The New Water Heater That Spilled at the Draft Hood
Why this matters
A like-for-like gas water heater swap is the job most likely to be done on autopilot, and the venting is the part most likely to have changed under it. When a furnace leaves a shared masonry chimney for its own sidewall vent, the water heater left behind is called orphaned: it is now one small appliance on a flue sized for two, and it often cannot draft. The old heater spilled combustion products into the basement for a year and nobody knew. The new one was caught in the first minute because the tech ran the spillage test before leaving, which is the only reason this is a case write-up and not a carbon monoxide call.
Stop first: what the tech did in the first two minutes
The test was the one in Water Heater Changeout: Day of Install: personal CO monitor clipped at the breathing zone before the burner lit, house set to worst case (dryer running, bath fans and range hood on, basement door shut), burner fired, smoke held at the draft hood relief opening.
At 60 seconds smoke was still rolling outward along the hood skirt. The personal monitor had climbed from background to 15 ppm standing at the heater.
Contain before thinking. The tech closed the heater's gas control and then its appliance shutoff, which stopped the source. The CO reading was the reason not to stay and experiment: the monitor was rising and the tech was breathing it. Then:
- Ventilated the basement: exterior door and a window opened, the dryer and fans turned off so they stopped pulling air back down the chimney.
- Checked the rest of the house. The monitor at the top of the basement stairs read background, and so did a walk through the main floor. Had it read above background upstairs, the occupants would have gone outside first and the diagnosis waited.
- Red-tagged the heater off at the gas control and the shutoff, and told the homeowner face to face that the heater was installed but could not be left running, why, and that the house would have no hot water until the vent was fixed.
Three containment options existed, and only one was acceptable. Leaving the heater running "until the chimney guy comes" was not one of them. Running it with the basement window open was not one either; it may help on one day and not the next. The acceptable choice was off and tagged, with hot water lost until the vent was right.
Is it the house or the vent?
A spilling draft hood has two families of cause, and they get fixed by different people. The house can be pulling air down the chimney (exhaust fans, a dryer, a stack effect in a tight house) faster than the cold flue can push up. Or the vent cannot establish draft at all: blocked, too small, too big, too cold, or the connector is wrong. The test that separates them is the same test run with the house neutral.
After the basement cleared to background, the tech reset the house to neutral: dryer off, every fan off, basement door open to the main floor, a basement window cracked to give makeup air. Burner lit, monitor on.
- At 60 seconds: still spilling, though less than before.
- At about 4 minutes: smoke began to draw in. By 5 minutes it drew steadily.
That pattern is the vent, not the house. (The NFPA 54 annex procedure reads draft after 5 minutes of main burner operation, so by that test alone this heater would have scraped by; the shop holds a stricter 60 second pass point, and the flue was over the code's size limit either way.) A house-depressurization problem goes away or nearly away once the depressurizing appliances stop. A flue that spills for minutes and then drafts once it warms up is a cold flue: it cannot move combustion products until the heater's own exhaust has heated the masonry enough to create buoyancy, and until then the products come out of the hood. The burner was shut down at 5 minutes and stayed tagged.
Had the neutral test drawn at once, the call would have gone the other way. A heater that drafts in seconds with the house neutral and spills only with the fans and dryer running is a makeup-air problem: the house is starving the appliance, and the fix is combustion and makeup air, sized by the method in Combustion Air Requirements Reference, not a liner. That case still leaves the heater off until the air is provided, because worst case is how the house actually runs on a winter evening. And had the neutral test never drawn, even at 10 minutes, the next suspect would have been a blockage (a fallen tile, a nest, a cap that collapsed) rather than a cold flue, which is why the cleanout mirror check below matters even when the size alone looks like the answer.
What the chimney showed
The flue. The chimney ran up the outside of the north wall, exterior on three sides, with a nominal 12 in by 12 in clay tile liner, measured at the cleanout at about 10.5 in square inside, so roughly 110 square inches of flue. The heater's draft hood outlet was 4 in, about 12.6 square inches. The flue was about 110 divided by 12.6, or 8.7 times the draft hood outlet; going the other way, 12.6 times 8.7 is 110.
The fuel gas code limits how large a flue may be relative to the outlet of a single draft-hood appliance venting into it, in substance to a multiple of that outlet area; NFPA 54 and the IFGC, as adopted, carry the rule in their chimney and vent sizing chapters, and the vent tables restrict exterior masonry chimneys further. Check the current edition. This flue was over the multiple the code allows, before counting the exterior exposure that makes it colder still.
The cleanout. At the chimney base, the cleanout door opened onto a puddle of water and a crust of white efflorescence on the tile. A cold, oversized flue condenses the heater's water vapor before it reaches the top, and the condensate runs down and leaches the mortar. A mirror at the cleanout showed daylight; the flue was not blocked.
The connector. Correct for the heater per its maker's installation instructions. The connector was not the problem, which is the point Atmospheric Water Heater Vent Connector Rules makes about a connector that is right on its own and still spills into a chimney that is wrong.
Here is the whole setup as the tech found it, with the spot where the smoke was held:
| exterior | chimney:
connector, 4 in single-wall, | masonry | clay tile,
up 1/4 in per ft to thimble | flue, | about 110
+-----------------------+ draws up | sq in inside
| sealed thimble | only once |
+-+ hood outlet, 4 in | it warms |
/ \ draft hood | |
<-- --> relief opening: | |
+-------+ smoke held here,| |
| water | spill rolls out | |
| heater| +[cleanout]--+
+-------+ water, white crust
Why the old heater never gave it away
The furnace that had shared the chimney was replaced the winter before by a heating contractor with a sealed-combustion condensing furnace vented in PVC through the rim joist. Its old thimble was patched with a cap. That left the water heater alone on the flue.
The old heater spilled too, every cold start, for about a year. Three things hid it.
- The spillage is front-loaded. Most of it happens in the first few minutes of a burn on a cold flue, then the flue warms and drafts. Nobody stands at a water heater during the first minutes of a burn unless they are testing it.
- Nobody tested it. The heating contractor removed the furnace and left. The fuel gas code, as adopted, requires in substance that when an appliance is removed from a common vent, the remaining vent system be checked and resized for what is left; check the current edition's chimney and vent chapter. That check was the one step that would have caught it, and it is a step the departing trade owns.
- The evidence was on the old heater, but it went to the curb. On the old unit in the driveway, the tech found what had been missed: rust streaks running down the jacket below the draft hood, the hood's paint browned on one side, and the top of the tank corroded where condensate from the spilling products had settled. A tech reading the removed heater before setting the new one would have predicted the spillage test.
The new heater was not worse than the old. It was tested.
The fix and who owned it
The options. Three fixes address an orphaned heater on an oversized exterior chimney:
- A listed chimney liner sized to the heater by the adopted edition's vent tables, insulated where the tables require it for an exterior chimney.
- A power-vent or direct-vent water heater with its own sidewall vent, abandoning the chimney.
- A heat pump or electric heater, where the electrical service allows, also abandoning the chimney.
The pick. The homeowner chose the liner because the heater was new and paid for. A 4 in insulated stainless liner sized from the tables was quoted, with a permit, and installed two days later by the shop's lead tech, who holds the license and the liner maker's training for it; where a chimney contractor does liners in your area, that is the handoff point. The old furnace thimble was sealed at the liner. The clay tile was left in place as the chase.
The cost conversation. The extra work was not caused by the plumbing install, and the customer was told so plainly. The tech's ticket noted the furnace replacement date and that the remaining-vent check had not been done when the furnace came out. Whether the homeowner pursues the heating contractor is the homeowner's call; the shop's job was the facts on paper.
How they confirmed it
After the liner went in, the full worst-case test was rerun: dryer, all three fans and the range hood on, basement door shut, personal monitor on. Burner lit.
- Smoke drew into the relief opening within about 10 seconds, and stayed in for the full run.
- The personal monitor stayed at background for 10 minutes at the heater.
- The neutral-house test drew in from the start.
The red tag came off, the heater was set to 120 F at the nearest fixture, and the record carried both spillage results (failed at 60 seconds before the liner, drawing at about 10 seconds after), the monitor readings, the flue and draft hood areas, photos of the cleanout, the removed heater's rust streaks, and the homeowner's signature on the off-and-tagged notice from the first day.
The rule the shop took from it. Before a gas heater comes out, the tech looks at what else is, or used to be, on the chimney. A patched thimble beside a heater's vent means an appliance left, and the heater is now orphaned until a test proves otherwise.
References
- NFPA 54 or IFGC 2021, as adopted by your jurisdiction: chimney and vent sizing, the single-appliance flue limit, and resizing a vent system after an appliance is removed.
- The water heater maker's installation instructions, part of its listing, for venting and draft requirements.
- See related: Atmospheric Water Heater Vent Connector Rules; Water Heater Changeout: Day of Install; Combustion Air Requirements Reference.