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Kitchen and bathroom exhaust fans solve different problems. A bathroom fan removes moisture from the room; a kitchen range hood should capture cooking emissions at the appliance and discharge them outdoors. In both rooms, the installed duct route determines how much air actually moves. Select airflow and sound together, use a duct sized for the equipment, keep the route short and smooth, and never terminate the exhaust in an attic, crawlspace or floor cavity.
Kitchen exhaust and bathroom exhaust are not interchangeable
What a bathroom fan is designed to do
Bathroom ventilation controls water vapor generated by showers, baths and sinks. The fan removes humid air before it condenses on mirrors, paint, insulation and framing. Its effectiveness depends on running it during bathing and long enough afterward, not merely on the number printed on the housing.
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For a remodel, inspect the existing opening, wiring and duct route before choosing a replacement. A fan that fits the ceiling cutout may still perform poorly if the old duct is undersized, crushed, disconnected or routed through an unheated space without insulation.
What a kitchen hood is designed to do
Kitchen exhaust is source capture. The hood should collect smoke, grease aerosol, heat and combustion products above the cooking surface and send them directly outdoors. The U.S. Environmental Protection Agency describes the range hood as the most common kitchen ventilation strategy and says it should capture combustion products and vent them directly outdoors.
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A ceiling or wall exhaust fan elsewhere in the kitchen cannot reliably capture a plume at a range. Hood geometry, distance above the appliance, cooking style and the duct route all affect capture. Island and peninsula installations generally need more careful hood selection than a hood against a wall because air can spill around more sides of the canopy.
Bathroom fan sizing: use the room volume, then verify the installation
The older DOE rule of thumb
The U.S. Department of Energy’s 2002 Spot Ventilation brochure suggests choosing at least 50 CFM or 8 air changes per hour (ACH), whichever is larger. This is older design guidance, not a current universal code minimum. The brochure’s calculation is:
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For example, a bathroom measuring 8 feet by 10 feet with an 8-foot ceiling has 640 cubic feet of volume. Eight air changes per hour produces 85.3 CFM, so a nominal 90-CFM or 100-CFM fan would be the relevant size range before considering duct resistance and the manufacturer’s performance data.
| Bathroom condition | How to use the guidance | Important qualification |
|---|---|---|
| Small room where the calculation is below 50 CFM | Use at least 50 CFM under the DOE brochure’s approach | The 2002 figure is guidance, not a universal present-day code requirement |
| Room where 8 ACH exceeds 50 CFM | Use the calculated airflow or the next available product size | Actual delivered airflow falls when the duct has resistance |
| Continuous or demand-controlled ventilation | Follow the system design and the current product instructions | Intermittent spot-ventilation numbers do not automatically describe a whole-house system |
Do not select a fan solely by its free-air rating. Ask for the airflow at a stated static pressure, and confirm that the rating applies to the speed and installation you will use. A product advertised at one CFM in an open test may deliver substantially less through a long, flexible or elbow-filled duct.
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Kitchen hood sizing: capture area and placement matter
DOE’s older capacity guidance
The same 2002 DOE brochure gives a kitchen range-hood guideline of 100 CFM or 40 CFM per linear foot of hood, whichever is larger. It also notes that a hood mounted higher above the range and island or peninsula geometry affect selection. Treat these values as context-specific design guidance rather than an adopted code requirement.
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For a 30-inch hood (2.5 linear feet), 40 CFM per linear foot equals 100 CFM, so the rule produces 100 CFM. For a 48-inch hood (4 linear feet), the calculation produces 160 CFM. Those calculations do not account for unusually high-heat appliances, a very tall mounting position or a difficult duct route; the appliance and hood manufacturer’s instructions take precedence.
Other published reference points
- EPA’s 2023 Indoor airPLUS Version 2 criterion calls for a verifier-measured 100 CFM exhaust for a range hood or appliance-range-hood combination, subject to the program’s exceptions and verification conditions. It is a voluntary program criterion, not a universal code statement.
- The DOE Building Science Education kitchen-exhaust page offers at least 5 ACH as kitchen fan guidance. The page does not establish that value as an adopted code requirement.
Use these figures to frame a design conversation, then check the current mechanical code adopted where the house is located, the hood installation manual and the actual duct layout. A local authority may require a different approach.
Why the duct changes the airflow you receive
Fans are commonly rated at different static pressures. The DOE brochure explains that a 0.25-inch static-pressure rating is generally more representative of an installed system; its 0.1-inch rating may fit only a short, smooth, nearly straight duct. Because that brochure dates to December 2002, use current product performance tables and installation instructions rather than transferring its thresholds mechanically to a new product.
Every foot of duct, elbow, transition, grille, damper and exterior cap adds resistance. Corrugated flexible duct creates more resistance than smooth metal duct and is easier to kink or compress. An undersized duct can make a powerful fan noisy while reducing delivered airflow.
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- Match the duct diameter to the fan or hood outlet; do not reduce it to fit an existing opening without confirming that the manufacturer permits the transition.
- Keep the route as short and straight as the building allows.
- Use smooth, rigid or appropriately supported duct where feasible, with gradual turns instead of sharp bends.
- Seal every joint with a product approved for duct sealing; ordinary cloth duct tape is not a durable air seal.
- Insulate ducts that pass through unheated spaces and slope them away from the fan where the installation permits, limiting condensation drainage back toward the housing.
- Finish with an exterior termination cap or wall/roof termination designed for exhaust and equipped with backdraft control where specified.
The duct must terminate outdoors. The DOE Building Science Education guidance specifically says kitchen exhaust should not discharge into an attic, crawlspace or space between floors. Moisture and pollutants in those cavities can damage insulation and framing and can spread through the building.
How to choose a bathroom exhaust fan
Compare products using the installation you actually have, not just the free-air CFM. These are the decision points that affect performance and daily use:
| Comparison point | What to verify | Why it matters |
|---|---|---|
| Delivered airflow | CFM at the manufacturer’s stated static pressure | Shows what the fan can move through a real duct system |
| Sound | Sones at the operating speed you will use | A noisy fan is less likely to be run long enough to remove moisture |
| Duct connection | Outlet diameter, allowable duct length and required fittings | Prevents an incompatible transition or restrictive route |
| Mounting and dimensions | Housing depth, ceiling opening and joist or truss clearance | Determines whether the old opening and structure can be reused |
| Controls | Wall switch, timer, humidity sensor or continuous operation | Supports the run time the room needs without relying on memory |
| Documentation | Current installation manual, warranty and efficiency information | Confirms permitted installation methods and service support |
If a bathroom has no working exhaust fan, EPA remodeling guidance recommends installing one. Place the intake where it can collect humid air, typically near the shower or tub but outside the direct splash zone, and follow the listed clearances and electrical requirements for the product.
How to choose a ducted kitchen range hood
Start with the appliance and hood geometry
Record the cooking surface width, the hood capture area and the planned mounting height. A hood that is narrower than the appliance or mounted higher than its instructions assume can allow emissions to escape before they enter the filter and blower path. Island and peninsula locations deserve particular attention because there is no wall to contain cross-drafts.
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Measure the proposed duct diameter and route, count elbows and identify whether the discharge will be through a wall or roof. Compare the hood’s performance curve at the expected resistance. A higher nominal CFM is not automatically better if the duct cannot carry it quietly and safely.
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Plan for noise and pressure
Compare sound levels at the speed you expect to use. Also determine whether the house has naturally drafted or atmospherically vented combustion appliances. A large exhaust flow can depressurize the building and pull combustion gases back down a flue.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Installation sequence for a reliable exhaust path
Bathroom fan
- Confirm the fan’s required CFM, duct diameter, electrical circuit and mounting clearances from the current manual.
- Inspect the existing duct from the housing to the exterior. Replace crushed, disconnected or undersized sections rather than attaching a new fan to a defective route.
- Run the shortest practical smooth duct to an exterior wall or roof termination. Minimize elbows and seal each joint.
- Insulate duct in unheated space and arrange the run so condensation does not drain toward the fan housing.
- Install the exterior cap or termination with backdraft control as specified, then verify that the damper opens when the fan runs.
- Set the timer or humidity control so the fan operates during bathing and afterward. For Indoor airPLUS Version 2 demand-controlled designs, the program guidance calls for operation during occupancy and for at least 20 minutes after vacancy, or until relative humidity falls below 60%; this is not a universal residential rule.
Kitchen hood
- Confirm the hood is approved for the cooking appliance and note the required mounting height and duct size.
- Lay out the duct route before framing or cabinet installation. Avoid unnecessary offsets and keep transitions gradual.
- Install the specified smooth duct, sealing joints and supporting it so sections cannot sag or separate.
- Provide the required exterior wall or roof termination; never stop the duct in an attic, crawlspace or floor cavity.
- Test the hood at each speed with the exterior termination installed. Listen for rattling, observe damper operation and check that capture is acceptable at normal cooking height.
- Before commissioning high-flow equipment, review makeup-air and combustion-safety requirements with the designer, installer or authority having jurisdiction.
Makeup air and combustion safety
Exhausting air faster than it enters can create negative pressure. In a tight house, that pressure may increase drafts, pull air through unintended leakage paths or reverse the flow in a combustion flue. EPA remodeling guidance links larger fans with depressurization and possible backdrafting.
EPA’s 2023 Indoor airPLUS advisory says that at exhaust rates of 200 CFM or greater, makeup air should be provided at rates equal or similar to the exhaust. The same material makes clear that the appropriate solution depends on the system and applicable requirements. This is an advisory for that voluntary program, not a statement that every jurisdiction universally requires makeup air at exactly 200 CFM.
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Troubleshooting: match the symptom to the likely cause
| Symptom | Likely causes | Checks and corrective action |
|---|---|---|
| Fan runs but the mirror or ceiling remains wet | Insufficient delivered airflow, short run time, blocked duct or poor room air path | Verify the exterior damper opens, inspect the duct, measure or confirm airflow at stated static pressure, and use the timer or humidity control |
| Hood is loud and capture is weak | Undersized or corrugated duct, excessive elbows, dirty filter or restrictive termination | Clean filters, compare duct size with the manual, remove avoidable restrictions and check the termination |
| Condensation appears in the duct or fan housing | Uninsulated duct in a cold space, improper slope or leakage at joints | Insulate, reseal and arrange the duct to limit drainage toward the fan |
| Cold air enters when the fan is off | Missing or stuck backdraft damper, leaking termination or pressure imbalance | Inspect the damper and exterior cap; investigate makeup air and house pressure if the issue persists |
| Combustion appliance spills or shows unstable draft when the hood runs | Excessive depressurization | Stop using the high-flow exhaust until a qualified professional evaluates combustion safety and makeup air |
A practical specification checklist
- Room or appliance: bathroom volume, or cooking-surface width and hood geometry.
- Airflow basis: required CFM and the static pressure at which it is measured.
- Duct path: diameter, material, approximate length, elbows, transitions and exterior termination.
- Sound: sones at the normal operating speed.
- Controls: timer, humidity sensing, manual switch or continuous operation.
- Structure: housing dimensions, joist or truss clearance and access for service.
- Safety: combustion appliances, pressure effects and any makeup-air provisions.
- Compliance: current locally adopted code, permit requirements and the manufacturer’s installation instructions.
The best exhaust fan is therefore not the one with the largest headline CFM. It is the system whose delivered airflow, sound, duct route, controls and safety provisions fit the room and the way occupants will use it. Verify current local requirements before ordering equipment, because the DOE figures cited here are older guidance and the EPA Indoor airPLUS values are voluntary program criteria rather than universal code.
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