How can I tell if an exhaust hood or fan must be vented outside instead of recirculating?

Short answer

• Bathroom/toilet exhaust fans must be vented outdoors—recirculating is not allowed by code.
• Kitchen range hoods can be either ducted (vented outside) or ductless (recirculating) depending on the model’s listing/label and your local code. High-output or pro-style setups and most downdraft systems typically require venting to the exterior.

How to know which you have (and what’s required)

1) Identify the appliance and check labeling

• Bathroom exhaust fan: These are designed only to expel air. By code, they discharge to the outdoors. If yours isn’t ducted outside, it needs correction.
• Kitchen hood (under-cabinet, wall-chimney, island, OTR microwave): Look for labels such as “Convertible,” “Ducted,” or “Ductless/Recirculating.” The installation manual will explicitly state whether recirculation is allowed and what kit/filters are required.
• Downdraft ventilators: Most are designed for exterior venting. Recirculating kits are uncommon and often not permitted for gas cooktops.

2) Look for physical clues

• Duct collar/knockouts: A round or rectangular outlet at the top/rear indicates duct capability. Many hoods include a damper for outside venting.
• Charcoal filters and recirculation baffles: Presence of a charcoal filter or a recirc deflector suggests the unit can recirculate (kitchen only). Bathroom fans do not use charcoal filters.
• Discharge point: Trace the duct. It must terminate outdoors with a wall or roof cap—not in an attic, soffit cavity, crawlspace, or garage.

3) Check local code and typical standards

• Bathrooms: Exhaust air must go outdoors. No exceptions for recirc.
• Kitchens: Many jurisdictions follow IRC/IMC. They allow ductless range hoods only if the hood is specifically listed for recirculation. Grease-laden vapors must never travel through plastic or flexible dryer-style duct.

Kitchen ducts: smooth-walled metal (galvanized/stainless/copper), sized to the hood outlet (often 6–10 in.), minimal elbows, sealed with foil tape, and terminated outdoors with a backdraft damper.
Bathroom ducts: dedicated run to exterior, insulated if passing through unconditioned space to prevent condensation.

Why some must be vented outside

• Moisture control (bath): Recirculation would dump humidity back inside, driving mold and damage.
• Combustion/odor/grease (kitchen): Charcoal filters reduce odor but don’t remove moisture or combustion byproducts. Gas cooktops produce water vapor and small amounts of NOx/CO—best practice is ducting outside. High-BTU ranges are often manufacturer-required to be ducted.

Step-by-step: Determine your configuration

1. Kill power at the breaker and remove the grease or grille filters.
2. Inspect the interior: Look for a damper/duct adapter and a clear outlet path. On OTR microwaves, check the damper orientation—these units ship “convertible” and can be rotated for updraft/out-the-top, rear-vent, or set to recirc.
3. Trace the duct: Use a flashlight from the hood/fan to verify a continuous duct to a roof or wall cap. Attics often hide disconnected ducts.
4. Check outside: Find the exhaust cap. Run the fan and feel for airflow. A tissue test or smoke pencil helps confirm.
5. Read the model manual: Search the model number online. It will state if recirculation is allowed and list kit/charcoal filter part numbers. If it says “ducted only,” you must vent outdoors.
6. Evaluate CFM and makeup air: If your hood is 400+ CFM, many codes require a makeup air device. The manual and local code office can confirm.

Tools and materials for inspection or conversion

• Screwdrivers (Phillips/flat), nut driver set
• Flashlight/headlamp; inspection mirror
• Ladder/step stool
• Smoke pencil or incense stick (for airflow testing)
• Foil HVAC tape (UL 181), not cloth “duct tape”
• Sheet metal snips and crimping tool (for ducting changes)
• Appropriate metal ducting (6–10 in. round or rectangular), elbows, and an exterior wall/roof cap with damper
• Charcoal filter/recirculation kit (if your hood is listed for ductless)

Safety and best practices

• Power off before removing covers—sheet metal edges are sharp; wear cut-resistant gloves and eye protection.
• Never vent into attics or soffit cavities. Moisture and grease damage structures and invite mold/pests.
• Use proper ducting. For range hoods, use smooth metal duct only; avoid flexible plastic or foil for grease-laden air. Seal joints with foil tape; avoid screws protruding into the airstream where prohibited.
• Short, straight, and sized right. Match the hood outlet size (e.g., a 600 CFM hood often needs 6–8 in. duct). Minimize elbows to maintain performance and reduce noise.
• Insulate bath fan ducts in unconditioned spaces to prevent condensation and ceiling stains.
• Maintain filters. Wash metal baffle/mesh filters monthly; replace charcoal filters every 3–6 months if recirculating.

Common mistakes to avoid

• Assuming any hood can recirculate—many are ducted only.
• Using a charcoal filter over a gas range as a substitute for ducting, against manufacturer instructions.
• Venting a bathroom fan into the attic or soffit.
• Undersizing the duct or using too many elbows, causing poor capture and noise.
• Skipping makeup air on high-CFM hoods, leading to backdrafting of fireplaces or water heaters.

When to call a pro

• You need to cut and flash a roof or exterior wall for a new vent.
• The hood is 400+ CFM and may require a makeup air system.
• You have masonry/brick penetrations or limited duct routing options.
• You suspect combustion appliance backdrafting or have indoor air quality concerns.

Cost and time snapshots

• Recirculation kit (if allowed): $20–$80; 15–30 minutes to install.
• Ducting materials and cap: $100–$300 for typical 6–8 in. run.
• Professional install: $300–$1,200+, depending on complexity and exterior penetration.
• Makeup air kit: $300–$800+ plus labor.

Bottom line

• Bathroom fans: always vent outdoors.
• Kitchen hoods: follow the unit’s listing and local code. If your model is ducted-only—or you have a high-output/gas setup—plan for exterior venting for performance, safety, and code compliance.