Condensing boiler replacement with an orphaned water heater
A contractor is replacing a failed 140,000 Btu/h atmospheric gas boiler in a 10 ft by 12 ft basement mechanical room with a 120,000 Btu/h condensing boiler. The new boiler will be direct-vented through the sidewall with 3-in. PVC. The existing 40-gal natural-draft gas water heater, also in the room, shares an 8 x 12 in. masonry chimney with the old boiler. The mechanical room has a full-lite louvered door to the basement; there are no high or low outdoor combustion-air openings. The chimney is on an interior wall and terminates above the roof. The homeowner wants to keep the water heater and avoid a chimney liner. A bedroom window is 4 ft from the proposed PVC termination, and a clothes dryer vent is 3 ft below it.
1. After the old boiler is removed, what combustion-air and venting problems does the water heater present? What would you require before signing off?
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Once the boiler is removed, the water heater is orphaned in a chimney sized for a much larger appliance. A 40,000 Btu/h natural-draft water heater in an 8 x 12 in. masonry chimney may have too little draft, especially in a cold exterior chimney, leading to spillage, condensation, and deterioration. I would require a chimney sizing calculation and, if the chimney is oversized, a listed liner sized for the water heater or a change to a direct-vent or power-vent water heater. I would not accept the existing chimney without verification.
Combustion air is also inadequate. The room is 10 x 12 x 8 = 960 ft3. Using the conventional indoor-air method, a 40,000 Btu/h appliance needs about 2,000 ft3, so the room alone is too small. The louvered door to the basement is not a guaranteed outdoor-air opening. I would require two permanent openings sized per the gas code to outdoors or a listed combustion-air system, and I would verify draft with a combustion analyzer and spillage test. CO alarms should be installed.
2. Evaluate the proposed PVC termination clearances and material. What corrections are needed?
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The PVC termination must follow the boiler manufacturer's listing and the gas code, not just a generic rule of thumb. A bedroom window 4 ft away may satisfy a minimum distance, but it is still a poor location because exhaust plume, odor, and recirculation can affect the window. A clothes dryer vent 3 ft below is also a concern: lint can be drawn into the combustion air, and moisture can freeze on the termination.
I would relocate the termination to a location with proper clearance above grade and snow, away from windows, doors, operable openings, dryer vents, and forced-air intakes. I would verify the PVC is the manufacturer's listed material and that the termination is the listed type. The vent must slope back to the boiler, be supported, and have a condensate trap and neutralizer. In Massachusetts, also confirm local amendments and any historic-district or sidewall restrictions before cutting the wall.
3. The owner asks whether the new boiler can use the existing masonry chimney instead. Explain your answer.
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A condensing boiler produces low-temperature, acidic flue gas. It cannot be vented directly into an unlined masonry chimney because the gas will condense on the masonry, corrode the chimney, and may cause draft problems. The chimney is not automatically usable just because it exists.
If the owner wants to use the chimney as a chase, the boiler must be vented with a listed liner or listed vent system approved by the manufacturer for condensing service. The liner must be sized for the boiler and installed with the required clearances and termination. The water heater would still need its own venting solution; the two appliances generally cannot share a chimney unless a listed common-vent system is designed for that arrangement. I would require the manufacturer's venting instructions and a code-compliant design before proceeding.