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

Related Tech Tips

Simple Pump Selection Exercise
Here is another excellent article from Michael Housh, owner of Housh Home Energy and a regular contributor to HVAC School. Thanks, Michael! I thought I would go through a simple example of sizing a hydronic circulator for an application. This is a made-up scenario, but I sketched out a 20’ x 20’ square home with […]
Read more
Simple Pump Selection Part 2
Another follow-up article by Michael Housh… Enjoy! In this article, I thought I would show a pump curve and match it up with our system head-loss curve that we created in the last article, but before I do that, I thought I would talk a little bit more about the system head-loss curve and why […]
Read more
Pre-Cleaning Copper
Sometimes, we can focus on the more complicated aspects of a system and parts installation—like evacuation and flowing nitrogen—and forget the simple and critical common-sense steps to keep contaminants out of the system. One of these is pre-cleaning the tubing and connection before cutting or unsweating. For example: If you are replacing a coil, use […]
Read more

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.

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

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