The Basement Finish That Closed In the Furnace
Why this matters
Finishing a basement almost always means building a room around the furnace and water heater, and that room is where combustion air comes from and where flue gas goes if the vent stops working. In this case the remodeler built the room with a solid door and the drywaller, trying to help, cut a return grille into the duct inside it so the "utility room wouldn't get stuffy." Nobody on the remodel thought of either as mechanical work. The water heater spilled flue gas into that room every time the furnace blower ran, through most of a heating season, until a maintenance visit caught it.
The safety handling comes first in this write-up because it came first on the visit.
What the tech saw, and the first ten minutes
A fall maintenance visit. The HVAC shop's tech, who is also the shop's gas tech, wears a personal CO monitor clipped at the chest on every combustion call. The basement had been finished since his last visit. The furnace and water heater were now inside a new room off the family room, with a solid slab door, closed.
Inside, before touching anything: rust streaks down the water heater's jacket below the draft hood, a light soot line at the hood's rim, and condensation marks on the vent connector. The furnace was running on a heating call. His monitor read 22 ppm and climbing.
His first moves, in order:
- Propped the door wide open and turned the furnace off at the thermostat, which stopped the blower and its pull on the room. The water heater was not firing at that moment.
- Asked whether anyone in the house had headaches, nausea or dizziness that eased outdoors. No one did. If anyone had, or if his monitor had kept climbing with the door open, everyone would have left the house and he would have called 911 from outside.
- Waited for the monitor to fall back to zero with the door open before continuing, and checked that the house had a working CO alarm on each level. The basement had none; he placed a plug-in alarm from the truck in the family room and wrote it on the ticket.
Only then did he start working out why.
The room as built
FINISHED BASEMENT PLAN (not to scale)
+----------------------------------------------+
| FAMILY ROOM (finished) |
| |
| +----------- MECH ROOM -------+ |
| | [FURNACE] 80,000 Btu/h | |
| | return trunk with new | |
| | grille cut in: [RG] | |
| solid -> | | |
| door | [WATER HEATER] 40,000 Btu/h | |
| (shut) | draft hood, common vent | |
| | to masonry chimney | |
| +-----------------------------+ |
| room 9 x 8 ft, 7.5 ft ceiling |
+----------------------------------------------+
Both appliances are draft-vented: they take combustion air from the room and send flue gas up a common vent by buoyancy alone. The return grille, about 14 by 6 inches, was cut into the side of the return trunk a few feet from the furnace.
Working through the suspects
The tech ran each test with his monitor on, the door propped open between tests, and a smoke pencil at the water heater's draft hood rim while the water heater fired on its own call. Reading the room's pressure against the family room with a manometer, tube under the door:
| Condition | Room vs family room | Water heater draft hood |
|---|---|---|
| Door open, blower off | about 0 Pa | smoke drawn in, draft established promptly |
| Door shut, blower off | about -0.4 Pa | smoke drawn in, slower to establish |
| Door shut, blower on | about -4.5 Pa | smoke pushed out, continuous spillage |
| Door open, blower on | about -1.0 Pa | drawn in, established |
| Door shut, blower on, grille covered with taped sheet metal | about -0.6 Pa | drawn in, established |
Blocked vent or chimney. Ruled out first: with the door open and the blower off, the water heater established draft promptly, and a mirror at the vent connector showed no obstruction. A blocked vent spills whatever the room is doing.
A burner fault. Flame pattern normal; the CO came from spilled flue gas, not from a sooting burner, since it appeared only under the conditions that reversed the draft.
The door alone. Shutting the door with the blower off brought the room to -0.4 Pa and slowed the draft but did not reverse it. The door was a real problem (see the combustion air arithmetic below), but on its own it was not what produced continuous spillage.
The return grille. Blower on, door shut: -4.5 Pa and spillage. Same conditions with the grille covered: -0.6 Pa and an established draft. The grille was the driver. A return opening inside a closed appliance room pulls air out of that room every minute the blower runs, and the only large path back in is down the water heater's vent. See related: The Combustion Appliance Zone and the Door That Closes It, which owns the idea of the room as its own pressure zone.
The combustion air arithmetic
Even with the grille gone, a closed room this size cannot supply these appliances. The fuel gas code's standard method asks for about 50 cubic feet of room volume per 1,000 Btu/h of total appliance input (IRC G2407 as adopted by your jurisdiction, in substance; check the current edition, its method for houses of unusually tight construction, and the appliance manufacturers' installation instructions).
- Total input: 80,000 plus 40,000 is 120,000 Btu/h, which is 120 thousands.
- Volume needed: 120 times 50 is 6,000 cubic feet.
- Room volume: 9 times 8 times 7.5 is 540 cubic feet.
540 is 9 percent of 6,000, so this is a confined space. The usual remedy is to connect it to the rest of the house with two permanent openings, one within 12 inches of the ceiling and one within 12 inches of the floor, each with at least 1 square inch of free area per 1,000 Btu/h and never less than 100 square inches. Here that is 120 times 1, or 120 square inches each. The spaces they open into have to make up the 6,000 cubic feet together; the finished basement, about 1,000 square feet at 7.5 feet, is about 7,500 cubic feet, plus the room's 540, so the volume is there once the openings are.
The return grille also ran into the IRC's limits on where return air may be taken. M1602.2 restricts return air from a room containing a fuel-burning appliance, with conditions and exceptions for things like direct-vent appliances, so read your adopted text for its exact reach; whatever it says in a given edition, the spillage test above is the reason this grille had to go.
The interim condition
Shop policy: an appliance that spills under normal operating conditions is shut off unless a temporary correction, verified on the spot, stops the spill. Here the correction held. The tech left the grille covered with taped sheet metal, wedged the door fully open with a written tag on it ("Do not close: heating appliances need this air"), reran the blower-on test with the door open and the grille covered (-0.3 Pa, draft established), set the thermostat back to its normal heating schedule and watched a full heating cycle with his monitor at zero, and gave the owner a written notice listing both conditions and the CO alarm.
How the remodeler and the HVAC shop split the fix
The remodeler had built the enclosure and its crew had cut the grille, so the shop called the remodeler with the owner's permission and laid out the readings. The split they agreed:
- HVAC shop: permanently closed the grille opening in the return trunk with a sheet metal patch sealed with mastic, sealed the trunk's other seams inside the room, and ran the final combustion safety test. Duct and appliance work is mechanical scope.
- Remodeler: replaced the slab door with one carrying two listed grilles of at least 120 square inches free area each, set high and low on the door to meet the 12-inch placement, patched the drywall around the old grille cut, and paid for both trades' return trips. The remodeler had pulled a building permit for the finish; the change went to the inspector as a revision.
- Owner: kept the plug-in CO alarm and agreed to hardwired or sealed-battery alarms on every level, which the shop quoted separately.
The remodeler also changed its own process, which is the part that prevents the next one. Any finish plan that puts a wall around a furnace, boiler or water heater now gets an HVAC walk-through before framing, with three questions written on the plan: what each appliance's input and vent type is, where its combustion air will come from once the wall is up, and whether any return or supply opening ends up inside the new room. And a standing rule for the crews: nobody but the HVAC contractor cuts into ductwork, for any reason, comfort included. The drywaller's grille took ten minutes to cut and most of a heating season to find.
The proof before the tag came off
Door shut, furnace blower on high, water heater firing on its own call: room at about -0.3 Pa against the family room, smoke drawn into the draft hood, draft established promptly, monitor at zero through the test. Then the same with the family room's bath fan on as well: -0.5 Pa, same result. Only then did the door tag come off and the job close.
What would have changed the call. If the furnace and water heater had been sealed-combustion, direct-vent units, the room's pressure would not have reversed anything, and the grille would have been a code and filtration question rather than a carbon monoxide one. If the basement had been finished with the appliances left open to the family room, the volume test would have passed outright and there would have been no door to close.
References
- See related: The Combustion Appliance Zone and the Door That Closes It; How a House Pulls Flue Gas Back Down a Vent
- See related: Which Trade Jobs Change House Pressure and Moisture; Combustion Safety After a Partial Conversion
- IRC G2407 (combustion, ventilation and dilution air) and M1602.2 (return air), as adopted by your jurisdiction; 2021 edition consulted
- Appliance manufacturers' installation instructions for clearances and combustion air