Refrigeration Pressure Switches | HVAC Systems

Richard - What are pressure switches used in refrigeration circuits and how do pressure switches protect the system from further damage?

Refrigeration Pressure Switches | HVAC Systems

Refrigeration Cycle Component Diagram for Air Conditioners and Heat Pumps

Thanks for the question and for being a reader of High Performance HVAC! First, there two types of basic refrigeration switches in the typical high-grade residential system. I say high grade because not every refrigeration system in air conditioners and heat pumps has switches to protect the refrigeration system.

High-Pressure Refrigeration Pressure Switches - The high-pressure switch will kill the compressor circuit if the pressure exceeds the rating on the pressure switch. The ratings will vary depending on the type of refrigerant the air conditioner or heat pump uses the system.

On some air conditioners or heat pumps, this switch is manually reset. Because if it trips there is definitely a problem that needs to be resolved before the system should run again.

On Rheem and Ruud heat pumps and air-conditioners, the reset button is usually red on located near the control panel. A simple push of the switch will reset the switch, and the operation of the air conditioner should return to normal until the pressure exceeds the rating of the switch.

Things that can cause the high-pressure switch to trip include:

• Bad condenser fan motor
• Condenser Coils plugged up with dirt and debris
• Fins on the coils bent
• Blockage of the refrigeration system

These issues, if they exist, should be taken care of before the switch is reset. The high-pressure switch prevents the system from operating at extremely high pressures. Continuously resetting the switch without taking care of the problem will lead to possible catastrophic failure of the system resulting in higher repair costs.

Low Refrigeration Pressure Switches - The low-pressure switch is typically an automatic reset switch that will reset itself when the pressure on the low-pressure side of the refrigeration systems returns to normal or above the setting on the switch.

When a refrigeration system operates at lower than normal pressures the system can freeze up with the freezing beginning at the evaporator coil and working its way back to the compressor via the suction line.

The freezing of the refrigeration system is bad and leads to liquid refrigerant slugging in the compressor which can lead to compressor failure. Things that can cause the low-pressure switch to trip include:

• Low refrigerant charge because of a refrigerant leak
• Blower motor failure in the air handler
• Plugged evaporator coils or a blockage of airflow
• Failure of a refrigeration component in the refrigeration system

Air Conditioner and Heat Pump Protection

Again, if these issues exist in the refrigeration system, they need to be addressed to prevent the low-pressure switch from tripping. In some cases, such as catastrophic failure or loss of the refrigerant charge, the system will not run. Simply because the controls were engineered correctly to prevent short-cycling.

However, it is possible the system can short cycle, especially if the condenser has been re-engineered with aftermarket parts by someone who knew enough to be dangerous.

That includes me in certain instances; however, I always do my best to think things through before making a change to the system. After all, solving one problem only to create another problem is not fixing the HVAC system. Number one is safety and reliability, and number two is protecting the system from permanent damage keeping number one in mind.

These switches are used for a specific purpose, and re-engineering the HVAC system can create other problems. It is also essential that the person making the changes is competent or has someone qualified to review the changes to make sure other issues will not be created.

When appropriately engineered, these devices can protect the system and save you tons of money in the event of a catastrophic failure of the system for any reason, whether it is a system defect or it is a child playing that damages the unit.

Pressure Switches for Air Conditioning Settings

The settings for refrigeration pressure switches will vary depending on the type of refrigerant is used in the system. For refrigeration low-pressure switch settings a refrigerant pressure-temperature chart should be used to determine the cut-off for the low-pressure side of the system. For most HVAC applications, this setting for the pressure control should be just above the freezing point so that ice cannot be formed on the evaporator coil.

There are some refrigeration pressure switches provided by the manufacturer that are preset non-adjustable switches. In many cases, these preset switches will automatically cut off the condensing unit before freezing can occur based on the setting of the preset switch.

The high side, if equipped with a high-pressure switch, should be set according to the manufacturer’s recommendation. This pressure will be based on specifications for maximum pressure for the refrigeration systems high side and the type of refrigerant used. The key is to protect the compressor from extremely high pressures that could damage the internal components of the compressor.

In some cases, the manufacturer supplies a high-pressure refrigeration switch with the condenser, and it is a manual resettable pressure switch. In any case, the pressure switches tripping the system will result in a trouble call from the customer where the pressure problem can be resolved by a technician.

Pressure Switches for Air Conditioning Wiring Diagram | Ladder Logic with Legend

In the following ladder logic schematic, you will see a red arrow on the bottom right. The red arrow points to the low-pressure switch, and just to the right of the low-pressure switch is the high-pressure switch. These pressure switches are wired through the Y wire or terminal leading into the heat pump. In this case, if one of the pressure switches were to trip, this control would kill the entire condensing unit.

Following the logic of the wiring from the diagram, the Y wire goes through the pressure switches and then through a compressor time delay. From the time delay, the logic follows the compressor contactor coil. If you kill power to the compressor contactor coil, you will kill the entire unit. The compressor contactor contacts open when the coil loses power. That is what would happen if one of the refrigeration pressure switches were to open.

The control in the above wiring diagram would be the same, whether it is a heat pump or an air conditioner. The basic premise for refrigeration pressure switches in HVAC equipment is to protect the equipment from catastrophic failure.

Some commercial refrigeration systems use pressure controls in the refrigeration system to turn the compressor and condenser fan motor on and off for control. In this instance, for comfort cooling and heating, it is to protect the equipment in the event of a high-pressure problem or low-pressure problem.

Refrigeration Pressure Switches for HVAC | Conclusion

It should be noted, if a pressure switch is used for either the high side or the low side of any refrigeration system that it should be coupled with a short cycle delay timer to prevent short-cycling of the compressor.

Resource: HVAC Refrigeration Pressure Switch

Refrigeration Pressure Switches for HVAC

Technical Resource: Refrigeration and Air Conditioning Technology