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A bathroom fan is not designed to be powerful enough to remove all of the steam while showering. Instead, it is designed to be kept running even after showering to dry out the bathroom and move moisture out of the building envelope.
A bathroom fan keeps your home free from moisture-related problems. Unfortunately, most people don’t use them regularly and not for long enough periods after taking a shower.
The exhaust fan would need to be many times as powerful as common bathroom fans in order for it to remove all of the steam while showering. This would waste an enormous amount of energy since much more heated or cooled air would be exhausted out of the house. Find out how exactly a bathroom fan works.
The bathroom fan should be kept on at least 30 minutes after taking a shower. This will clear any remaining moisture in the bathroom and eliminate the risk of peeling paint, mold, or mildew.
90% of problems with excess moisture in the bathroom are related to not running the bathroom fan long enough.
If you already keep your bathroom fan running for at least 30 minutes after taking a shower, and still get moisture problems, then keep reading. Let’s find out why the bathroom fan is not efficiently removing steam and moisture from your bathroom.
Bathroom Door Gap is Not Sufficient
First, let’s begin with the basics. In order for the bathroom fan to work, replacement air must come into the bathroom. If there is no gap under the bathroom door, fresh air can not easily flow back into the room.
If the fan is turned on and the door closed the fan will have to fight to pull air through small cracks. This will make the bathroom fan much less effective.
This gap will allow fresh air to flow into the bathroom when the bathroom fan is running. An easy way to test if the gap is sufficient is to close the door quickly, if there is air resistance when closing it, the gap is too small.
Often doors are originally installed correctly. However, during renovations, the floor height can rise. The extra height can be caused by underfloor heating, thicker tiles, or just the fact that new tiles were mounted on top of the old ones. It is actually quite common for contractors to forget to cut the door after work in the bathroom is completed.
How Can I Increase the Door Gap
Most doors can be safely cut by 1/2″ from the underside. Keep in mind that there are most likely nails or screws in there so don’t use your best saw blade to make the cut.
I recommend using a jigsaw with a clean wood blade. Jigsaw blades are relatively inexpensive compared to circular saw blades. If you are looking to buy a jigsaw, it’s ok to buy a cheap one, just get quality blades. The blade is what makes 90% of a difference.
If you don’t have any tools at home here is everything you need to cut the door by 1/2″
|Tool||View on Amazon|
|Clamps to hold the guide||View|
|Piece of straight wood for a guide|
If for some reason it is not possible to cut the bottom of the door to increase the gap, there is another solution. You could cut a hole into the door and install a vent instead. This will allow for plenty of air movement for even larger capacity exhaust fans. Make sure you get a vent grille that matches your door and comes with two sides. A plain white door vent will fit most doors.
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Bathroom Fan is Not Vented
Vented bathroom fans transport steam out of the building. Ductless fans circulate air through an activated carbon filter, this does not remove steam, instead, it will filter some of the odors to keep the bathroom smelling better. They are usually used for pantries and quarter baths where not a lot of moisture is created.
Ductless fans are sometimes falsely advertised as being able to eliminate moisture, this is simply not true. If you recently moved into a new place and are having moisture problems then this might be your issue.
The previous owner could have been fooled by the false advertising and bought a duct-free fan for the bathroom.
Even the name “duct-free bathroom fan” is misleading. It should be called a duct-free toilet vent instead.
How To Check If The Bathroom Fan is Vented
A duct-free bathroom fan will have a replaceable activated carbon filter. This is usually accessed by removing the plastic grille. If you find a filter inside the fan, then most likely it is a ductless bathroom fan.
The good news is that you have found the source of poor ventilation. The bad news is that if there is no duct behind the fan, installing one could be tricky and expensive.
Related article: 6 steps to finding the right bathroom fan
Bathroom Fan Not Powerful Enough
So, by now you have checked the door gap. If the gap is at least 1/2″ then the problem could be that the exhaust fan is not powerful enough to exhaust the moisture out of the bathroom before it starts causing problems.
I have made a very simple CFM calculator which will tell you the minimal CFM your bathroom fan should have. This is based on the size of the room and 8 air changes per hour as the Home Ventilating Institute recommends.
You can compare this number with the CFM of your bathroom fan. Keep in mind that this is the absolute minimum value. It is generally recommended to install an exhaust fan with slightly higher CFM.
You should be able to remove the cover from your exhaust fan to reveal the model number. It’s usually clipped on and can be removed by simply pulling down. By googling the model nr you will find out the CFM.
CFM – Cubic feet per minute
If you are unable to find the model number and the fan is very old, there is a good chance that it needs to be replaced anyway. Before jumping in and ordering a new exhaust fan, do some research. I have written a guide that will make choosing a bathroom fan as simple as possible without missing anything important.
Fan is Turned Off Too Soon
Some people think that it is enough to run the bathroom fan only when showering. This is simply not true. There will be so much moisture in the air after showering that the fan must run at least 30 minutes after taking a shower.
Take a look at the moisture level chart from my bathroom. You can see that it takes more than 1 hour for the moisture level to drop back down to normal. This is because water is slowly drying from all the surfaces and evaporating. By leaving a towel to dry in the bathroom the time will be extended even further.
Often the bathroom fan switch is right next to the light switch. This means that it will likely be turned off together with the light. Doing this will keep most of the humidity in the house. Eventually, this will lead to moisture-related damage all around the house. The most affected areas are the attic and cold corners of the bathroom where moisture condensates and creates a perfect breeding ground for mold.
Regular on/off switches are not suitable for bathroom fans, as people often forget to even turn it on. There is only one solution that will guarantee that the bathroom fan will be turned on. Regardless of human input, and that is a humidity-controlled switch.
A humidity-controlled switch is almost mandatory for kids’ bathrooms as they often forget to turn the exhaust fan on. It is also a wise choice in a rental property, as tenants usually don’t care about future moisture problems and don’t use the bathroom fan at all.
Install A Humidity-Controlled Switch To Keep The Fan Turned on as Long as Necessary
A humidity-controlled switch will automatically turn on the bathroom fan whenever the humidity level rises over a set point. The humidity level at which the fan turns on should be adjusted twice a year.
During wintertime, the relative humidity in the home will be much lower compared to the summer. This means that the humidity level must be adjusted in order for the bathroom fan to run when needed. My bathroom fan is adjusted to 45% relative humidity during the winter and 55% during the summer. This is just an example, don’t copy my settings, as it will be different from where you live.
An easy way to tell if the humidity sensor needs adjusting in the summer will be that the fan is running all of the time. This means that the turn-on humidity level must be raised.
A twice-a-year adjustment is a small price to pay if this means that the fan will work automatically for the rest of the year and don’t have to be turned on every time you take a shower. Most humidity-controlled switches have a button to turn the fan on manually as well. This is useful when cleaning or after using the toilet.
My Tips on Choosing a Humidity Controlled Switch
- Make sure that the humidity adjustment is easy since it has to be done twice a year
- A button for manual operation
- Don’t buy the cheapest switch because the humidity sensors in them don’t last very long
- Consider a motion sensor that will turn on the fan every time someone is in the bathroom (Avoid this if your bathroom fan is not quiet)
Topgreener TDHOS5 is an excellent choice. It will give your existing bathroom fan the same functionality as a $200 bath fan.
- FAN TIMER SWITCH Dual-tech in-wall humidity control switch that controls both bathroom fan and light simultaneously. Dual technology of passive infrared (PIR) motion sensor switch and humidity sensor...
- BATHROOM FAN OCCUPANCY DETECTOR Converts bathroom ventilation fan control wall switch with this smart humidity control switch, turning fan ON automatically when humidity exceeds preset level and...
- HUMIDITY SENSOR SWITCH features simple controls and operates in two modes: Occupancy Mode (auto on/off lights, auto on/off fan), and Vacancy Mode (manual on/ auto off lights, auto on/off fan)....
- MOTION SENSOR Two single pole switches to control bathroom light and fan separately; Compatible with LED light bulbs; Energy savings of up to 40% with motion sensing control; Complies with CA Title 24
The Bathroom Fan is Dirty
Dust and lint build-up will limit the amount of air that will pass through the duct. This will make the exhaust fan less effective. The fan blades are moist when exhausting steam out of the house. There is always some dust in the air. This dust will be deposited on the fan blades and over time it can cause problems.
To be honest, the fan would have to be extremely dirty for it to affect air flow enough to cause it to not function properly. However, if you have not cleaned out the fan in the last half a year, or have moved into a new place it’s a good idea to do it anyway since a dirty lint-filled bathroom fan is a fire risk.
Related Article: How to Clean A Bathroom Fan
Duct is Too Small or With Too Many Turns
The size of the duct and how complicated(how many turns) it is will increase the static pressure of the duct. Higher static pressure will make it much harder for a bathroom exhaust fan to push air out of the house.
If you pair a small complicated duct run with a bathroom fan that is barely powerful enough then the airflow will not be sufficient to remove moisture at an acceptable rate. This means that the fan needs to be turned on for more than the recommended 30 minutes to dry the bathroom. However, not many people leave their fans turned on for that long.
In some extreme cases, the bathroom fan could even struggle to overcome the static pressure and not exhaust any air at all. This can be tested by holding a tissue in front of the bathroom fan. The fan should easily be able to hold the tissue up. If it can’t there is a very good chance that the fan is underpowered for the current duct.
Using rigid smooth-walled duct is favorable compared to flexible ducting. The smooth-walled duct has a higher airflow rating compared to a similarly-sized flexible duct. However, if the run is short and the duct is correctly sized then it is perfectly fine to use flexible ducting.
High-end Panasonic bathroom fans(amazon link) use ECM motors. They will adapt to the higher static pressure of a complicated duct run and still provide the quoted CFM. This is done by increasing the amperage when the static pressure is high. They work effectively near the quoted CFM at up to 0.375 “w.g static pressure. Normally the static pressure is only given for 0.1 and 0.25 inches w.g
Note: It is always better to make the duct run shorter and more direct when practical. Installing a more powerful fan to an undersized, too long, and too complicated duct will waste energy as it requires more electricity to exhaust the same amount of air.
How to Stop Condensation on Bathroom Mirror
Even if the ventilation is working as it should, there is a good chance that the bathroom mirror will fog up after a long hot shower. This is especially true for small bathrooms as the steam does not have time to spread in the air before reaching the mirror.
Luckily there are simple steps that can be taken to reduce, or completely eliminate the bathroom fan from steaming up.
Increase Bathroom Temperature
The easiest method to reduce condensation on a bathroom mirror is to simply increase the bathroom temperature. Usually, it is enough to bump it up a couple of degrees.
Condensation occurs when moist air hits a surface cold enough for the moisture in the air to condensate. If the surface is sufficiently warm, condensation does not occur.
Increase Air flow
By exhausting the moist air before it has time to condensate will limit the amount of steam that will be on the mirror. However, if the bathroom is cold, it is nearly impossible to avoid condensation by increasing the airflow. Even the slightest amount of moisture in the air will condensate on the cold mirror.
If the mirror is fogging up just a little, then upgrading the bathroom fan could help. If the fan is to be replaced soon anyway then it’s a double win 🙂
Install a Defogger Pad
A defogger pad is an electric heater that is placed to the back of the bathroom fan. It will heat up the mirror so that the moisture won’t condensate on it.
This is the most effective solution, it will keep the bathroom mirror clear even if the room is filled with steam.
- ANTI-FOG MIRROR PAD: Turn any wall hanging mirrors into a anti-fog mirror
- EASY TO INSTALL: The self-adhesive backing of the mat allows for direct application to the back of any of your existing bathroom wall-hung mirrors in just minutes and uses heat to keep fog from...
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It should be wired to be turned on at the same time the bathroom light is turned on. This way it will turn on automatically each time you take a shower.
This will be more cost-effective compared to increasing the temperature of the entire bathroom. It will use fraction of the energy.
A bathroom fan is not designed to remove all of the steam while showering, instead, it will slowly exhaust the moist air during and after the shower. It will take between 30 and 60 minutes to clear the bathroom of the excess moisture created by an average shower. If the bathroom is still damp after 60 minutes there could be issues with fresh air flow, bathroom fan capacity, or ducting. Most people don’t run their bathroom fans long enough to exhaust all of the moisture.
I hope you found this article helpful, please vote below and let me know how I can improve in the future.
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