Please pay attention, there is a quiz at the end.

In a previous article I wrote about simple circuits – power, line, switch, and load. Most HVAC equipment have controls and safety switches that are activated by time, temperature, pressure, or in the case of hydronics – flow. Let’s take a look at two of the common HVAC switches.

Pressure Controlled Switches – In the diagram below, they are designated as Low Pressure Switch (LPS) and High Pressure Switch (HPS). These types of switches open or close based on pressure.

LPS – (Close on Pressure Increase) Examine the schematic symbol of the LPS. Notice it has as a semi-circle on the bottom. Imagine this is where the pressure pushes, into the bottom. On a LPS, as the pressure increases (pushes up) it holds the switch closed. If the pressure decreases, the switch will fall open. Actually the switch has spring tension that actuates the opening of the switch.

HPS – (Open on Pressure Increase) Look at the HPS. It is similar to the LPS but with one difference. When the pressure increases it pushes the switch open. When the pressure decreases it closes the switch. Can you see it?

Pressure controlled switches in residential HVAC systems are factory set to open or close within specific pressure ranges which varies somewhat from manufacturer to manufacturer.

 Fig 4

Temperature Controlled Switches – In the diagram above, they are designated as THER (Thermostat) . These types of switches open or close based on temperature.

There are two types of temperature controlled switches in residential HVAC systems: close on temperature rise and open on temperature rise.

Examine the symbol designated as THER. The zig-zag symbol indicates that this is a temperature controlled switch. In this instance it closes on temperature rise and opens when the temperature falls.  An open on temperature rise switch (not depicted), would look like the HPS symbol only it will have a zig-zag at the bottom rather than the semi-circle. Can you see it?

Remember if it is a close on temperature rise it also opens on temperature fall, same with the pressure switches. These types of switches also have a range.  For example the HPS could be designed to open at 400 psi and stay open until the pressure decreases to 325 psi, a range of 75 psi.

Pressure and temperature controlled switches are two of the most common controls found in a residential HVAC system. They are designed to either protect a load (motor or compressor) or activate/deactivate a load based on temperature and pressure conditions.

Quiz – Look at CFM2 THERM. How does it control the load (CFM2). Leave your answer in the comments below.

by Ron Walker


Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.