Updated: Apr 23, 2020
Pressure switches are typically installed in heat pumps and air conditioning systems to protect the compressor from a high pressure situation, to protect the compressor in a low refrigerant situation, and to control the condenser fan cycling.
High and low pressure switches may be installed on the refrigerant tubing of some air conditioning systems. However, they are commonly found on heat pumps. In the case of a heat pump in cooling mode, the high pressure switch is used to make sure that outdoor unit can reject heat with the outside air. In heating mode this same high pressure switch makes sure that the indoor unit is able to reject enough heat inside the building so that the discharge pressure does not exceed the internal relief pressure. This could happen if the duct collapsed, the filter was clogged, dust covered the return side of the indoor coil, the blower speed was too low or the blower motor was broken.
The fan cycling pressure switch which is found on the discharge gas line will also help if the indoor coil is having a hard time rejecting heat because it controls what pressure the outdoor fan will turn on at and what pressure it will turn off at. If the outdoor coil is absorbing more heat than the indoor coil can reject then the discharge pressure will start to rise.
In one example of an R410A heat pump with a fan cycling switch installed, when the discharge pressure rises to 425psi, the fan will shut off and when the pressure goes down to 350 psi, the fan will turn back on. Also, after the compressor initially turns on, the pressure must rise to 350psi to even turn on the fan.
The low pressure safety switch is normally closed when the system is off if the system has saturated refrigerant inside. When the unit turns on, one side of the system rises in pressure and one side lowers in pressure. If the pressure is too low on the low side of the system, this switch will open the 24v electrical circuit going to either the contactor or the defrost board safety switch terminal. This switch may open if the unit is low on refrigerant or if there is a major clog in the liquid line part of the system such as the filter drier, metering device, distributor tubes, or strainer.
To learn more about pressure switches and testing view this video- https://youtu.be/8VtbJldahOI
Check out our book “Refrigerant Charging and Service Procedures for Air Conditioning”.
The full outline is available at https://www.acservicetech.com/the-book
If you have already purchased our book, be sure to tell local HVACR Instructors about our book and what you think of it. We would love to get the book into the hands of the next generation of HVACR Technicians!
Author: Craig Migliaccio
About the Author:Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel