Backdrafting Water Heaters and Furnaces:
How Exhaust Ends Up in the House

How an older gas water heater vents

A standard tank water heater is “natural draft,” also called atmospheric. There’s no fan. The burner heats the air in the flue, hot air rises, and it pulls the exhaust up the vent pipe on its own.

The metal cone on top of the tank is the draft hood. It sits on a few short legs, leaving a gap between the hood and the tank. That gap is on purpose. Room air gets pulled in through it and mixes with the exhaust, which keeps the draft even when wind gusts across the top of the vent.

The same gap is where trouble shows up. If the vent stops drawing, exhaust has a ready-made exit into the room. The draft hood works only as long as the vent pipe above it pulls harder than the house pulls back.

Older gas furnaces vent into the same kind of metal pipe or chimney, often shared with the water heater. Newer high-efficiency furnaces don’t. They use a fan and plastic pipe out the side of the house, and that change sets up the orphaned water heater problem further down.

What backdrafting leaves behind

Exhaust is wet. When it spills out of the draft hood and hits cooler metal, the water condenses, and it’s slightly acidic. Over months, that leaves marks.

In Russ’s reports, the signs are rust on the top of the tank around the draft hood, a corroded band where the hood meets the vent, white or tan crusty deposits, and staining around the vent connection. One Portland report found corrosion on the metal band right next to the draft hood. The DOE’s Building America guide adds melted plastic on the water line fittings and soot spots on top of the tank to the list.

Those marks show the heater spilled at some point. They don’t say whether it spills today, under what conditions, or how often. A heater can draft fine on a mild afternoon and spill on a cold morning with the dryer running.

That’s where the visual inspection stops. Russ reports the signs and the likely causes he can see. A licensed HVAC technician can then run a draft or spillage test with the burner going, ideally with the house set up the worst way: exhaust fans and dryer on, interior doors closed. That test shows whether it spills now.

Loose draft hoods, taped joints and short vent runs

Most of the venting findings in Russ’s reports were mechanical. Somebody bumped something, or the install was never quite right.

The draft hood itself was the most common problem. Ten reports had a hood that was displaced, off center, not plumb, wobbling when touched, or held down with foil tape. In a Lake Oswego house, the hood sat loose and visibly misaligned over the flue opening. In a Hillsboro house it was unseated and taped to the top of the tank. A hood that isn’t centered on the flue can’t pull exhaust up evenly.

The vent connector, the single-wall metal pipe between the hood and the chimney or B-vent, is next. Each joint is supposed to be fastened with sheet metal screws, usually three. Russ has found pipes just resting on the draft hood with no screws (Canby) and joints wrapped in foil tape (Lake Oswego, Beaverton, Portland and Washougal). The tape hides whether the screws are there, and a pipe held only by tape can shift apart.

Above the ceiling, the vent becomes B-vent, and the post on gas vent pipes from ceiling to roof covers the rust, caps and clearance problems Russ finds there.

Then there’s the shape of the run:

• Slope. The connector has to rise all the way to the vent. A section running downhill, like the one in a Vancouver garage, makes hot exhaust work against itself.
• The first rise. Many water heater manuals call for a straight vertical run off the draft hood, often about a foot, before the first elbow. A Gladstone water heater had less than that and showed signs of backdrafting at the hood.
• Two appliances on one vent. When a furnace and water heater share a vent, the fuel gas codes have the smaller appliance enter above the larger one. In one Portland house the water heater joined the shared pipe below the furnace, and the furnace vent connector showed rust and moisture staining at its base.

They’re easy to miss at a showing. The top of a water heater is above eye level and usually dusty.

Where the air comes from: closets, louvers and big range hoods

A gas burner needs air to burn, and the draft hood needs more air to make the draft work. Codes call this combustion air. When the room can’t supply it, the heater pulls from wherever it can, sometimes down its own vent.

The model codes set a rule of thumb for a heater drawing from the room around it: 50 cubic feet of room for every 1,000 Btu per hour of appliance input. A typical 40,000 Btu water heater needs about 2,000 cubic feet on that basis. That’s a room about 16 by 16 feet with an 8-foot ceiling, and a lot more than a utility closet holds.

So closets get vents. The usual setup is two permanent openings, one within 12 inches of the ceiling and one within 12 inches of the floor, or louvered doors with enough open area. In a Canby house, the water heater closet had only one vent, down low. Russ’s report noted that a single low opening can leave the draft hood short of air.

Closets aren’t the only way to starve a heater. Anything that blows air out of the house pulls the house slightly negative: a clothes dryer, bath fans, and above all a large kitchen range hood. A tight, newer house feels it more than a drafty 1920s one. The International Residential Code calls for makeup air (a dedicated opening that brings outside air in) when a kitchen exhaust system can move more than 400 cubic feet per minute, because a big hood can overpower a natural-draft vent. In one Portland house, the client told Russ the makeup air damper wasn’t closing consistently, and the report sent it to a contractor.

The dryer vent post covers the dryer side of this. A crushed or clogged dryer vent doesn’t cause backdrafting, but a dryer running at full blast is one of the things that pulls a house negative.

The orphaned water heater

This one shows up in older houses that started out with a chimney. The furnace and the gas water heater used to vent together up a masonry chimney. Then the furnace got replaced with a high-efficiency model that vents out the side wall through plastic pipe. The water heater stayed on the chimney, alone.

Trade sources call that an orphaned water heater. The chimney was sized for two appliances. One small water heater sends a fraction of the heat up it, the flue stays cold, and cold flues draft poorly. The exhaust also cools enough to condense inside the chimney, which can rust a metal vent and break down mortar and clay liners over time.

The usual corrections are a properly sized metal liner run up the chimney for the water heater, or a water heater that doesn’t need the chimney at all, like a power-vent model. A licensed HVAC contractor or chimney professional sizes that. The unlined chimney flue post covers liners in more detail.

Russ’s reports show the neighbors of this problem. In one Portland house, the furnace and water heater connectors were combined and run into a larger, older flue, likely left from an oil furnace, with rust and white deposits on the joints. In another, the water heater and furnace vented through a flexible foil-type duct inside the chimney, a material not meant for flue gases. A home inspection sees the bottom of the chimney and the top of it from the roof. It can’t see down the inside, which is what a Level 2 chimney inspection is for.

Carbon monoxide alarms

A working CO alarm is the backstop for everything above. The CPSC says to put CO alarms on every level of the home and outside sleeping areas, test them every month, and have a professional inspect fuel-burning heating systems like furnaces and chimneys every year.

Alarms wear out. Most CO alarms have a replacement date printed on the back, and the post on expired smoke and CO alarms goes through how to read it and what Oregon and Washington require at sale.

If the water heater on the listing is old enough that you’re already wondering about replacement, the water heater serial number decoder gives an approximate age from the label.

Gas venting is one of the items Russ looks at on every home with gas appliances. To set up an inspection, you can book online.