Backdrafting Water Heater in a Shared Chimney
A 50-gallon natural-draft atmospheric water heater shares a 6 ft x 8 ft mechanical closet with a Category I 80% furnace. Both vent into a single 5-inch masonry chimney about 10 ft tall. The closet has a louvered door to the hallway and no dedicated combustion air duct. With both appliances firing and a 110 cfm downstairs bath exhaust fan running, a digital manometer reads -7 Pa in the closet relative to outdoors. A smoke pencil at the water heater draft hood shows spillage that has not cleared after three minutes. A CO meter reads 14 ppm in the closet and 6 ppm in the adjacent hallway; the water heater flue sample is 40 ppm undiluted. The contractor proposes adding a CO alarm and a 4-inch makeup air grille in the closet door.
1. Interpret the pressure, spillage, and CO data. Is this a pass or a fail, and on what basis?
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This is a clear fail, and the reasons are independent of each other:
- -7 Pa in the closet exceeds the usual -3 Pa worst-case limit used for a naturally drafted appliance in a combustion appliance zone (some programs allow -5 Pa). At -7 Pa the closet is competing with the chimney for air, and the chimney loses.
- Spillage persisting past three minutes at the draft hood is a fail by the standard spillage test. A brief puff at startup can be normal; sustained spillage is not.
- 14 ppm CO in the closet confirms combustion products are entering the space. That reading, not the flue reading, is the health-relevant number.
The 40 ppm undiluted flue CO is actually unremarkable for a natural-draft burner, and that is the trap in this scenario: good combustion does not mean safe venting. The defect is the venting path and the pressure environment, not burner adjustment. Note also that the louvered door is the only air path and that two appliances share a 5-inch flue, which is a classic combination for poor draft even without the fan.
2. Evaluate the proposed fix. What sizing basis would you use for combustion air, and what else must be addressed?
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The proposal is inadequate on two counts. A CO alarm is a warning device, not a repair, and a single 4-inch grille may not deliver enough air, may not relieve the fan-driven depressurization, and does nothing about the shared chimney.
Sizing basis: use the total appliance input and the applicable combustion-air method, then convert to free area. For example, two permanent openings connected to outdoors are commonly sized at 1 sq in of free area per 4,000 BTU/hr of total input when using vertical ducts, or 1 sq in per 1,000 BTU/hr when using horizontal ducts, with louver free area derated (wood louvers often taken at 25%). Confirm the method in the local code and the appliance instructions before cutting.
Other items to address:
- The fan interaction. Either provide makeup air near the exhaust fan or interlock the fan, then re-test.
- The shared chimney. Verify it is lined, correctly sized for both appliances, and not blocked or leaking; a 5-inch flue serving two appliances often needs a liner evaluation.
- The closet door. If it is the only air path, its free area must be included in the calculation or replaced with properly sized openings.
- Worst-case testing with all exhaust devices operating, not just the bath fan.
3. What test results and documentation must exist before the system is returned to service?
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Return to service should require a signed record containing:
- Worst-case depressurization test - closet pressure relative to outdoors with every exhaust fan, dryer, and vented appliance running, and with interior doors in their normal position.
- Draft and spillage test at each appliance - smoke pencil or draft gauge at the draft hood, with a stated observation time.
- CO readings - ambient in the closet and adjacent rooms, plus undiluted flue samples at steady state.
- Combustion analysis - before and after any adjustment, with O2 or CO2, CO air-free, and stack temperature.
- Combustion air calculation - total input, opening sizes, louver free area assumed.
- Chimney inspection - liner condition, sizing, blockage, and photographs.
The system should not be left running unless it drafts under the worst case. If it cannot, shut it down and tag it, specify the repair, and re-test after the repair using the same conditions so the before/after comparison is meaningful.