Where the Kitchen Exhaust Really Goes:
Range Hoods, Microwave Vents and Their Ducts

What a kitchen hood is supposed to do

Cooking puts out three things worth removing: grease, moisture and combustion gases from a gas range. A hood that’s ducted outside takes all three out of the house. A recirculating hood pulls air through a grease filter (and sometimes a charcoal filter) and blows it back into the kitchen. It catches some grease and some odor. The water vapor and the gas-burner exhaust stay in the room.

The residential code that most Oregon and Washington houses are built to says a range hood duct should be smooth-walled metal (galvanized steel, stainless steel or copper), airtight, fitted with a backdraft damper, and run to the outdoors. It allows a listed ductless hood when the manufacturer’s instructions are followed and the kitchen gets its required exhaust some other way. So a recirculating hood isn’t automatically wrong. It’s just a different thing than most buyers assume they’re getting.

When the air comes back into the kitchen

This was the most common kitchen exhaust finding in Russ’s reports, and it comes in a few versions.

The hood with no duct. In one Portland kitchen, the hood wasn’t connected to any duct and was blowing straight back into the room. Sometimes there’s a duct in the wall and the hood just isn’t attached to it.

The microwave set up the wrong way. Many over-the-range microwaves can either recirculate through the grille above the door or exhaust out the top or back into a duct, depending on how the blower is set during installation. Russ found microwaves in Vancouver, Lake Oswego and Beaverton blowing part of their air back into the kitchen even though a duct was there. That pattern usually points to a blower left in recirculating mode or a unit that doesn’t seal to the duct above it.

The hood that leaks at the top. A ducted hood that also blows air out around its top edge has a gap between the hood and the duct. Russ wrote this up in Salem, Beaverton and Vancouver.

A quick test works on all of them. Turn the fan on high and hold a strip of paper towel near the top of the hood or the top grille of the microwave. Air blowing out at you is air that isn’t going outside.

Ducts that end in the cabinet or the attic

The duct above the hood usually runs through the cabinet over it. In 10 kitchens Russ wrote up, that duct wasn’t sealed where it passed through the cabinet, or was venting into the cabinet itself. In a Damascus house, the range hood was blowing into the cabinet. The giveaway is a sticky film inside the upper cabinet, or steam drifting out of it while the fan runs.

Attic endings are worse. In a Portland house, the kitchen duct ended inside the attic instead of going out through the roof. In a Lake Oswego house, the kitchen fan vented straight into the attic, and the same report found a ceiling joist cut to make room for it. In King City, a rigid duct in the attic wasn’t sealed near the roofline.

Kitchen air is warm, wet and greasy. Dumped into a cold Portland attic, it condenses on the roof sheathing and leaves a grease film that collects dust and feeds mold-like growth. The post on bathroom fans that vent into the attic covers the same problem from the bathroom side, and the attic mold post covers what that moisture tends to leave behind.

Dampers, caps and the outside wall

The outside end of the duct has a cap with a flap (the damper) that should swing open when the fan runs and fall shut when it stops. Damper problems turned up in 12 kitchens: flaps stuck partly open in Troutdale, brittle dampers in Vancouver with wire mesh stuffed behind them, a missing damper in Happy Valley, and insect nesting blocking a damper in King City. One Beaverton cap had masking tape stuck inside it, likely left from painting.

An open damper is a hole in the house. Cold air comes in all winter, and birds and wasps like a warm duct. The cap itself can let water in too. In Canby and Vancouver, the hood vent cap was mounted right over the siding without proper flashing or sealant, which leaves a path for rain behind the siding.

What a home inspection checks

Russ runs the hood or microwave fan on each speed, checks the light and controls, and looks at the filter. He checks for air coming back into the kitchen, looks inside the cabinet above for the duct and any gaps, and finds the cap outside or on the roof. In the attic, he follows the duct when it’s visible and reachable. Many ducts run through closed walls or ceilings, so the full run can’t always be seen, and the report says when only the two ends could be checked.

He doesn’t judge whether a hood is big enough for the range, and he doesn’t test airflow with an instrument. A hood that’s loud, rattles, or has a dead speed setting goes in the report as a repair item for an appliance technician. A duct that ends in the wrong place, leaks into the cabinet or has no damper goes to an HVAC or ventilation contractor.

Big hoods bring one more question. Under the residential code, a hood that can move more than 400 cubic feet per minute generally needs makeup air, a way for outside air to come in to replace what it pulls out. Without it, a powerful hood in a tight house can pull exhaust down a gas water heater or furnace vent. The post on backdrafting and combustion air explains how that happens.

For the rest of the kitchen, the guide to what to look at in the kitchen before you make an offer covers appliances, plumbing under the sink and the electrical. To set up an inspection, you can book online.