Richard - What are pressure switches used for in refrigeration circuits and how do pressure switches protect the system from further damage?
Refrigeration Pressure Switches
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 have 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 in the system. On some air conditioners or heat pumps this switch is manually reset only simply 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 pressure switch to trip include:
- Bad condenser fan motor
- Coils plugged up with dirt and debris
- Fins on the coils bent
- Blockage in the refrigeration system
These issues, if they exist, should be taken care of before before the switch is reset. The high pressure switch prevents the system from operating at extremely high pressures.
Low Refrigeration Pressure Switches - The low pressure switch is typically an automatically 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. Freezing of the refrigeration system is bad and lead 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 from a refrigerant leak
- Blower motor failure in the air handler
- Plugged evaporator coils or a blockage of air flow
- Failure of a refrigeration component in the refrigeration system
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 at 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. This includes myself 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 important that the person making the changes is competent or has someone competent review the changes to make sure other problems will not be created. When engineered properly 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.