3 Reasons Why Dryer Booster Fan Keep Running

A dryer booster fan is an optional device that you may find necessary to install in your home. Having a dryer booster fan can make your clothes dryer work much more efficiently in some circumstances.

Dryer booster fans can malfunction and keep running even when the dryer is OFF. In 85% of cases, the cause of the problem is usually a bad pressure switch, but it’s also possible that the fan was installed incorrectly or that there is too much ambient airflow in the ductwork.

Today we’ll be going over some of the common issues that can cause your dryer booster fan to stay on indefinitely and the ways you can fix these issues.

We’ll also explain how exactly dryer booster fans work, and we’ll share with you some tips you can use to install your dryer booster fan correctly the first time.

Related: 3 Reasons Why Dryer Booster Fan Is Not Working

What Is a Dryer Booster Fan?

A dryer booster fan is a device that is used to increase the airflow in the exhaust ductwork of your dryer when the dryer is running. If you find your dryer struggles to dry your clothes completely within a reasonable amount of time, installing a dryer booster fan can sometimes make all the difference.

When your dryer is running, it generates exhaust, which contains hot air, excess moisture, and tiny particles of lint. This exhaust gets routed through ductwork which leads to a vent outside your home.

If the exhaust has trouble exiting the dryer, however, this will result in a much less efficient dryer. This usually happens when the ductwork to the exhaust vent is too long or contains too many bends. 

This is when a dryer booster fan comes in handy the most. It works by generating extra airflow in the ductwork, which helps the exhaust reach the outside vent more easily.

How Do Dryer Booster Fans Work?

Dryer booster fans are designed to only run when the dryer itself is running, which saves electricity and also prevents the motor from wearing out prematurely. Dryer booster fans work by using a switch that detects when the dryer is on. This is usually either a pressure sensor switch or a current relay switch.

A pressure sensor switch works by detecting when there is more air pressure in the ductwork, which is caused by the dryer exhaust moving through the ductwork. Air flows into a small tube in the sensor and expands a diagram that hits a contact point, which completes an electrical circuit. Completing this circuit is what turns the fan on.

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A current relay switch, on the other hand, works by detecting when the dryer is using power. When power flows through the current relay, it automatically switches on, and then automatically switches off after it goes a certain length of time without receiving power again.

There are a couple of types of relays you might find in such a device; electromagnetically relays, which use moving parts to physically open and close a circuit, and solid-state relays, which can create a circuit without using any moving parts.

Tips for Installing a Dryer Booster Fan

When connecting a dryer booster fan to your dryer for the first time, it’s a good idea to keep a few things in mind to ensure that you install it correctly the first time.

For one, it’s good to know when you’ll actually need a dryer booster fan to begin with. As we mentioned earlier, dryer booster fans are only really necessary if your dryer ductwork is too long or contains several bends.

In most cases, it’s likely that your dryer will work fine without a booster fan.

It’s pretty easy to determine whether or not you need a dryer booster fan based on the length of your ductwork and the number of bends in it. The maximum length that a straight piece of ductwork can be without requiring a dryer booster fan is generally about 25 feet.

Related: Do Dryer Booster Fans Really Work?

For each bend in your ductwork, you should subtract 5 feet from the maximum possible length. Therefore, if your ductwork has three bends in it, it can only be about 10 feet long before it’ll need a dryer booster fan.

If you do have to install a dryer booster fan, it’s important to install it in the right place along the length of your ductwork. You don’t want to install it too close or too far away from the dryer itself.

Installing it too far away will affect the fan’s ability to generate sufficient airflow in the ductwork; installing it too close to the dryer can cause it to pull lint into the ductwork, which will eventually clog the fan.

How Do I Fix My Dryer Booster Fan?

There are a few things that can potentially go wrong with your dryer booster fan that can cause it to become stuck on. Let’s take a look at some of these problems and go over how to solve them.

Bad Pressure Switch

Not all dryer booster fans use a pressure switch, but if yours does, this is what’s responsible for letting your fan know when to turn on. As we already mentioned, pressure switches work by sensing changes in air pressure caused by the dryer’s exhaust.

Why It Fails:

In most cases, pressure switches tend to fail if they get clogged with moisture or lint. When this happens, however, it usually causes the dryer booster fan to not run at all instead of running all the time.

If it is in fact your pressure switch that is causing the booster fan to run all the time, it’s most likely due to the fact that the pressure switch isn’t adjusted properly.

How to Fix It:

  • You may be able to fix the problem by adjusting the sensitivity of your pressure switch, if possible. Many dryer booster fans with pressure switches allow you to change this.
  • If this doesn’t work, however, then it’s likely that your pressure switch is completely shot, and you’ll have to get a new one.

Bad Current Relay

The current relay is, of course, the other type of switch used in dryer booster fans to tell them when to turn on and off. Current relays generally come in electromagnetically and solid-state varieties.

Why It Fails: 

Both electromagnetically and solid-state current relays can fail, although they tend to fail in different ways. Electromagnetically current relays contain moving parts, and generally fail when some of these parts break or seize up. Solid-state relays don’t use moving parts, and generally fail when they get burnt out.

If your dryer booster fan is stuck in its running mode, it’s far more likely that you have an electromagnetically relay that has gotten stuck in the “on” position.

How to Fix It:

  • If your current relay has failed, there unfortunately isn’t any feasible way to repair it. You’ll have to buy and install a new one if this happens.

Fan Is Installed Improperly

Sometimes, there may not be anything wrong with the fan itself at all; you may have just installed it incorrectly.

Why It Fails:

If you have a dryer booster fan with a pressure sensor switch, it’s important that you install it in the correct position. The orientation of the switch and the sensor tube connected to it can make a big difference when it comes to correctly determining changes in air pressure.

How to Fix It:

  • Double-check to make sure that you installed the fan correctly to begin with.
  • If not, you’ll have to remove it from the ductwork and reinstall it in the correct orientation.

Too Much Ambient Airflow

Once again, this is a problem you’ll only have if you have a dryer fan with a pressure sensor switch. This isn’t a problem with the switch or the fan itself, but more likely a problem with the ductwork or the outside vent.

Why It Fails: 

For a pressure sensor switch to work properly, there needs to be a distinct difference in air pressure within the ductwork when the dryer isn’t running versus when it is. If these two pressures are too similar, the sensor won’t be able to differentiate between when the dryer is running and when it’s off.

Related: Dryer Vent Repair – Should You Fix It By Yourself?

This might happen if there are leaks in the ductwork that are causing more airflow than would otherwise be present. If the outside vent is facing in a direction that tends to get a lot of wind, it’s also possible that wind is actually blowing into the vent, and if the wind is strong enough it could potentially set off the pressure sensors.

How to Fix It:

  • First, check the ductwork and make sure there are no leaks in it. If you find any, seal them with some aluminum tape.
  • If you determine that the problem is in fact being caused by wind getting into the ductwork via the outside vent, the solution will probably be to install new vents that will do a better job of blocking airflow from the outside. 


It’s not too common for dryer booster fans to keep running whether the dryer is on or not, but it does happen on occasion. We hope that after learning what we’ve had to share with you in this article that you have an easier time troubleshooting any issues with your dryer booster fan in the future.

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