The Dryer That Pulled Flue Gas Into the Laundry
Why this matters
A gas water heater and a clothes dryer sharing a closet is a common arrangement in townhouses and smaller homes, and it works for years as long as the closet can breathe. Change the doors and the same two appliances become a carbon monoxide problem: the dryer is a fan that pulls air out of the closet, and the water heater's vent is the biggest hole the air can come back through. In this case the first repair was correct, the symptom came back anyway, and the person who finally connected it was an appliance tech who came to fix a squealing dryer belt.
The house and the change nobody counted
A two-story townhouse, about 1,400 square feet. On the second floor, a hallway laundry closet about 5 feet wide, 3 feet deep and 8 feet tall: an electric dryer vented outdoors, a washer, and a 40,000 Btu/h natural-draft gas water heater with a draft hood, venting up through the roof. For fourteen years the closet had louvered bifold doors.
In the spring, the owners had the hallway repainted and the handyman swapped the louvered bifolds for solid ones to match the new bedroom doors. Nobody on that job had a reason to think about the water heater.
The first repair, and why it was right
In early summer the owners called a plumber because the water heater "made a sooty smell" and there was black residue on top of it. The plumber found soot on the draft hood and the flue baffle, a yellow, lazy flame, and a burner clogged with soot. He cleaned the burner, the baffle and the flue passage, confirmed a blue flame, and closed the ticket: "Burner sooted. Cleaned. Flame normal."
Every word of that ticket was correct. A sooted burner makes incomplete combustion and more carbon monoxide, and cleaning it restores a good flame. What the ticket did not ask was why a burner that had been clean for fourteen years sooted in a few months. A burner soots when it is starved of oxygen, and one of the ways it gets starved is by breathing its own exhaust: flue gas that spills out of the draft hood into a small room mixes with the air the burner draws in, the burner gets less oxygen, combustion worsens, and the CO in what spills rises again. See related: How a House Pulls Flue Gas Back Down a Vent, which owns that mechanism.
By September the soot was back, and one of the owners had started mentioning headaches on laundry days.
The visit that connected it
The appliance tech was there for the dryer: a squealing drum belt. Her shop has every tech who works within reach of a gas appliance wear a personal CO monitor, and she had hers clipped on.
Two things caught her eye before she pulled the dryer out. A fresh soot line at the water heater's draft hood rim, and the solid doors, which did not match the closet's age. She asked how long the doors had been there, and when the water heater had been cleaned.
She replaced the belt, pushed the dryer back, closed the doors the way the closet normally runs, and started a test cycle to listen for the squeal from the hallway. One of the owners was in the shower at the time. About four minutes in, her monitor, clipped at her chest a foot from the closet doors, went from zero to 30 ppm and kept climbing.
What she did, in order
- Stopped the dryer and opened both closet doors wide. That removed the pull on the closet and gave it the hallway's air.
- Asked about symptoms. The owner in the hallway felt fine; the owner who reported headaches was out. Had anyone been dizzy, confused or sick at that moment, she would have had everyone leave and called 911 from outside.
- Opened the hallway window and watched her monitor fall back to zero with the doors open.
- Ran one screen, with the doors open, and no further. Doors open, dryer running, the shower still calling for hot water so the water heater was firing in ordinary use: she held a smoke pencil at the rim of the draft hood. The smoke was drawn into the hood, meaning the vent was carrying the flue gas up. Same conditions, dryer off: drawn in more briskly. With the doors open, the water heater vented. With them shut and the dryer running, the reading on her chest said it did not.
She did not close the doors again to confirm it. A test that closes the doors to make the water heater spill is a deliberate worst-case test, and it belongs to a qualified gas or combustion-safety tech with an analyzer and the authority to shut the appliance off. Her job was to recognize the pattern, stop the exposure, and hand it off with what she had seen.
- Told the owners plainly, and left the doors open with a note on them. "The new solid doors keep the water heater from getting the air it needs. When the dryer runs with the doors shut, it pulls air down the water heater's vent and the exhaust comes into the closet. Leave these doors open until a gas tech has been out, and get a CO alarm in this hallway today." The townhouse had a smoke alarm in the hall and no CO alarm; she wrote that on the ticket.
- Referred it to a gas-certified plumbing and heating shop the owners chose, with her ticket notes: the door change and its date, the earlier soot cleaning, the monitor reading and conditions, and the doors-open smoke result.
What the gas tech confirmed
The gas tech ran the worst-case test that the appliance tech had correctly stopped short of. With a manometer reading the closet against the hallway, the doors shut and the dryer running, the closet read about -6 pascals and the water heater spilled continuously at the draft hood, with CO in the spillage rising over the minutes it ran. Dryer off, doors shut: about -0.5 pascals, and the draft established, slowly. Doors open, dryer on: about -0.4 pascals, draft established.
The arithmetic for combustion air told the rest. The fuel gas code's standard method asks for about 50 cubic feet of room volume per 1,000 Btu/h of input (IRC G2407 as adopted by your jurisdiction, in substance; check your edition). For 40,000 Btu/h that is 40 times 50, or 2,000 cubic feet. The closet is 5 times 3 times 8, or 120 cubic feet, 6 percent of what the water heater needs. The louvered doors had been the opening to the rest of the house; the solid doors closed it.
The dryer was the multiplier. An electric dryer of this kind commonly moves somewhere around 150 to 200 cfm out of the house when its vent is clean, and in a closet with solid doors the air it exhausts has to come from somewhere. See related: The Clothes Dryer as the Fan Nobody Counts.
The fix
The gas tech gave the owners three options, in writing:
- Restore a combustion air path into the closet. Two permanent openings to the hallway, one within 12 inches of the top of the enclosure and one within 12 inches of the bottom, each with at least 1 square inch of free area per 1,000 Btu/h and not less than 100 square inches. At 40,000 Btu/h the per-input figure is 40 square inches, so the 100 square inch minimum governs: 100 square inches each. The openings have to lead to space that, with the closet, makes up the 2,000 cubic feet; the hallway alone does not, but the hallway open to the bedrooms and stair does.
- Go back to louvered doors with a published free area that meets the same figures, high and low.
- Replace the water heater with a sealed-combustion direct-vent unit, which takes its combustion out of the closet's air altogether, or a heat pump water heater, which burns nothing but needs the room air volume or ducting its installation instructions call for, more than a closet this size gives.
The owners kept the solid doors for the look and had the handyman, working to the gas tech's spec, cut two listed grilles into the doors, each 100 square inches of free area, one high and one low. The gas tech inspected and cleaned the burner again, since it had sooted a second time, and reran the test: doors shut, dryer running, water heater firing, closet at about -0.8 pascals, smoke drawn into the hood, draft established, CO in the spillage zone at zero. A plug-in CO alarm went in the hallway that afternoon.
Why the first repair could not have held
The plumber's diagnosis was right and his repair was right, and it was always going to come back, because the soot was a symptom of the closet's air, not of the burner. The detail that would have sent him upstream was on the ticket the whole time: a burner that sooted in months after fourteen clean years. A burner that soots without a change in the burner points to a change in what it breathes, and the question that finds it is "what changed in this house recently?" The answer, solid doors, was standing in front of him.
The trap in a clean vent. The appliance tech's shop now tells its techs one thing that sounds backwards: in a closet like this, cleaning a clogged dryer vent can make the spillage worse. A lint-packed vent moves little air and pulls little on the closet; the same vent cleaned out moves its full flow and pulls the closet harder. A dryer vent cleaning in a closet with a draft-vented water heater now gets a check of the closet before the tech leaves: soot at the draft hood, the tech's own CO monitor during the post-cleaning test run, and a written referral to a gas tech if the closet has no combustion air path.
The door swap is mechanical work in disguise. The handyman's shop, told what happened, added a line to its own job sheet: before changing a door on any closet, check whether there is a gas appliance or a dryer inside, and if there is, keep the existing louvers or vents or call the owner before closing them up. Ten seconds of looking would have saved two service calls and a season of headaches.
What would change the read. If the dryer had been gas, it would add its own input to the combustion air figure and its own vent to the list of things the closet's pressure affects. If the water heater had been sealed-combustion and direct-vent, the same door change would have been a lint and dryer-performance issue, not a CO issue.
References
- See related: How a House Pulls Flue Gas Back Down a Vent; The Clothes Dryer as the Fan Nobody Counts
- See related: The Combustion Appliance Zone and the Door That Closes It; Counting Every Exhaust Device in the House
- IRC G2407 (combustion, ventilation and dilution air), as adopted by your jurisdiction; 2021 edition consulted
- Water heater and dryer manufacturers' installation instructions for clearances and combustion air