How Long Should Bathroom Fan Run After Shower


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The Home Ventilation Institute recommends running a bathroom fan for at least 20 minutes after showering. This ensures all the moisture has been pulled from the bathroom and vented out of the ducts. Using a bathroom fan timer switch can ensure that your fan always runs the optimal amount of time.

We all know the importance of running a bathroom exhaust fan during a shower or steamy bath. But did you know that turning your fan off right after you have finished showering means that it is only partially effective?

What makes running a bathroom fan after a shower necessary? What does it mean if a bathroom is still steamy after the recommended time period? Should a bathroom fan be run continuously for the best protection against moisture build-up? I provide you with answers to these questions and more in the following sections.  

Recommended Bathroom Fan Ventilation Duration 

According to the Home Ventilation Institute (HVI), for a bathroom to be adequately ventilated, it needs to undergo approximately 8 air changes per hour. The HVI recommends 2-3 air changes or running the fan for 20 minutes after showering to remove all the steam from the bathroom after a shower.

Why Should the Fan Be Left to Run After a Shower?

There are two primary reasons why the fan should be allowed to run once you are done in the shower.

Steam Is Typically Generated Faster Than It Is Exhausted

Most people switch on the fan right before getting in the shower (or a few minutes after realizing they forgot) and switch it off immediately after the shower. However, this would only work if the rate of steam production directly matched the rate at which air is exhausted. In reality, steam is generated faster than it is exhausted.

Furthermore, it is not only the steam you have to consider. If there is moisture on the walls or other surfaces as a result of condensation or splashing, this moisture can evaporate into the air in the time following your shower.

This does not look like rolling clouds of steam, but it is, nonetheless detrimental to your bathroom. Running the bathroom fan for 20 minutes longer than you are in the shower allows moisture removal while the bathroom surfaces dry.

If there’s anything that can cause a list of damages to your bathroom, it is moisture, so if an extra 20 minutes can save us from warped doors, rusted fixtures, peeling paint and wallpaper, and growth of mold, then I, for one, am not complaining!

Running the Fan Ensures the Air Is Exhausted Outside

The International Residential Code, Section M1505.2 prohibits exhausting bathroom and toilet room air to anywhere other than outside of the house. This involves the use of various lengths of ducting.

Turning the bathroom fan on creates the pressure system required to move the air along the duct and out of the house. However, if you turn off the fan prematurely, the moist air remains in the duct and doesn’t get completely exhausted, or it may take longer to exit the ductwork.

Over time, this can result in issues like rust or mold inside the ventilation system. This can end up compromising indoor air quality and putting your family members’ health at risk of all kinds of diseases.

What if the Bathroom Is Still Steamy After 20 Minutes?

The exhaust fan is there to remove the steam completely from your bathroom. If the bathroom is still steamy after 20 minutes, then it indicates a problem with the ventilation in the bathroom. Let us look at the various reasons that could cause this.

Check out the graph below. This is from my home. I used to have an underpowered bathroom fan. It took nearly 2 hours for the humidity level to normalize after a shower.

Is the Fan Strong Enough for the Bathroom?

When it comes to deciding on an exhaust fan, you must keep the size of your bathroom in mind. If a fan is too small for your bathroom, it will be incapable of removing the steam efficiently within the 20 minutes.

The size of your bathroom determines the appropriate CFM (cubic feet per minute) rating of the fan you need in your bathroom. The ideal airflow needs to be at least 1 CFM per square foot of bathroom area.

But don’t be frightened off by the idea of doing math! I have created an easy-to-use CFM Calculator that will tell you in a few seconds the exact size you need to install for efficient ventilation.

Fan May Be Functioning Sub-Optimally

If your bathroom fan is sized correctly, but there is still steam lingering after 20 minutes, it can indicate that your fan is not functioning correctly.

Sometimes this is the result of age (10 years is the average lifespan of a bathroom exhaust fan), which causes natural wear and tear on the motor, electrics, blades, etc. But it also might be something as simple as dirt.

Like every other electrical appliance, your bathroom fan needs proper care and regular cleaning. The fan blades are usually moist while venting the steam out of the house. Over time the dust in the air deposits on the blades and affects the fan’s efficiency.

You must clean your bathroom fan at least once every six months. Don’t forget that a dirty and lint-filled bathroom fan is not just inefficient, but it is also a fire risk.

Fan Location Can Make a Difference

Technically, the bathroom fan will vent the moist air outside no matter where it is placed. But for maximum efficiency, it’s best to put the bathroom fan on the ceiling in or near the shower.

Hot air rises, so ceiling insert fans make a lot of practical sense. And placing the fan above or near the shower is also logical as this is the main source of steam, so it is exhausted before it can spread too far.

However, it should be noted that placing the fan directly above the shower can make showering chillier, especially in winter, as it creates a breeze.

If you want to find out more about the best location for your bathroom fan, then check out this article that will surely benefit you.

Temporary Solutions to Ineffective Bathroom Fans

You need to address the underlying cause as to why your bathroom is still steamy after 20 minutes, but until you are able to do so, here are a few temporary solutions that can help:

  • Open the door and window during and after showering.
  • Use a floor dehumidifier.
  • Put out containers of moisture-absorbing powders.
  • Wipe the walls after showering.
  • Use a regular pedestal fan to create airflow.
  • Dry your towels outside the bathroom so that they do not contribute to the moisture content of the air.

Problems With Running Your Bathroom Fan for Too Long

Although running your bathroom fan during your shower and for 20 minutes afterward is important, this does not mean that leaving it on indefinitely is the best option.

There Is a Financial Cost

If used when necessary, the average cost of running a modern, correctly sized fan is $3.5 per year, and for a heated bath fan, it is $113 per year. But if the fan runs longer than required, then your electricity bill will surely rise.

Furthermore, overtaxed fans draw more energy, and continuous use will shorten the fan’s lifespan, necessitating early replacement. 

Using a bathroom exhaust fan too long when the home is being heated or cooled will drastically increase the energy bill. There is considerable heat loss from the running fan. A 20-30 minute runtime to exhaust moisture is OK. However, when the fan is running for hours, there will be significant heat loss.

There Is a Safety Risk

It has been proven that running a fan 24/7 can potentially cause a fire!

In 2017, the US Consumer Products Safety Commission (CPSC) did a detailed study on exhaust fan fires. Based on that study, it’s been made clear that exhaust fans can be dangerous and can cause a fire.

 According to the CPSC study:

 “More than 75 percent (318/423) of the known incidents occurred in a bathroom or restroom. The most likely origin of the incident in these cases was the exhaust fan. The incidents ranged in severity from minor smoke to fire spreading through the structure. Of the 318 incidents that occurred in a bathroom or restroom, almost 80 percent (254/318) occurred in a residence.”

But how does it cause fires?

Well, excessive running of a bathroom fan results in thermal wearing. This, in turn, will eventually cause the fan’s thermal cutoffs to fail. Thus, the fan motor temperatures are no longer controlled or capped, and they can rise high enough to ignite and cause a fire.

In addition, worn bearings and lint build-ups cause the motor to overheat and catch fire.

Install a Timer

Some people purposefully run the exhaust fan excessively under the impression that it is beneficial, while others just forget to turn it off. The best solution is installing a quality timer that will save money and energy and also ensure your fan lasts longer. 

Install a timer switch that is easy to operate and has multiple timers along with up-to-date wiring to comply with the building codes.

Here’s a list of the best bathroom fan timer switches that will surely cater to your needs.  

Dual Speed Bathroom Exhaust Fans

If you want to run your bathroom fan continuously, then you need to purchase a fan that is specifically designed for that purpose. A great choice is a dual speed bathroom exhaust fan.

As the name suggests, these fans function at two speeds. The low speed is two or three times lower than the high speed and is used for continuous ventilation. The high speed is used as a normal bathroom exhaust fan. It is switched on during the shower and switched off 20 minutes after showering. 

An advantage of having a fan run continuously is better indoor air quality. The constant ventilation brings fresh air from outside inside the home while pulling out the humid and stale air from the inside. 

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