A Liquid Line Solenoid and What it Does

200rb-with-coil

Depending on what segment of the business you are in and your location, you either work on pump down solenoid systems all the time, or YOU HAVE NO CLUE what they are.

A liquid line solenoid is just a valve that opens and closes; it has a magnetic coil. Depending on whether the valve is normally open or normally closed, it opens or closes when the coil is energized.

If you work on refrigeration or straight cool units up north, you are likely very well acquainted with “pump down” solenoids. If you do residential HVAC in the south, you may have never seen one.

Do you know that you pump down a system by closing the liquid line? That's all a pump down solenoid does. It closes when the system is running, causing the system to pump all of the refrigerant into the condenser and receiver (if there is one).

The trick is that in order for it to pump down, the compressor needs to be running, and then it needs to SHUT OFF once it is done pumping down. That means that it needs a good-quality, properly set low-pressure switch near the compressor to shut it off when the suction pressure gets low enough—but not TOO low. The goal is to get all the liquid pumped into the condenser, not pump down to zero.

There are a few benefits of a pump down solenoid. First, it helps prevent liquid refrigerant migration down the suction line into the compressor. When liquid refrigerant migrates into the compressor, it dilutes the oil and can cause a “flooded” start.

The other cool thing is that you don't need any low-voltage controls between the indoor and outdoor unit (in some cases). The solenoid is in the liquid line near the air handler inside, so by opening the valve, the suction pressure increases, the compressor turns on, and when it closes, the compressor pumps down and shuts off.

Obviously, this would not work on a heat pump system because it would attempt to pump down into the indoor coil in heat mode, which would not work. They also won't work in most cases when you have complex or proprietary controls.

In some cases, the liquid line solenoid is not used to “pump down;” it simply closes during the off cycle, preventing refrigerant flow and migration in that way.

So, there are places where liquid line solenoids make sense and applications where they don't, but they are fairly simple and easy to understand.

—Bryan

Related Tech Tips

Reach-In Temperature Controls
  This article is written by Christopher Stephens of JVS Refrigeration in California, with just a few additions by me (Bryan) in italics. Thanks, Chris! Reach-in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Since we usually see reach-ins in kitchens or convenience stores, the refrigerators are never […]
Read more
How to use an Adjustable (Crescent) Wrench, Pipe Wrench and Tongue & Groove (Channel Locks) Wrenches
My grandfather is a really interesting guy. He grew up working in the Pennsylvania coal mines starting at the age of 7 or 8 and then worked as a well driller, was a plumber, went to HVAC school, did some gas work, worked a while as an electrician, became a welder, then was a diver, […]
Read more
Diagnosing ECM & X13 Motors
First, let's give proper credit. Most of the best practices and tools for diagnosing ECM and X13 motors come from Regal/Genteq and their site, thedealertoolbox.com, and their app, The Dealer Toolbelt. Your best bet is to follow the practices shown there and use their TECInspect diagnostic tool shown below. Here is the general process to […]
Read more

One response to “A Liquid Line Solenoid and What it Does”

  1. I know this is an older one but since it’s been highlighted on the facebook page I’d just like to take a second to make a point. When sizing these pump down valves they are sized by the size of the system (tonnage/hp) not the size of the pipe.

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