Common Venting and Combustion Air for a Condensing Boiler and Draft-Hood Water Heater
A 100,000 Btu/h condensing boiler, Category IV, and a 50,000 Btu/h natural-draft water heater are installed in a 10 ft by 12 ft mechanical room with a 7 ft ceiling. The room has two louvered openings to a corridor: one 12 in by 12 in opening 6 in above the floor and one 12 in by 12 in opening 6 in below the ceiling. Each louver has 75 percent free area. The boiler is vented with 2-in PVC through a sidewall. The contractor proposes to connect the water heater draft hood to the same PVC vent with a wye fitting. The water heater's existing vent is a 6 in by 8 in masonry chimney with a 10-ft exterior offset. The building official has asked for a combustion-air and venting evaluation before issuing the permit.
1. Determine whether the two openings provide adequate combustion air. Show volume and opening calculations and state the assumption about the corridor.
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Room volume = 10 x 12 x 7 = 840 ft^3. For 150,000 Btu/h total input, the unconfined-space threshold is 50 ft^3 per 1,000 Btu/h = 7,500 ft^3, so the room is a confined space. Two permanent openings: each opening = 1 in^2 per 4,000 Btu/h = 37.5 in^2. Each 12 x 12 louver has 144 in^2 gross and 75% free area = 108 in^2, so the opening area appears adequate. However, the openings connect to a corridor, not directly outdoors. If the corridor does not freely communicate with outdoors, the indoor-air method applies: 1 in^2 per 1,000 Btu/h = 150 in^2 free area per opening, and 108 in^2 is insufficient. The AHJ must determine whether the corridor qualifies as outdoor air; otherwise, enlarge the openings or provide direct outdoor air.
2. Can the water heater be common-vented with the condensing boiler? Explain.
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No. A Category IV condensing boiler is positive-pressure and produces acidic condensate; a natural-draft water heater with a draft hood is negative-pressure and relies on dilution air. Common venting is limited to appliances of the same category and listed for common venting; the boiler manufacturer's instructions will almost certainly prohibit it. Connecting the draft hood to the PVC vent would create spillage at the draft hood, possible condensate damage, and unsafe flue-gas release. The water heater needs a separate vent sized per its listing and the UMC vent tables. The boiler's sidewall PVC termination also needs to meet clearance and condensate-management requirements.
3. The contractor proposes a 4-in PVC liner in the masonry chimney for the water heater. Evaluate.
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PVC is not acceptable as a liner for a natural-draft water heater. PVC is limited to low flue-gas temperatures (roughly 140 F or per listing), while natural-draft flue gases can be much hotter. A masonry chimney that is not gastight needs a listed liner; use a properly sized stainless-steel or other listed metal liner, installed with the required clearance, support, and termination. The 4-inch size may or may not be correct; verify against the vent tables for the appliance input, height, and connector rise. The boiler's sidewall PVC vent is a separate Category IV system and should not be connected. The chimney's exterior offset may also affect draft and needs to be included in the vent sizing.