Home ventilation

Everyday life keeps adding things to the air you breathe at home. Bathing, cooking, washing dishes and even breathing all produce moisture. Exhaust fans in kitchens and bathrooms are there to push moisture, odors and particles out. Many homes are missing a whole-house fresh air system that keeps moisture, dust, pollen and fine particles diluted. Those contaminants can trigger asthma and allergy attacks, as well as other health problems.

EPA puts the order of things simply. The most effective ways to improve your indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air. Filtration can be a useful addition to those two steps. When the weather and the outdoor air allow it, and it is safe, opening doors and windows brings in fresh air. So the fans and vents on this page are a good place to start.

Source: University of Minnesota Extension · U.S. Department of Energy · U.S. EPA

A bathroom fan is there to clear shower steam. Whatever ventilation the rest of the house has, exhaust fans are recommended in bathrooms to remove extra moisture and fumes from cleaning products. An open window should not be relied on for steady bathroom ventilation. By keeping humidity down, the fan helps discourage condensation and the growth of mold, mildew and dust mites.

Fans are rated by how much air they move, in cubic feet per minute (cfm). To meet ASHRAE 62.2 (2010) and the International Residential Code (IRC 2021, Section M1507), a bathroom fan should move at least 50 cfm if it runs now and then, or at least 20 cfm if it runs all the time. ENERGY STAR recommends some headroom: a fan rated at least 70 cfm for the 50 cfm rate, and one that moves more than 50 cfm for the continuous 20 cfm rate. Fan noise is rated in sones, and ASHRAE 62.2 requires 3 sones or less for a kitchen or bath fan you switch on and off, and 1 sone or less for one that runs all the time.

How long the fan runs matters as much as its size. It is recommended to leave the fan on for one hour after a shower, and to run a fan you switch on and off from a timer switch rather than a standard wall switch. A timer lets you set the run to 1 hour, and then the fan shuts itself off. Bathroom fans can also run all the time, and some models have several speeds so they can work either way.

Source: U.S. Department of Energy

A fan only helps if its air actually leaves the house. The fan should be ducted outside, not into the attic, crawlspace, garage, the space between floors or wall cavities. Moisture from bathroom and kitchen fans that empty into an attic or crawlspace can cause mold and damage the wood there. The moist air can condense on cold rafters and roof boards, producing water droplets. Illinois's mechanical code says exhaust air must go outdoors, and may not be exhausted into an attic or crawl space or directed onto walkways. Chicago's energy code likewise says exhaust must leave the building, not end in an attic or crawl space, and cannot sit near a fresh air intake.

The duct itself shapes how well the fan works. Long ducts, many bends and heavily ribbed duct material all cut airflow, and a duct with no insulation can gather condensation. At a minimum, the first three feet of duct from the fan should run straight, since an elbow right at the fan can significantly reduce airflow. A smooth duct such as galvanized sheet metal or PVC, in the size the maker specifies, is the choice, and flexible aluminum duct should be stretched tight. Outside, the wall cap should have a damper that closes when the fan is off. The outlet should be at least 10 feet from any mechanical air inlet or at least 3 feet from any gravity air inlet, under Section M1504.3 of the IRC.

Some checks are easy to do yourself. You can look in the attic or crawlspace to make sure your bathroom and kitchen fans vent outside. Vacuuming the fan cover and blades, and making sure the outside end of the duct is not clogged, can help the fan pull better. If a fan empties into the attic, crawlspace or the space between floors, the duct should be replaced or rerouted to the outside. A roofer is typically the best trade for a vent through the roof or a gable wall, and an HVAC technician or skilled handyman can route insulated duct from the fan to that vent.

Source: U.S. Department of Energy · UpCodes (code text) · University of Illinois Smart Energy

Cooking puts steam and odors into the kitchen air. A kitchen exhaust fan sends steam and cooking odors outdoors. EPA suggests turning the range hood on whenever you cook and leaving it on for 10 to 20 minutes after you finish. Cooking on the back burners when you can helps capture more of what cooking gives off. Clean the hood's grease filter routinely, along with your cooktop and cooking surfaces.

Not every hood removes air from the house. Building science guidance from PNNL says recirculating fans, which pull air through a filter and blow it back into the room, are not acceptable. Illinois's mechanical code does allow a listed ductless range hood when it is installed to the maker's instructions and the room is ventilated by other means. Where a hood vents outdoors, the code requires a sheet metal duct of galvanized steel, stainless steel, aluminum or copper, with smooth inner walls, airtight, and with a backdraft damper. Kitchen exhaust must also stay separate from all other exhaust systems. A duct that is too small can trap grease, creating a fire hazard.

Size matters here too. To meet the IRC (2015), a kitchen fan you switch on and off should move 100 cfm or more, and one that runs all the time 25 cfm or more. The duct, the outside cap and the installation can bring the measured airflow below the factory rating. The most direct route outside, with as few bends as possible, helps.

Source: U.S. Department of Energy · U.S. EPA · UpCodes (code text)

Air pushed out of a house has to be replaced from somewhere. Illinois's mechanical code requires makeup air, at about the same rate as the exhaust, for a hood system that can exhaust more than 400 cfm. That makeup air system must be able to close, and must start and run automatically with the hood. Some homeowners install commercial-style ranges with hood fans rated as high as 1200 cfm. Fans that strong can easily backdraft a furnace, water heater or fireplace that is not sealed combustion. Backdrafting happens when more air is exhausted to the outside than is brought in.

The code states the general rule too. Where a mechanical exhaust system removes more air from a room than a supply system brings in, adequate makeup air must be provided. If you are planning a powerful hood, ask about makeup air before it goes in. The page on venting and chimneys explains how gas appliances send their exhaust outside.

Source: UpCodes (code text) · U.S. Department of Energy · U.S. EPA

In a Chicago winter, every bit of outdoor air you bring in has to be heated. A heat-recovery ventilator (HRV) is a packaged unit with fans that moves heat between two separate airstreams. An energy-recovery ventilator (ERV) is a heat-recovery ventilator built to move both heat and moisture. In winter, they cut the cost of heating fresh air by passing heat from the warm air going out to the cold air coming in. Because an ERV also passes along some water vapor, it helps the humidity inside stay steadier.

A few points matter if you are considering one. To keep its electricity use down, a stand-alone, fully ducted installation is recommended. Units with electric resistance heaters will not qualify as ENERGY STAR. If a fresh air intake is ducted to your furnace or air handler, a filter belongs at that intake, in a spot that is easy to reach. Filters should be replaced or cleaned often. A technician can tell you how one would fit with your ducts.

Source: ENERGY STAR · U.S. Department of Energy