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 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

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. Required fields are marked *

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

Related Tech Tips

Wide, Narrow, Wide Diagnosis
When you walk up to a piece of equipment, you want to follow a process to ensure that you accomplish five things: #1 – You diagnose the fault correctly. #2 – If possible, you find the “why” of the failure. #3 – Find any other problems or potential problems with the system that can cause […]
Read more
What is the "Mid Point" of a Refrigerant Blend?
As we have mentioned in several previous articles (such as this one HERE), many blended refrigerants have glide, which simply means they boil and condense over a range of temperatures instead of just one temperature. As an example, consider refrigerant R407c. It is a zeotropic blend, which means it has enough glide to make a […]
Read more
3 Bad Techs Who Don't Know It
First, let's state the obvious and clear the air a bit. The photo above is SUPER CHEESY! But this story is about three bad techs who don't know it, so a photo of three models clearly posing in clean clothes makes as good of a proxy for a bad tech as anything else. First off, I'm […]
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