pressure switch or the oil pressure switch opened. A3 is a start-up failure after four unsuccessful restarts.
• Heatcraft Smart Controller: SHTDN means compressor shutdown on high/low pressure or oil pressure.
• Dixell XR60CX: CA can indicate a pressure-switch alarm, with all outputs turned off; on the pressure-switch setting, it recovers only by cycling power.
• Carel ir33: LP indicates low pressure.

Code meanings differ by brand and even by model, so always name the controller when looking one up. See controller alarm codes.

Why bypassing or jumping a control is dangerous

Every so often someone "fixes" a tripping walk-in by jumping out the pressure control with a wire. The compressor runs, the box cools for a while, and then:

• On a high-pressure problem, head pressure and discharge temperature climb with nothing to stop them. That can overheat the compressor, break down oil, and stress components and joints that were never meant to run at those pressures.
• On a low-pressure problem, the compressor keeps pulling on an empty or starved evaporator. It overheats from lack of cooling vapor, and a system with a leak can pull in air and moisture.

It can also cost you the warranty. Heatcraft's condensing unit warranty conditions require controls and safeties to be wired per the diagram and prohibit factory wiring changes without written approval.

Safety: Never jump, bypass, wedge or repeatedly reset a pressure control, and never let a service provider leave one bypassed "until the part comes in." If the system can't run safely with its safeties in place, move product and wait for the repair.

Checks staff can do safely

1. Note what the controller displays, exactly as shown, and when it happened.
2. Check the condensing unit's airflow. Is the condenser fan running? Is the coil clogged? Is anything blocking the unit?
3. Look at the evaporator for heavy ice.
4. Record the weather. High-pressure trips cluster on hot days; low-pressure trips on outdoor units can cluster on very cold ones.
5. Don't press reset buttons on the condensing unit. Tell the technician if you see one.

The what staff can safely check page has the full do and don't list.

What a technician will check

A technician will connect gauges to read suction and discharge pressures, compare them with the control settings and the refrigerant's normal range for the conditions, and check superheat and subcooling to find the cause. They'll test that the control actually opens and closes at its settings and replace it if it's drifted or stuck. On Heatcraft equipment, the site's Heatcraft troubleshooting guide covers common system faults.

Questions to ask after a pressure trip

• Which control tripped, low or high? The answer points to very different causes.
• What were the pressures and the settings? A tech should be able to show you.
• Was a manual reset pressed, and why was it safe to do so?
• What was fixed to stop it happening again? A reset alone is not a repair.

The repair is whatever caused the trip: cleaning the condenser, replacing a fan motor, fixing a leak, clearing a restriction, or correcting the charge.