Each unit in a new condominium had its own heating system. A sealed-combustion high-efficiency gas water heater connected to the outdoors by polyvinyl-chloride flue and combustion air-intake pipes running up through the roof provided domestic hot water and hot water for a heating coil in the air-conditioning unit.

Residents had moved in just a few days prior, but now the building stood empty by order of the fire department. Carbon-monoxide (CO) detectors had gone off in a third-floor unit the previous afternoon. Two residents had been hospitalized, and firefighters had detected CO in the apartments above and below the affected unit, so the fire department evacuated the building until the problem could be corrected.

The flue and combustion air-intake piping looked OK, but it did not take long to find the problem. Opening the ceiling and taking down and inspecting the offending unit's flue piping released about a half gallon of water. The vent pipe made a long horizontal offset before going up through the roof. No pipe hanger was supporting it, only the friction of the high hat flashing. Snow and water must have accumulated in the pipe during winter storms when the heater was off, and the piping sagged so that the horizontal pipe formed a trap at the elbow. Eventually, the trap had filled with enough water to block it off, and the flue gases no longer were able to vent from the heater. Instead, the gases escaped into the equipment closet.

This explanation solved the third-floor unit's problem, but the second- and fourth-floor flues were fine. Those heaters worked. However, the overflow pans under the water heaters were connected by an open vertical drain pipe that provided a direct path for the flue gas escaping from the third floor to migrate to the second and fourth floors. Therefore, CO increased to detectable levels in those units. That narrowed the problem to one water heater. Or did it?

When I went up to the roof and pushed down on each flue and combustion air-intake pipe, three others moved down several inches and conceivably could form similar traps. With a few pipe hangers in strategic locations, the residents could come back with their confidence restored.
David M. Elovitz, PE
Energy Economics Inc.
Natick, Mass.

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