How Solenoid Valves Work

Solenoid valves are used to remotely or autonomously control the flow of fluid or gas. Their main purpose is to replace manual valves that require a human operator to open and close. Solenoid valves are particularly advantageous in large and complex projects. Indeed, being able to remotely control the flow of substance is much more efficient than having workers manually open and close valves at different points in the system.

Because of their efficiency, solenoid valves have a wide range of residential, commercial and industrial applications.

To help you get a better understanding of what a solenoid valve is, we’re going to take a deeper look at its inner workings. We’re also going to be discussing a few examples of how it’s used in HVAC and industrial settings.

How solenoid valves work

Solenoid valves have two overall components: the solenoid coil and the valve itself. If we break down the parts even further, we can say that the major aspects of a solenoid valve include the coil, the armature, the plunger and the spring.

First, let’s talk about the solenoid coil. A solenoid is a type of electromagnet that can create a magnetic field via the use of electric current. The coil shape is used so that the magnetic field lines become tightly packed at the center of the object. Compact field lines result in a stronger magnetic force. Depending on the direction of the electric current, this magnetic field can either have a pushing or pulling effect.

The solenoid coil is placed around a cylindrical armature made of soft iron. The armature sits on top of the valve body where the fluid flows. Inside this cylinder sits both the plunger and the spring.

The plunger is used to open or close the valve. Meanwhile, the spring pushes the plunger up or down depending on the valve type.

Solenoid valve types

There are two common types of solenoid valves: the normally closed valve and the normally open valve.

In the normally closed valve, the spring pushes the plunger downwards towards the valve body, blocking the fluid. When you run an electric current through the solenoid coil, the magnetic field will begin to pull the plunger upwards. Since the armature is at the center of the coil, the pulling force that is being placed on the plunger is much stronger than the spring pushing it down. Once the plunger is up, the flow of fluid continues.

In the normally open valve, the spring pushes the plunger upwards, opening the valve. In these types of valves, the electric current is run in the opposite direction. As a result of this, the magnetic field creates a downward pushing force. Once the plunger is down, the valve becomes closed.

To revert the valve back to its normal state, you simply have to stop the electrical current and the spring will push the plunger back to its original position.

Other variations

  • Direct acting

Direct acting solenoid valves do not use pressure to keep the valve closed. It simply relies on the spring and the plunger to block the flow of fluid. Once the plunger is pushed up the fluid will simply flow from the inlet port through to the outlet port.

These simple valves are more cost-effective compared to their pilot operated counterparts.

  • Pilot operated

Pilot operated valves use an additional chamber to assist in the closing and opening of the valve. This chamber sits above the inlet and an outlet port. It consists of a small pilot hole, which allows the fluid to come in, and a diaphragm, which separates the inlet port from the outlet port.

When the fluid flows from the inlet and through the small pilot hole, the pressure created by the fluid pushes the diaphragm down. This reinforces the separation between the two ports. However, when the plunger is raised, the fluid from the diaphragm chamber will flow through to the outlet port. This reduces the pressure in the chamber and increases the pressure in the outlet port. The result is the diaphragm lifting, finally allowing a full flow of fluid through the valve.

Due to its use of pressure, pilot operated valves require less energy to operate. Therefore, they are much more efficient compared to direct acting valves.


Now that you know how solenoid valves work, let’s take a look at how they are used.

  • HVAC Systems

You’ll find solenoid valves used in many refrigeration systems. Most commonly, they are used to stop or start the flow of refrigerant through a thermal expansion valve (TEV).

As an example, let’s say an air conditioning system has two TEVs and two evaporators. TEVs directly regulate the amount of refrigerant that goes into their respective evaporators. In short, it’s the valve that controls the temperature of the air that circulates a building or a home.

If the weather isn’t too hot, an air conditioning system can usually provide enough cool air through one TEV and evaporator. During such conditions, a solenoid valve will need to stop the flow of refrigerant through to the second TEV. However, once summer arrives, the cooling load will simply be too much for one TEV and evaporator. As a result, the solenoid valve will need to open, allowing the flow of refrigerant through both TEVs.

  • Production lines

Solenoid valves are also used in production lines. To give you an example, let’s take a look at their use in the drinks industry. Solenoid valves are useful in this setting as it can autonomously control the amount of fluid that goes into a bottle. Workers can set up a valve so that it closes after a bottle reaches its capacity and opens up again once the next bottle is ready. This speeds up the process and offers a level of consistency that manual labour cannot.

Solenoid valves are also used to mitigate leakages. If the system detects a leak somewhere in the line, a signal can be sent to a nearby solenoid valve to stop the flow of fluid. This reduces the amount of wastage but it also ensures that the workers and equipment are safe. This is particularly important if a system is handling harmful substances.

That was just a quick guide on solenoid valves. Given the right application, solenoid valves can make a system safer and more efficient.

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Shlomi Atash

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