Is it safe to repurpose an unused HVAC duct for a kitchen range hood, and what precautions?

Short answer

Usually no. You generally should not repurpose an unused HVAC duct for a kitchen range hood. A range hood needs its own dedicated, smooth-metal duct that vents directly outdoors. You can only reuse an existing duct if it is completely isolated from the HVAC system, correctly sized, unlined (no internal insulation), smooth-walled metal, in good condition, and routed with proper termination. In many homes, the old HVAC branch won’t meet these requirements, so installing a new, dedicated duct is the safer, code-compliant path.

Why repurposing is often a bad idea

• Code and safety: Most mechanical codes require range hoods to discharge outdoors through dedicated ducting with a backdraft damper. They cannot share or connect to any HVAC supply/return plenum or ductwork.
• Grease and lint buildup: Any duct with internal insulation (lined) or flex sections will trap grease, increasing fire and odor risk.
• Performance: Undersized or convoluted duct runs create high static pressure, reducing capture and increasing noise. Many capped HVAC branches are 4–5 inches, while range hoods often need 6–8 inches.
• Contamination: Cross-connecting to HVAC can spread cooking odors and moisture through the house.

When repurposing might be acceptable

Only consider reuse if the duct is:
- A truly standalone duct not connected to any HVAC plenum or other exhaust system.
- Rigid, smooth-wall metal (galvanized, stainless) with no internal insulation or flex sections.
- Proper diameter to match your hood outlet.
- Able to terminate outdoors with an appropriate wall/roof cap and backdraft damper.
- In a route with limited elbows and acceptable total equivalent length per the hood’s manual.

If any of the above aren’t true, install a new dedicated duct.

Sizing and layout basics

Typical residential range hood ducting
- Duct type: Round, smooth metal (26–28 ga) preferred; rectangular OK if cross-sectional area matches manufacturer’s specs.
- Minimum diameter: 6 in for many 250–400 CFM hoods; 7 in for ~600 CFM; 8 in for 700–900+ CFM.
- Limit elbows: ≤2–3 90° bends; use long-radius elbows when possible.
- Equivalent length: Follow your hood’s manual (often 35–50 ft max). Deduct for fittings (e.g., 90° elbow = 10–15 ft, 45° = 5–7 ft).
- Termination: Listed roof/wall cap with backdraft damper; do not vent into attic, crawlspace, soffit, or chimney.

Step-by-step: Evaluate and convert (if feasible)

1. Identify the duct: Confirm it’s not connected to any active HVAC supply/return, bath fan, or other system. If it ties into a trunk, it’s not suitable.
2. Inspect interior: Look for internal insulation/liner, flex sections, screws protruding into the airstream, or corrosion. Any lining or flex? Do not use.
3. Measure diameter and route: Verify the duct matches the hood outlet and provides a reasonable path with few elbows. Plan termination outdoors with proper clearances from windows/soffit vents per local code.
4. Clean or replace sections: If the metal is clean, smooth, and sized right, clean dust and debris thoroughly. Replace any damaged or undersized sections.
5. Connect to hood: Use a listed transition that matches your hood outlet. Install a backdraft damper (often integrated in the hood or cap).
6. Seal joints: Use UL 181 aluminum foil tape and/or HVAC mastic. Avoid cloth “duct tape.” Minimize or avoid fasteners protruding into the duct; follow your hood’s manual.
7. Support and insulate: Strap the duct every 4–6 ft. In unconditioned spaces, insulate the duct to prevent condensation. Slightly pitch horizontal runs toward the exterior or follow manufacturer guidance.
8. Install termination: Use a range-hood-rated wall/roof cap. In many areas, bird screens are allowed if part of a listed cap; don’t add fine mesh that clogs with grease.
9. Test airflow: Run the hood on high. Check outside for strong discharge and proper damper operation. Indoors, use a tissue or smoke pencil at the hood to confirm capture.

Time/cost: Expect 4–8 hours DIY for a straightforward run. Materials usually run $150–$400; professional installation can range $500–$1,500+ depending on access and roof work.

Tools and materials

• Tools: Sheet-metal snips, 4-in-1 crimper, drill/driver, step bit or hole saw, rivet tool, tape measure, level, aviation snips, caulk gun, utility knife, PPE (gloves, safety glasses, respirator for attic work), headlamp.
• Materials: Smooth-wall metal duct and elbows (6–8 in typical), listed transition, UL 181 foil tape, HVAC mastic, sheet-metal screws or rivets (if allowed), hanger strap, pipe insulation or insulated duct wrap, roof/wall cap with damper, exterior sealant.

Safety and code considerations

• Make-up air: Many jurisdictions require make-up air for kitchen exhaust systems over 400 CFM. Check local code and your hood’s manual; a powered or automatic damper may be needed.
• Combustion appliances: Strong exhaust can backdraft gas water heaters or furnaces. Test with a pro if you have atmospherically vented appliances.
• Terminations: Vent outdoors and maintain clearances from openings and soffit vents; follow local code and the cap’s listing.
• No shared ducts: Do not connect to HVAC, bath fans, dryers, or other exhausts.
• Attic/roof work: Use fall protection; watch for live wiring; be cautious with old insulation.

Tips for best results

• Match the duct size to the hood outlet—upsizing is OK, downsizing is not.
• Prefer straight, short runs with long-radius elbows.
• Keep the hood’s grease filters clean to maintain airflow.
• If noise is high, your duct may be undersized or too long; correct the run rather than living with the noise.

Common mistakes

• Using flexible foil or plastic duct—it traps grease and violates most manuals.
• Venting into an attic or soffit instead of outdoors.
• Undersizing the duct or stacking too many tight elbows.
• Skipping the backdraft damper or using a non-rated termination cap.
• Relying on cloth duct tape instead of foil tape/mastic.

When to call a pro

• You need to cut a new roof or wall penetration.
• The run is long/complex or through multiple stories.
• Your hood is >400 CFM and may need make-up air.
• You have gas appliances or a fireplace and want a backdraft safety check.
• You suspect the existing duct is part of the HVAC system or can’t confirm isolation.

Bottom line: A dedicated, properly sized, smooth-metal duct to the exterior is the right way to vent a range hood. Repurposing an old HVAC branch is rarely a clean fit, but if it truly meets the criteria above, you can adapt it safely with careful inspection and upgrades.