The Condensing Furnace on the Old Masonry Chimney
A 1955 house has an unlined exterior masonry chimney serving a 40,000 Btu/h natural-draft water heater and an 80,000 Btu/h natural-draft furnace through a wye common vent. The furnace fails and is replaced with a 60,000 Btu/h Category IV condensing furnace. The contractor proposes to keep both appliances on the chimney, run 2-inch PVC from the new furnace into the chimney breech, and says the furnace blower will 'push the flue gases up.' The existing single-wall connectors slope 1/4 inch per foot away from the chimney toward the appliances, and the water heater connector is 3-inch diameter while its draft hood outlet is 4 inch. The chimney is 30 feet tall on an exterior wall, unlined, with its top 2 feet above the roof surface and 8 feet from the ridge.
1. Can the Category IV furnace be common-vented into the masonry chimney with the natural-draft water heater?
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No. Category IV appliances operate under positive pressure and produce acidic condensate; they must be vented with a venting system listed for Category IV and installed per the manufacturer's instructions. Common venting requires each appliance to be listed for common venting with the other, and a natural-draft Category I water heater cannot share a vent with a positive-pressure condensing appliance-the draft characteristics are incompatible and the chimney is not a listed vent for the furnace. Running PVC into the chimney breech does not create a listed vent system and would discharge into an unlined flue subject to condensate damage. The furnace needs its own listed vent (PVC, CPVC, or polypropylene as the instructions allow) terminating per the manufacturer and the code.
2. If the water heater stays on the chimney, what must be done to the chimney and the water heater venting?
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The chimney must be lined. An unlined exterior masonry chimney serving a gas appliance is not acceptable, and the 30-foot flue becomes grossly oversized once only a 40,000 Btu/h appliance remains-insufficient heat input to warm the flue, causing condensation and poor draft. A listed liner sized for the water heater (typically 3 or 4 inch) must be installed for the full height, with a listed cap and a proper connector. The flue should be inspected for integrity and clearance to combustibles before lining. Verify combustion air for the water heater in its space and confirm the draft hood connector is correctly sized and supported. The furnace is vented separately.
3. Identify the vent connector defects and how each must be corrected.
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Three defects. (1) Slope: the connectors run 1/4 inch per foot away from the chimney; they must slope upward toward the chimney not less than 1/4 inch per foot over the entire run, with no sag. (2) Sizing: the water heater connector is 3 inch but the draft hood outlet is 4 inch; a connector must not be smaller than the appliance outlet, so it must be 4 inch (or the full outlet size) to the chimney. (3) Termination height: the chimney top must be at least 3 feet above the roof penetration and at least 2 feet above any part of the building within 10 feet horizontally; with the ridge 8 feet away, the top must be 2 feet above the ridge, which must be verified. Also verify support, the thimble at the chimney, and that no single-wall connector passes through a concealed space.