What is backdrafting on a water heater?

Exhaust gas coming out of the draft hood into the room instead of going up the flue. A conventional water heater has no fan — the exhaust rises because it is hot. Anything that pulls air out of the house faster than it can leak back in, such as bath fans, a kitchen hood or a dryer, can reverse that flow and bring air down the chimney instead.

Why would it only fail when the dryer is running?

Because that is when the house is pulling hardest. A water heater tested in a quiet house can draft perfectly and still spill the moment the laundry and two bath fans are on. That is why the test is repeated under worst-case depressurization rather than in ideal conditions.

What is an orphaned water heater?

A tank left alone on a masonry chimney that was sized for it plus a furnace, after that furnace was replaced with a sealed unit venting out a side wall. One small appliance cannot warm that much flue, so the exhaust cools, drafts poorly and condenses inside the chimney. It is a common and entirely accidental consequence of a good furnace upgrade.

Does air sealing my house make this worse?

It can, and that is not an argument against air sealing — it is the argument for testing combustion appliances afterward. Tightening a house reduces the accidental leakage that used to supply replacement air. The work is still worth doing; it just means the appliances need checking once it is done.

Do you replace water heaters?

No. Replacement, gas piping and chimney lining are licensed trade work and are referred out. What we provide is the measurement and a written report, so whoever does the repair is working from numbers rather than a hunch.

Your Water Heater Might Only Be Unsafe When the Dryer Is On

The appliance passes. The house fails.

A conventional gas water heater has no fan. Nothing pushes its exhaust up the flue except the fact that hot gas is lighter than cold gas and wants to rise. That is the entire mechanism, and on a still day in a leaky house it works perfectly.

Then somebody runs a shower with the bath fan on, starts the dryer, and switches on the kitchen hood. Those three together can pull several hundred cubic feet a minute out of the building. That air has to be replaced, and it comes back in through whatever opening offers the least resistance. In a tightened-up house, that opening is often the water heater flue.

Air then comes down the chimney, and the exhaust that was supposed to go up comes out of the draft hood into the room instead. It is called spillage, and what spills includes carbon monoxide.

Why this is getting more common, not less

Every improvement that makes a house cheaper to heat makes this more likely. Air sealing, new windows, insulation, a tighter attic hatch — all of them reduce the leakage that used to supply replacement air without anyone noticing. The work is still worth doing. It just means the combustion appliances need checking afterward, which is why any honest energy assessment includes a combustion test and why rebate programs require one.

The house that backdrafts today is frequently the house that was improved last year.

The orphaned water heater

Here is a specific version worth knowing about, because it follows directly from a good decision.

A furnace is replaced with a modern sealed unit that vents through a plastic pipe out the side wall. Excellent — it is more efficient and it takes its combustion air from outside. But that furnace used to share a masonry chimney with the water heater, and the chimney was sized for both. Now one small appliance is trying to warm a flue built for two.

It cannot. The exhaust cools on the way up, loses its buoyancy, drafts badly and condenses inside the chimney. That condensate is acidic and it works on the liner. The water heater has been orphaned, and nobody did anything wrong.

What a test actually looks like

Run the appliance until the readings settle, measure carbon monoxide in the undiluted flue gases, and check that draft establishes from a cold start within a reasonable time. That is the easy half.

The half that matters is doing it again with the house in the worst state it realistically reaches: every exhaust fan running, the dryer on, interior doors set to make it as hard as possible. A water heater that drafts beautifully in a quiet house and backdrafts when the laundry is going has not passed anything. It has simply not been asked the right question.

What you can check yourself this week

Carbon monoxide alarms on every level, none past its replacement date. This matters more than any test anyone could sell you.
Rust streaks or soot on the vent pipe above the water heater, especially at the draft hood. Rust there is exhaust condensing where it should not be.
Does the burner or pilot ever go out when the dryer or a bath fan is running? That is the whole problem announcing itself.
Is the water heater in a closet with louvered doors? Do not stack storage against them. Those louvers are its combustion air.
Is the furnace a sealed unit with a plastic pipe through the wall, while the water heater still goes up a brick chimney? If so, the tank may be orphaned.

If you smell exhaust or anyone has symptoms, stop reading

Leave the building. Call 911 or Consumers Energy from outside — they run a 24-hour emergency line and they come out free. Headaches, dizziness or nausea that clear up when people leave the house and come back when they return is the pattern worth taking seriously.

A scheduled test is for finding a problem before it has symptoms. It is not what you need when something is happening now.

What we do and what we do not

We measure and we report, with the readings in writing. Replacing a water heater, altering gas piping or lining a chimney is licensed trade work and goes to somebody who holds that license — with our numbers handed over, so they are not starting from nothing.

Our BPI Building Analyst certification is in progress through the Consumers Energy program at Jackson College and completes in November 2026. We do not claim to hold it yet, here or anywhere else on this site.

When it does: combustion safety testing across Greater Lansing, and it is worth doing at the same time as the furnace, because appliances sharing a flue affect each other and testing one alone can pass a pair that fails.