In a field study of 10 homes, every chimney-vented gas water heater spilled some exhaust when its burner lit, and five of the six long spills came from the house with the shortest chimney. Have the draft checked with the fans running, and keep a CO alarm near every bedroom.
If the gas water heater in your basement has a metal vent running into a chimney, it depends on hot exhaust rising to get its fumes out of the house. Nine to 12 months of monitoring in 10 homes found that this does not always happen. Every one of these heaters spilled some exhaust at startup, and the long spills clustered at the house with the shortest chimney.
About 25% to about 95%:
the share of firings in which each chimney-vented water heater spilled exhaust at startup, depending on the heater.
The source
Scott Pigg, Dan Cautley and Andy Mendyk of the Energy Center of Wisconsin published “Energy Use by Residential Gas Water Heaters: A technical field study in 10 Wisconsin homes” in December 2010 (ECW Report 254-1). It was part of a national project on gas water heater performance.
They monitored the existing natural-draft gas water heaters in 10 homes in southern Wisconsin, all wood-frame houses with full basements, for about 9 to 12 months each. The heaters were made between 1977 and 2003, and three shared a chimney with a furnace or boiler. One short-term replacement heater of the same chimney-vented type was also monitored.
To catch exhaust spilling out, they sampled the air right at the draft hood, the open collar on top of the heater where the vent begins. A rise of 1000 ppm of CO2 over the room’s level before firing counted as a spill.
What they found
- Every chimney-vented heater spilled at startup. In about 25% to about 95% of firings, depending on the heater. Normal draft usually took over within two minutes.
- About 30 spills beyond the first five minutes of firing, mostly short, and many apparently caused by gusty wind.
- Six serious spills: after the first two minutes, lasting at least a minute. In several, the vent ran backwards, starting while only the pilot light was burning and continuing through up to at least 15 minutes of main burner firing.
- Five of the six happened at one house, Site G, which had the shortest chimney in the study (17 ft). The sixth was at Site A, where the authors say they could not identify a cause.
- Idle backdrafting: at Site G the vent ran backwards in nearly 2% of all monitoring periods when the heater was idle. At most homes it was far under 1%.
The draft test itself depended on the weather. The Wisconsin weatherization program sets a minimum draft that rises as it gets colder: 4 Pascals at 21 to 40 F outside. Most heaters reached 4 to 6 Pascals in moderately cold weather. Two, at Sites E and G, routinely failed the standard in cold weather and passed in warm weather.
Why it would work this way
A chimney works because hot exhaust rises. Our reading, not the report’s: a cold chimney needs a minute or two of heat before it pulls, so the first exhaust spills out the draft hood.
For the long spills, the authors point to a chimney filled with cooler outdoor air combined with a basement under suction. In the three serious spills they graphed, basement pressure dropped relative to outdoors around the time the vent began running backwards, which in those cases was hours before the spill. They name the dryer and range hood as devices that can overpower a chimney’s natural draft. Similar suction at other times did not trigger backdrafting, so something else has to line up too.
The limits, stated plainly
This was 10 volunteer homes, chosen to cover different venting setups. The authors say the sample is not random and does not represent the average Wisconsin home. Wisconsin is also colder than Portland.
Spills were measured at the draft hood, not in a bedroom or a living room. The report gives no carbon monoxide level for any spill and measured no health outcome. It shows that exhaust can come back into the house. It does not say how often anyone is harmed.
The manufacturer of the replacement water heaters supplied them for the project.
What survives: a chimney-vented water heater does not always vent, and the chimney and the basement decide how often.
What happens when you fix it
After the monitoring period, every heater was replaced with a power-vent or tankless unit that pushes exhaust out with a fan. The authors stopped watching those for spills, describing them as having “no pilot lights” and being “not susceptible to spillage.” That is a design judgment, not a measurement from this study.
What gets lost in the water heater sales pitch
Power-vent and tankless heaters are sold on energy savings. This study estimated about $20 to $25 a year for a power-vent heater and $50 to $60 for a tankless unit. For the air in the house, the bigger difference is the fan-driven vent.
CO alarms are sold as protection. The EPA’s page “What about carbon monoxide detectors?” (updated September 16, 2026) calls them “a back-up to BUT NOT A REPLACEMENT for proper installation, use, and maintenance of fuel-burning appliances,” and lists “backdrafting appliances or fireplaces” among the causes of CO build-up.
What the data supports
- Get fuel-burning appliances inspected, water heaters included. The EPA page carries this CPSC advice.
- Ask for the draft to be checked with the dryer, range hood and bath fans running, since those can pull the basement into suction.
- Put a CO alarm in the hallway near every separate sleeping area, certified to UL 2034, per the EPA.
- At replacement time, a fan-vented model no longer depends on the chimney.
Bottom line
In 10 homes, every chimney-vented gas water heater spilled some exhaust when it lit, and the long spills clustered at the house with the shortest chimney. If you want to know what is in your basement air, see how our room-by-room testing works, or book indoor air quality testing in Portland or Vancouver.
First step: go look at your water heater. If its vent runs into a chimney, book a draft check with the fans running.
Indoor air quality testing in Portland and Vancouver, room by room.
illumenair does indoor air quality testing and mold testing in Portland and Vancouver. We test and report. We do not do remediation.
971-363-5626 · info@illumenair.com · www.illumenair.com
Sources: Pigg S, Cautley D, Mendyk A. Energy Use by Residential Gas Water Heaters: A technical field study in 10 Wisconsin homes. Energy Center of Wisconsin, ECW Report 254-1, December 2010. Read October 31, 2026. U.S. EPA. What about carbon monoxide detectors? Last updated September 16, 2026; read October 31, 2026.