Common Ventilation Questions People Ask
I get asked about ventilation systems constantly on forums. Most people want quick answers, so here are the ones that come up most often. I deal with this stuff daily and I am going to give you the straight version without any fluff. Q: How do I size a bathroom exhaust fan? The standard rule is 8 air changes per hour for a bathroom. Take your bathroom volume in cubic feet and divide by 60 to get CFM. So a 7ft by 10ft room with an 8ft ceiling is 560 cubic feet. Divide by 60 and multiply by 8, which gives you about 75 CFM. Round up to the next standard fan size, which is usually 80 or 100 CFM.
Q: What about kitchen ventilation? Kitchens need a lot more airflow than bathrooms. The rule of thumb is 100 CFM per linear foot of cooktop. A standard 30 inch range needs at least 300 CFM. If you have a serious cooktop or a downdraft system, you might need 600 CFM or more. Range hoods that duct outside are way better than recirculating ones. Recirculating hoods just move the same greasy air back into your kitchen and do nothing useful. Q: Can I use a bathroom fan to ventilate my whole house?
No. A bathroom fan moves a tiny amount of air and is designed for spot ventilation. Whole house ventilation requires either an HRV or ERV system, or at minimum a properly sized central exhaust strategy. You need to understand the difference between exhaust ventilation and supply ventilation. Exhaust creates negative pressure which can pull moisture into walls. Supply creates positive pressure which can blow conditioned air out through cracks. HRVs and ERVs handle both and recover energy in the process. An ERV is better in humid climates because it retains some moisture in the incoming air. An HRV strips all moisture, which can dry out your house too much in winter. Q: What is the minimum CFM for a bedroom window AC unit vent? Window units don't really need dedicated ventilation ducts if they're installed correctly. They reject heat out the back. The real issue is making sure your room has enough infiltration or a separate intake so the unit isn't working against a sealed room. In practice, I found this matters more when you seal a room tightly for efficiency. You drop below 0.35 air changes per hour and the AC starts struggling. You can add a louvered vent near the ceiling on the opposite wall of the window. A 12 inch by 12 inch passive vent handles about 50 CFM of natural infiltration, which is usually enough.
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I learned this the hard way. I sealed up a second floor bedroom completely for a home office and the AC would short cycle on high humidity. The space felt muggy even though the temperature read 70 degrees. The problem was that the room had zero fresh air exchange because every crack was caulked. I cut a 6 inch hole in the drywall, ran a short piece of 6 inch flex duct through the exterior wall, and added a backdraft damper. It solved the humidity issue immediately. The AC stopped short cycling and the room actually felt drier and more comfortable. Q: Should I vent my dryer outside or use a recirculating dryer? Vent outside. Always. Recirculating dryers are a scam. They filter lint but they do not remove moisture. You will get mold in your walls within a year. Dryer vents should be as short and straight as possible. Every elbow you add reduces airflow significantly. A 90 degree elbow is worth about 5 feet of straight duct in terms of friction loss. If your vent run is longer than 25 feet, you need to go up a diameter or add a booster fan. Lint buildup in long runs is a fire hazard. Clean the vent pipe at least once a year. I once inspected a house where the dryer vent went through the attic and was completely blocked with lint. The dryer was overheating and the lint was warm to the touch.
Q: How do I know if my HVAC ventilation is balanced? You check by measuring airflow at each register. A proper balancing act means every room gets roughly the airflow it was designed for. The problem is most installers just set the registers to equal open positions and call it done. Open a register fully and you might get 80 CFM. Close it halfway and you might still get 50 CFM because the duct is undersized. The real test is using a anemometer at each register and comparing it to the design specs. If one room is consistently 20% over and another is 30% under, you have an imbalance. Adjust the dampers in the ductwork, not the registers at the wall. Register dampers cause turbulence and reduce airflow more than you think. Q: Is a whole house fan worth it?
They work well in dry climates where the night air is cooler than the day. You run the fan for 15 to 20 minutes in the evening and you can drop the indoor temperature by 10 to 15 degrees. This cuts AC usage significantly. In humid climates, whole house fans pull in moist air and make your house feel like a swamp. They also require enough attic ventilation to handle the volume of air being moved. A 2000 CFM fan needs at least 200 square inches of net free vent area in the attic. If your soffit vents are blocked by insulation, you have a problem. Check your attic vents before buying one. Q: What size fan do I need for a 12x14 utility room? That room is 1344 cubic feet at 8 foot ceilings. At 6 air changes per hour for a utility room, you need about 134 CFM. A 150 CFM fan would be the right choice. Utility rooms often have water heaters or furnaces that need combustion air. Check local code requirements. Some jurisdictions require make-up air for any exhaust fan over a certain size. If you exhaust more air than you replace, you create negative pressure that can backdraft your water heater. That is dangerous. Carbon monoxide from the water heater can enter the living space.
Q: How often should I replace HVAC filters? It depends on the filter and your environment. A basic 1 inch fiberglass filter in a normal house needs replacement every 30 days. A 4 inch pleated MERV 8 filter might last 90 days. MERV 13 and higher can clog faster because they trap more particles. If you have pets, replace filters more frequently. I once saw a system with a MERV 16 filter that was completely clogged after 6 weeks in a household with two dogs. The airflow dropped so low the heat exchanger cracked from overheating. Never run a system with a restricted filter. The cost of a new filter is nothing compared to replacing a heat exchanger. Q: Do I need a ventilation fan in a closet?
Not normally. Closets are not habitable spaces and they do not generate moisture or contaminants. The exception is if you are storing chemicals or have a built-in dehumidifier. For a standard walk-in closet, the air exchange from the room is sufficient. Some people install small fans to prevent mustiness, but that is solving a problem that usually comes from poor closet organization, not lack of ventilation. Q: What is the difference between natural and mechanical ventilation? Natural ventilation relies on wind and thermal buoyancy. Open windows, louvers, and stack effect move air without fans. It works but it is unreliable. You cannot control the airflow rate. Mechanical ventilation uses fans to move a known quantity of air. It is predictable and code-compliant. The best systems combine both. You use natural ventilation when conditions allow and mechanical when they don't. Modern tight houses absolutely require mechanical ventilation because there is not enough infiltration to maintain indoor air quality. A house with triple pane windows and spray foam insulation might have less than 0.1 air changes per hour at 50 Pascals. That is far below the ASHRAE 62.2 standard of 0.35 ACH.
If you are dealing with a specific ventilation problem, the first thing I always check is whether the homeowner has confused filtration with ventilation. Changing the air in the room is not the same as cleaning the air that is already there. They serve different purposes and you usually need both.
