Solenoid Facts

Do you know how a solenoid valve works?


On the surface, I think we all understand how a solenoid valve works. The coil energizes, creating an electromagnet. That temporary magnetism lifts an iron plunger within the valve allowing refrigerant to flow.

But is it really that simple?

It turns out that the answer isn't as straightforward as you'd expect.

The simplest type of solenoid valve is the direct-acting solenoid valve. These valves precisely match the description above. The iron plunger directly controls the flow of refrigerant through the valve. Every single solenoid valve you see incorporates a direct-acting valve, but there is more than what meets the eye.

Courtesy of Sporlan

Direct-acting solenoid valves have an inherent limitation. If the force created by the fluid flowing through the valve that is acting on the iron plunger is enough to lift that plunger, then it isn't going to close regardless of what the electromagnetic coil tries to tell it to do. That means that direct-acting solenoid valves are limited in size, and that size is pretty small.


So, how can we control the fluid flow in larger lines with solenoid valves?

We start to use the pressure within the system actually to force the valve closed.

Say what???

These are called pilot-operated or pilot-actuated valves. The direct-acting solenoid doesn't try to control the entire flow; it only acts to control a small portion of the fluid, which acts on a diaphragm or other device to open and close the valve.

Courtesy of Sporlan

Let's see if we can start to understand how these valves work in practice.

First, a few basics:

  1. Solenoids, like most valves, are directional. If you install it backward, it isn't going to work correctly. This is why.
  2. Solenoids must be sized properly. You can't just go buy a ½” solenoid valve and expect it to work because your line is ½”. This ensures a small pressure drop across the valve, which is what actually makes the valve work.

Ok, so the refrigerant flows through an energized solenoid. Now, the coil de-energizes, causing the iron plunger to drop and seal a tiny port.  This stops a small amount of flow from inlet to outlet, preventing that small flow from leaving the valve body. That small port being blocked causes pressure to build on top of the diaphragm or valve seat disc, forcing it down to seal the valve. The small iron plunger and spring don't have the force required to force the valve closed, but by utilizing system pressure, we have a much larger amount of force available.

In truth, the large majority of solenoid valves a technician sees are pilot-operated valves.

—Jeremy Smith CM




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

Remove "Weep" Plugs on Motors
  In this 60-second tech tip video by Brad Hicks with HVAC in SC, he shows us how and why to remove the weep port plugs on a condensing fan motor. I know from experience that motors can fail prematurely when this practice isn't followed. Remember that motor orientation dictates which weep port plugs are […]
Read more
Why Discharge Line Temperature is a Useful Reading
Since I started in the trade, we would take discharge line temperature in the winter on a heat pump system. You can easily check the discharge line in the winter, while suction superheat and even subcool can be more difficult to access. The old-timers who trained me would say that the discharge temp will be […]
Read more
Gluing Tubing Insulation
I see a lot of techs and installers use tape over foam tubing insulation joints (Rubbatex, Armaflex, etc.) rather than using the glue that is designed for it. Some of them use tape because they don't know the glue exists; some use tape because it's what they happen to have on the truck, and others […]
Read more

To continue you need to agree to our terms.

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