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Bathroom ventilation is very important to prevent any moisture damage to the building. It will also help improve indoor air quality and keep the bathroom smelling fresh.
The bathroom fan capacity(CFM) should be matched to the size of the bathroom itself. One might think that just buying the strongest fan possible is best. But there are many reasons why this is not true.
A Bathroom fan should not be overpowered since this will waste energy, create excessive noise and could even pull noxious carbon monoxide into the home. It is OK to get a 20-30 % higher capacity exhaust fan than specified by the CFM calculator.
A 20-30% stronger bathroom fan will exhaust the moist air slightly faster. Anything above that is simply wasting energy since the water can’t evaporate fast enough. It will take at least half an hour for the moisture to clear. It makes no difference if the bathroom fan is 80 or 300 CFM.
Now let’s find out all of the reasons why too much CFM can be bad.
High Power Fans Will Be Noisier
Most people prefer quiet bathroom fans. Since bathrooms are often next to the bedroom the noise from a loud bathroom fan can disturb sleep.
A whisper-quiet bathroom fan is especially important for people who shower right before bed and find it difficult to fall asleep.
My top 9 quiet bathroom fans (all fans on the list are under 0.5 sone)
High powered fans tend to make more noise. There is nothing to do about it. Moving air makes noise, and moving more air makes more noise 🙂
Even if the fan motor itself is whisper quiet, the moving air from a high CFM fan will make considerable noise.
Let’s say you have used the CFM calculator and found out that the optimal bathroom fan CFM for your bathroom is 110 Cubic feet per minute(CFM).
A quality 110 CFM fan(amazon link) will be almost silent. This particular fan I have linked to is rated at 0.3 Sones. There is absolutely no way it will be heard in the next room.
Compare that to the noise level of 2.9 Sones for a 300 CFM fan and you can easily see the massive difference.
2.9 Sones is about 10 times louder compared to 0.3 sones.
Wasting Energy With Excessive Ventilation
If you live in a climate where your home does not need to be neither cooled or heated you can ignore this.
Increased CFM will not dry out the bathroom any faster, so the fan will still have to run for the same amount of time. However, since the airflow is higher a lot more energy is lost. This is true whenever the home is cooled or heated.
One might think that installing a 200 instead of a 100 CFM fan will dry out the bathroom twice as fast. Unfortunately, it does not work that way. While the bathroom will dry out slightly faster, it will not be enough to justify doubling the airflow and at the same time wasting a lot of precious heated or cooled air.
Think of it like this; Say outdoor temperature is 10°F(-12°C) and indoor temperature is kept at 74°F (23°C).
Increasing the bathroom fan capacity by 100 CFM over what is recommended will rush 100 cubic feet of 10°F(-12°C) air into the house.
This will cool down the house and the heating system must work much harder to keep up. It will put a significant dent in your energy bill if the fan is used for extended periods of time.
One might even stop using the bathroom fan to save money on the energy bill. This will render the bathroom fan totally useless. It is much better to get an exhaust fan with the correct CFM. This way no energy is wasted and the bathroom is nice and dry.
The water that is left on the walls and floor must evaporate before the bathroom fan can be turned off. Installing a stronger exhaust fan will not make the water evaporate any faster.
This is why there is an optimal bathroom fan CFM that can be calculated. By increasing the CFM by more than 20% to what the calculator suggests, you will see diminishing returns when it comes to bathroom drying time.
During wintertime when the outside temperature drops below freezing any outside air blowing into the house will be very uncomfortable.
Since the same amount of air that is exhausted out of the house via the bathroom fan must be replaced the returning cold fresh air will create cold drafts.
If the bathroom fan is reasonably powered the cold draft will be so small that it won’t be noticeable.
However, if the bathroom fan is way too powerful. Say something like 300-400 CFM instead of 100 CFM. Then the cold draft will be more noticeable. Depending on how the air moves in your home, it could even draft right over you when relaxing on the sofa.
Backdrafting Fireplace, Water Heater or Furnace
A bathroom fan exhausts hot humid air out of the house. However, the same amount of air must come back somehow. If there are not enough vents to provide fresh air, the air will be pulled in from the chimney.
Old buildings were not built very airtight since energy costs were much lower compared to what they are today. This means that the fresh outdoor air was coming in from cracks in the windows and walls. This wastes a lot of energy, but in this case it will eliminate the danger of backdrafting.
It is very difficult to create enough of a vacuum in an old building to cause back-drafting since there are so many places where air can enter the house..
However, it is a different story with new buildings.
Modern homes are built very airtight to meet strict energy efficiency standards. This means when using a powerful bathroom exhaust fan there will be a vacuum inside the house. When you combine this with a fuel-burning appliance such as a fireplace, you will end up with a room filled with smoke and smoke alarms going off.
The problem is even worse when combined with a range hood. Since they tend do be very high capacity.
If you are trying to replace a bathroom fan in a modern home with one that is much more powerful than the previous, consider adding an extra fresh air intake. This will eliminate the back drafting issue.
Things To Consider When Installing A Stronger Bath Fan
If you are still considering replacing your current bathroom fan with one that is much more powerful, there are some things that you need to keep in mind.
Make sure you check out my list of top 9 high-power bathroom fans. Just a friendly reminder, most of the bath fans on the list are WAY too powerful for any normal bathroom 🙂
A higher capacity bathroom exhaust fan will need a larger duct. You can’t just slap a 300 CFM bath fan on a 4″ duct.
Well…you could, but it will be loud, inefficient and it won’t meet the specified CFM.
There is no point in connecting a high capacity bathroom fan to a small duct. If you don’t want to replace the duct, it is best to install a smaller bath fan instead.
Related Article: Where Should The Bathroom Exhaust Fan Be Placed
Extra Fresh Air Vent
As we discussed earlier, installing a larger bath fan without considering where the extra fresh air will come from can lead to serious problems.
This can be quite easily remedied with an extra fresh air vent(amazon link). Just make sure you don’t install this right next to the bathroom fan, as this would just circulate the air right in front of the fan.
Generally, it is good to place a fresh air vent in the living area, such as the living room or the bedroom. This way the moist air will be sucked out by the bathroom fan, and the fresh air will come directly into the living area via the fresh air vent.
Calculate The Correct CFM
Even if you are set on installing a high power bathroom fan, it is still a good idea to check what is the optimal CFM for your bathroom.
This can be calculated by various methods. I have created a calculator that will calculate the correct air flow needed based on the square footage and height of the bathroom. The calculator uses guidelines set by the Home Ventilating Institute.
Why Stronger Bathroom Fan Could Be Beneficial
So far we have discussed all the reasons why an overpowered bathroom fan can be bad. As it is with everything, there are some benefits of installing a high-power bathroom fan.
While a stronger bathroom fan will not dry out a bathroom considerably faster, it will remove odors much faster. And by odors I mean the human kind 🙂
If you install a speed controller, you could use the bathroom fan at lower CFM most of the time, and when needed would have the option increase the ventilation. I have written an extensive article about how to slow down bathroom fans.
Bathroom Cleaning Chemicals
When using harsh chemicals to clean the bathroom it is nice to crank up the high CFM bathroom fan and have instant fresh air.
Exhausting Paint Fumes
Even when painting in another room, a high power fan can exhaust the paint fumes.
All you have to do is close all other windows/doors turn on the bathroom fan and open one window in the room that is being painted. This will effectively pull fresh air into the room and paint fumes out.
It is generally not a good idea to install a bathroom fan that is much higher capacity compared to what is optimal. Doing so will waste energy, create cold drafts and could even cause a fireplace to backdraft and pull smoke into the house.
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