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

Why Selecting Equipment is Important (and Hard)
I get questions all the time about performing “load calculations” and “rules of thumb” or how to do it properly. This article isn't about load calculation, but the only good answer is to find a quality ACCA-approved Manual J software and get used to using it. You may have heard from others in the field […]
Read more
Trades and the Skills Gap - A Manifesto
This article is a year old, and I'm recycling it because it's on my mind today. I had a fun conversation with Richard Trethewey on the podcast that has me thinking along these lines today. The link is HERE if the player isn't showing up. I have a confession to make. I'm a bit of […]
Read more
Defrost Time & Temperature
The most common method to defrost appropriately in refrigeration involves both time for initiation and a combination of time and temperature for defrost termination (ending defrost). But why can't we just use temperature or time alone? (You may wonder.) Imagine a common freezer with a designed box temperature of -10°F and a coil temperature of […]
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