Q&A / 

Exhaust Fans for Kitchen and Bath

Indoor air pollution is real! All you have to do is stand in my kitchen on a Sunday morning when I cook breakfast for my family. There is smoke everywhere. In fact, the EPA has instructed me to install scrubbers on my kitchen exhaust stack. Evidently the people downwind from me are complaining.

Seriously, cooking, bathing and sanitation activities create airborne pollutants that should be exhausted to the exterior of your home. This is especially true if you live in a modern airtight home.

This is demonstrated to me every winter when my wife cooks my favorite vegetable soup. The aroma lingers in the house for days. This is an indication that these "favorable" pollutants are constantly circulating throughout the house. That's OK for vegetable soup, but not for the bacon smoke I produce!

Minimum Standards

Certain rooms of your home require different ventilation practices. Kitchens need more ventilation than any room in the house. The Home Ventilating Institute recommends that the air in your kitchen should be replaced 15 times per hour. Bathrooms require a minimum of eight air changes per hour. Other rooms, such as laundry, family room and basements require a minimum of six air changes per hour.

Note that these recommendations are minimum standards. If you want to ventilate more, feel free to do so.

Replacement Air

All too often people install exhaust fan systems without any thought as to replacement air. Replacement or makeup air is the air which must enter your house to replace the air you are exhausting. If you didn't do this, your house might implode. Just kidding! However, problems can arise, especially if you live in a modern airtight house.

If you do not provide for adequate makeup air, your fan will satisfy the pressure difference by sucking air from unwanted locations. For example, you may turn on that new high powered exhaust fan and draw furnace or hot water fumes right back down your chimney! This is called backdrafting.

I made a fool of myself one Thanksgiving at my sister's house demonstrating this phenomenon by mistake. For some reason, I decided to turn on her whole house fan. These are powerful exhaust fans. Well, since all the windows were closed, the fan decided to get its makeup air from the path of least resistance. That happened to be the fireplace chimney. It just so happened that a fire was in progress. Smoke and ashes were sucked into the dining room! What an idiot I was!

You can provide for makeup air in many ways. Some companies ( Tamarack Technologies) make nifty devices that you can install in a side wall to provide for this purpose. You can also make your own like I did. I simply cut a slot in the side of my band board (floor rim joist on top of foundation) and covered it with a screened air register cover.


Click here to watch the video on makeup air vents.

Types of Systems

There are two basic types of ventilating systems: point of use and centralized or remote. Most people are familiar with the point of use fans. These fans ventilate a specific room or area. Most kitchen range hoods or bath fans work this way. They have a motor in the fan housing (or it is sometimes in the wall or at the end of the duct run). This fan motor spins and exhausts that area.

Other systems are available that work like my central vacuum cleaner. In these setups, there is a centralized exhaust motor and fan located in a remote location. When turned on, it sucks the air out of every room that is equipped with a suction duct.

I have found that it is often best to vent fans and dryers through the roof. I urge you to watch this video of mine to see how easy it is to install the correct vent-cap flashing on a roof. Have no fear - if done right you will have no leaks.

Where does the air go?

In many instances, the air from exhaust fans goes to the wrong place. I have seen both kitchen and bathroom exhaust fans that simply discharge the air into an attic space. This is a huge mistake.

In the case of a kitchen exhaust fan, this practice represents a major fire hazard. If you have a fire develop on your stove while the fan is on, the flames and fire are directed into the attic. If the duct and the roof rafters, etc. are greasy the fire will spread quickly. The fan actually begins to operate like a blow torch.

Bathroom and kitchen exhaust fans also can transmit large volumes of moist humid air into the attic or other confined spaces. This can lead to wood rot, mold and mildew.

The trick is to simply exhaust this air from the fans directly to the exterior of your home. Every major fan manufacturer makes special termination "caps" that attach to the final piece of ductwork. These caps can be installed in a roof, sidewall or a soffit.

Ducting Practices

The fan that you purchase has only so much power. It is often rated as so many cubic feet per minute (CFM) at a given static pressure. Static pressure is a concern. It is a measurement of the power of the fan. In other words, if two fans are rated at the same CFM but list different static pressures, the fan with the higher static pressure rating will be more powerful.

The concept of static pressure is really quite easy. Think of the fan and duct assembly just before the fan turns on. Between the fan blades and the exterior of your home (end of the duct) lies a volume of air (the calm air in the exhaust duct). This duct may be short, long or have a number of bends. The more air that is in this system, the greater the load (static pressure) on the fan. If there is too much air to push, the fan blade will simply sit there and spin.

This is why you must follow the manufacturers instructions regarding duct size, total length of duct, number of bends, etc. If you decide to become an amateur mechanical engineer when installing your system, don't complain to me or the manufacturer if your fan doesn't seem to work.

In Conclusion

Exhaust fans will help you to maintain a great indoor environment. You just need to select the proper sized fan, install it correctly and provide for some makeup air. Sounds too tough? Not really.

Column B98


3 Responses to Exhaust Fans for Kitchen and Bath

  1. I was thinking of adding a second booster fan to the attic duct work for my micro wave/stove exhaust fan. My concern is first, is this a practical idea and secondly, should both fans be of equal cfm. I'm thinking that if the booster fan is more powerful it may cause a cavitation in the first fan and cause noise across the blades as well as a stalling (BLOCKAGE) of the air flow. If the more powerful fan is on the hood end I may have a reversal of this problem (if indeed there would be a problem).

    Another option suggested by a builder I know was to vent the air down through the kitchen floor and out through the basement wall. I live in a raised bungalow. Any feed bac would be most appreciated.

    Thank you.


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