Clifton furnace and water heater sharing a flue: getting the sizing right
Two gas appliances into one big old flue is the most common carbon monoxide setup we see in older Clifton homes. The math on why, and the liner that fixes it.
The setup
A pre-1950 house in Clifton, the original chimney built for a coal furnace. Today a mid-efficiency gas furnace and a natural-draft water heater both connect to it. The chimney flue is a 12 by 12 clay tile, which gives more than a hundred square inches of area. The two appliances together need something like a five- or six-inch round vent, about twenty to thirty square inches. The flue is four times bigger than it should be.
Why oversizing is dangerous
Draft depends on the flue gases being hot and moving. In a flue that big and that cold, the small volume of exhaust from a gas appliance cannot heat the flue; it cools, slows and condenses. The water heater is the victim: when the furnace fires, it may set up enough draft to pull the water heater exhaust along, but when the water heater runs alone its exhaust stalls in the cold flue and spills back out the draft hood into the basement. That spillage is carbon monoxide. Over time the condensate also eats the mortar joints, and debris starts falling into the flue and blocking it further.
The fix
Sizing tables in the fuel gas code give the right liner diameter for the combined input of both appliances and the height of the flue. In this house that came to a five-inch AL29-4C stainless liner, with the water heater connector entering above the furnace connector and both rising properly to the liner tee. We tested draft at both draft hoods with a smoke pencil and a CO meter after installation: no spillage with either appliance alone or both together. That is the only test that counts. If your furnace and water heater share an old flue and nobody has ever measured spillage, it is worth an inspection before winter.
If this sounds like your house: we handle chimney relining for gas appliances across Kankakee County, and our Clifton page covers what we usually find there. Call (815) 449-4264, 7am to 9pm, any day.
Quick questions
How can I tell if my water heater is spilling exhaust?
Soot or melted plastic around the draft hood, a headache in the basement, or a CO alarm. We test with a smoke pencil and CO meter.
Can two gas appliances share a liner?
Yes, if the liner is sized for the combined input and the connectors are arranged correctly.
Why not just use the big flue?
Because gas exhaust cannot heat an oversized masonry flue; it stalls and condenses.
Does a high-efficiency furnace vent into the chimney?
No. Condensing furnaces vent through PVC to the outside, which leaves the water heater alone in the old flue and makes the sizing problem worse.
Published October 3, 2026 · By Jacob Dove, Friendship Chimney Sweep, Bourbonnais IL