Water Heater Backdrafting or Improper Venting: What It Means on Your Home Inspection Report

On a standard-vent gas water heater, the burner's exhaust rises through the tank's flue into the draft hood, the fitting between the heater's flue outlet and the vent pipe, and then up the vent to a chimney or the outdoors. A.O. Smith calls the draft hood "critical to the operation of a gas water heater": properly installed, it "cools the combustion products and allows them to flow safely from the building."

Backdrafting, or spillage, is when that exhaust comes out at the draft hood into the room instead of going up the vent. Your report may say the heater was backdrafting, or name a venting defect that can cause it or leak exhaust on its own: a vent pipe that's disconnected, loose, sloped the wrong way, undersized, or rusted, a draft hood that's missing or loose, or a vent too close to wood or other combustibles, which A.O. Smith calls a fire hazard. Under the InterNACHI Standards of Practice, one of the standards inspectors work under, inspectors check the venting connections but not the inside of flues and chimneys or the adequacy of combustion air.

Why It Matters

The exhaust contains carbon monoxide. A.O. Smith's manual for these heaters says correct combustion, vent action, and vent pipe installation "are very important in preventing death from possible carbon monoxide poisoning and fires," and warns to be sure the vent pipe is properly connected "to prevent escape of dangerous flue gases which could cause deadly asphyxiation." The CPSC calls carbon monoxide "a deadly, colorless, odorless, poisonous gas."

The makers test for it. A.O. Smith's check is a worst case: doors and windows closed and every exhaust fan and air handler running, from the furnace and dryer to the range hood and bath fans. With the burner firing, a match flame should still be drawn into the draft hood; if it isn't, shut off the water heater and correct the air supply. That's a technician's test. Bradford White's rule: "DO NOT use the water heater or connected equipment if spillage is detected until the problem is corrected."

What the makers require:

• Tight, screwed joints. Every joint secured with sheet metal screws, including at the draft hood (A.O. Smith), and fitting tightly (Bradford White).
• An upward slope of at least ¼ inch per foot toward the outdoors (A.O. Smith, Bradford White). The CDC says this "prevents CO from leaking if the joints or pipes aren't fitted tightly."
• The right size: no smaller than the draft hood outlet (both makers). Adding or removing another appliance on a shared vent "may adversely affect the operation of the water heater," A.O. Smith warns.
• Clearance: at least 6 inches between single-wall vent pipe and anything combustible; without the required clearances, A.O. Smith says, the result is "a fire hazard."
• The original draft hood, secured, unmodified, and unobstructed, with nothing stored or insulation on top of the heater (A.O. Smith).
• Sound pipe and enough air. Replace rusted or perforated sections; too little combustion air causes "recirculation of combustion products resulting in air contamination that may be hazardous to life" (Bradford White).

What makes it more urgent: a CO alarm sounding or anyone with headaches, dizziness, or nausea (get everyone outside and call 911; see CO alarm going off); a vent pipe hanging loose; or soot, which A.O. Smith says "requires correction before further use": turn off the gas to the heater until repairs are made, because failing to correct the cause can result in a fire.

What to Do Before Closing

• Ask the inspector exactly what they saw: spillage during a test, or a specific joint, slope, clearance, or draft hood problem, with photos.
• Have a licensed plumber or HVAC contractor evaluate the whole vent, including the chimney or vent it joins and whether it's sized for what's left on it, and repeat the spillage test with the fans running.
• Talk to your real estate agent about asking the seller to correct it or offer a credit within your inspection contingency.

What to Do After You Move In

If it wasn't corrected before closing, don't use the water heater until a licensed plumber or HVAC contractor has fixed it; Bradford White says not to operate a heater that spills or has an unsealed joint. Then:

• Put CO alarms on every level and outside sleeping areas, as the CPSC recommends; see our CO alarm guide.
• Keep the top of the heater clear and combustibles away from it (A.O. Smith, Bradford White).
• Have it serviced yearly. Bradford White says a qualified service technician should inspect the heater at least annually for damaged components or joints not sealed, and the CDC says to have the water heater and other fuel-burning appliances serviced by a qualified technician every year.

Can You Fix It Yourself?

The repair, no. Bradford White recommends that its heaters be installed and serviced by a plumbing professional, and the CPSC warns never to install or service combustion appliances without the proper knowledge, skills, and tools. The looking, yes: A.O. Smith says to inspect the venting visually at least once a year for obstructions, damage or deterioration, and rust flakes around the top of the heater, and that if you find soot or deterioration, "something is wrong" and must be corrected before you use the heater again. Look without touching; Bradford White warns the ventilation air system may be hot.

Frequently Asked Questions

Can an exhaust fan make a water heater backdraft?

It can. A.O. Smith's draft test is run with every exhaust fan on, and Bradford White says that when an exhaust fan is in the same room as the heater, the walls need enough openings for air; undersized openings "will cause air to be drawn into the room through the chimney, causing recirculation of combustion products."

Is a backdrafting water heater a deal breaker?

It depends on the cause. A loose joint or bad slope is corrected in the vent pipe; an air supply or chimney problem can mean more work, since Bradford White says the chimney must be lined and the vent sized for what's on it. Get a written estimate before your contingency ends.

The Bottom Line

A gas water heater's exhaust is supposed to go up the vent, not out at the draft hood into your house, and the makers tie bad venting to carbon monoxide poisoning. Don't use a heater that spills; have a licensed plumber or HVAC contractor correct it, put CO alarms on every level, and look over the vent yourself once a year. See more findings in our inspection report decoder.

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