Turning the heat on this week? Check your water heater's vent first.
If you have a gas water heater with a metal vent pipe on top, its exhaust should rise up that pipe and out the chimney every time it fires. When something blocks or overpowers that draft, the exhaust spills into the basement instead, carbon monoxide included. The first cold week of fall is when it's most worth checking, because that's when the furnace starts sharing the chimney again.
Tuesday morning is forecast to be the coldest of the week, in the mid 30s to low 40s with frost possible, and the northern suburbs, Milford included, usually run colder than the rest of Metro Detroit on mornings like that. For a lot of houses it's the first time the furnace kicks on since spring, which makes this a good week for a two minute look at the vent on top of the water heater and the carbon monoxide alarms that back it up.
How the vent is supposed to work
Most gas tank water heaters, especially in older homes, are atmospheric units, which means the exhaust rises up the vent on its own because it's hot. The burner fires under the tank, the exhaust travels up through a flue in the middle of it, and at the top it passes through the draft hood, the bell shaped metal piece with a gap around its base, into the vent pipe and up the chimney. That gap lets room air get pulled in along with the exhaust, which is what keeps the draft steady.
Spillage, which you'll also hear called backdrafting, is that process running the wrong way, with exhaust coming out of the gap at the draft hood and into the room. A little spillage for a few seconds when the burner first lights can be normal. Exhaust that's still spilling after the burner has been going for a minute or two isn't.
The two minute smoke test
A little smoke held at the edge of the draft hood shows you which way the exhaust is going. Set the house up the way it is on a cold evening, because that's when the draft has the most working against it:
- Close the windows and outside doors, and turn on the bathroom and kitchen exhaust fans, plus the clothes dryer if it vents outside.
- Turn the thermostat up so the furnace runs.
- Run hot water at a faucet until you hear the water heater's burner light.
- Give it a minute or two, then hold a stick of incense, or a match you've just blown out, about an inch from the gap at the bottom of the draft hood.
The smoke should get pulled up into the hood all the way around it, so check every side, not just the front. If it's drawing everywhere, repeat the test once with the furnace off, since the furnace changes how the chimney behaves. If the smoke drifts out into the room or blows away from the gap at any point, the water heater is spilling, and you should stop there: turn the water heater's gas knob to Pilot so the burner stays off, open a window, and call us.
Signs it's been spilling for a while
The water heater often shows it before a test does. Look for rust, scorch marks, or melted plastic around the top of the tank and the draft hood, and for a vent pipe that's loose, sagging, or slipped off the hood. Water dripping out of the vent, or condensation on nearby windows and cold pipes when the water heater runs, is another sign, and so is a stale, sharp smell in the basement when the furnace and the water heater are both going. Any of these is worth a call even if the smoke test looks fine.
Why the furnace changes things
In a lot of older homes the furnace and the water heater vent into the same chimney, and the furnace's bigger, hotter exhaust is a big part of what keeps that chimney warm and drawing. When the furnace starts up again in October, everything about that shared chimney matters again, from a nest or debris that collected over the summer to a crumbling liner or a connector pipe that got knocked loose. Large exhaust fans in a tight house can also pull air down the chimney instead of letting it rise, which is why the test runs with the fans on.
The other common case is a house where the old furnace was replaced with a high efficiency one that vents out the side wall through plastic pipe. That leaves the water heater as the only appliance using a chimney that was sized for two, and a small amount of exhaust in a big, cold chimney can struggle to draw and leaves condensation behind. Plumbers call that an orphaned water heater, and it usually needs a liner or a different vent. If your furnace was replaced and nobody looked at the water heater's venting at the time, it's worth having checked.
The carbon monoxide alarm is the backup
A working carbon monoxide alarm is what catches spillage you can't see or smell, because carbon monoxide has no color and no odor. The U.S. Consumer Product Safety Commission recommends one on every level of the home and outside sleeping areas. Press the test button on each one this week, and replace any that are past the replace by date printed on them.
If a carbon monoxide alarm goes off, get everyone outside first, then call 911. If you smell gas, leave without flipping any switches, and call your gas utility from outside.
When to call us
Call us if the smoke spills, if you see any of the warning signs above, or if your furnace was replaced and you're not sure what the water heater vents into now. We check the venting on every water heater we install, and checking yours is a short visit. Our water heater page covers repairs and replacements in Milford, Novi, and Farmington Hills. If your tank is getting up there in years, how long water heaters last in Michigan helps you decide whether a fix is worth it, and flushing your water heater before winter covers the rest of the fall check.
Quick answers
What is water heater backdrafting?
It's when a gas water heater's exhaust spills out of the draft hood into the room instead of rising up the vent. The exhaust includes carbon monoxide, so it should be fixed before the heating season.
How do I test my water heater for backdrafting?
With the windows closed, the exhaust fans on, and the water heater running for a minute or two, hold smoke from incense or a blown out match about an inch from the draft hood. Smoke pulled into the hood is normal, and smoke that drifts out means it's spilling.
Can a new furnace affect my water heater's venting?
Yes. If a new high efficiency furnace vents out the side of the house, the water heater may be left alone in a chimney that's too big for it, which can make it draft poorly. Have the venting checked when the furnace is replaced.
Countryside Plumbing is a licensed master plumber serving Milford, Novi, Farmington Hills, and southeast Michigan since 1979. Water heater repair and replacement are a big part of what we do. Free quotes at 248-705-7543.