The “Other” Pressure Switch

The chart above is from a Carrier heat pump, and it lists some common accessories.

I have heard this common pressure switch (last on the list) called a low-pressure switch (because it opens on fall in pressure) and a high-pressure switch (because it's in the liquid line), but RARELY do I hear it called its true name.

Loss of charge switch

In a heat pump system, the operational suction pressure can vary greatly from the suction you will run on a hot day in cool mode to the suction you will run on a cold winter day in heat mode. That makes a traditional low-pressure switch a challenge.

The loss of charge switch performs much the same role as the low-pressure switch, but since it is in the liquid line in cool mode (before the metering device and after the condenser) and the expansion line in heat mode (after the metering device and before the outdoor coil), it will not be as prone to fluctuations. It will only keep the system off in cases of very low charge or “loss of charge” in cool mode.

In heat mode, it will open during a loss of charge but also if there is a line drier or heat mode metering device restriction.

All in all, it's pretty easy to understand, so long as you know its true identity and purpose.

—Bryan

One response to “The “Other” Pressure Switch”

  1. Does this Switch-Loss of Charge works through temperature? I mean, does it detects the loss of charge by sensing the temperature in the liquid line. We have seen it in MrCool ACs (Lennox) cooling only units, just besides the HPS.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Related Tech Tips

Troubleshooting Advanced Systems
This article was written by Senior Refrigeration Tech Jeremy Smith. Before we get to it, I want to remind you that ALL of the tech tips are available in chronological order HERE, and you can sort through them alphabetically by selecting the letters and numbers above the tips. It's a great link to share with […]
Read more
What is "Cascade" Refrigeration?
Photo Courtesy of Emerson What is Cascade refrigeration? “Cascade refrigeration” is a term you will hear more and more over the coming years, and while some of the systems may be very complex, the concept is actually pretty simple. Some refrigerants are well-suited for high and medium-temperature applications, and some are better suited for lower-temp […]
Read more
Weighing Refrigerant In and Out
If you don't use a scale every time you add or remove refrigerant, I suggest you begin doing so immediately, if not sooner. Weighing in while charging is fairly obvious, and it's useful so you can keep track of what you are using and how much to charge a customer. When you have a system […]
Read more
loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from