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Heating · VentingChimney Liner Installation for Gas Appliances
Chimney liner installation for gas appliances puts a correctly sized metal liner inside an old masonry flue so a natural-draft water heater or boiler can vent safely. The rule: if a high-efficiency furnace now vents out the wall and a gas water heater is the only appliance left on the chimney, the flue is almost always too big for it. It needs a liner, or the water heater needs to come off the chimney. The liner diameter comes from the CSA B149.1 venting tables, not from guesswork.
The job exists because of the orphaned water heater. For decades a furnace and a tank water heater shared one clay-tile flue, and the furnace's hot exhaust did most of the work of keeping that flue warm and drawing. Replace the furnace with a condensing unit that vents through plastic pipe, and the water heater is left alone on a flue built for several times its input. Its small, cool exhaust cannot warm that much masonry, so it either spills back out of the draft hood into the basement or condenses on the flue walls.
Condensation is the slow damage. Flue-gas moisture is mildly acidic; it soaks into the clay tile and mortar, freezes in winter and pushes the brick apart from the inside. The signs show outside as white staining and flaking brick near the top of the chimney, and inside as damp patches on the chimney breast or rust on top of the water heater. Chimneys built on an outside wall are the worst case, because three sides of the flue sit in the cold.
A liner fixes the size mismatch. We size it from the CSA B149.1 tables using the appliance input in BTU per hour, the height of the chimney and the length of the connector between the appliance and the flue. For natural-draft gas appliances the liner is flexible aluminium, lowered from the roof, joined at the base to the vent connector and finished at the top with a sealed plate and rain cap. Aluminium liners are for gas only: an oil or wood appliance on the same chimney needs stainless steel and a different trade.
Lining is not the only answer, and sometimes it is not the best one. If the tank is over ten years old, replacing it with a power-vent water heater or a direct-vent model takes it off the chimney entirely, and the flue can be capped and abandoned. We install liners but do not sweep chimneys or rebuild masonry, so a flue blocked by fallen tile or nests needs a chimney sweep or mason first. Whether your house needs a liner at all is covered in our guide to chimney liners and furnace replacement.
When a gas chimney needs a liner
- A new high-efficiency furnace vents through the wall and the gas water heater is still on the old chimney
- White powdery staining, flaking brick or crumbling mortar near the top of the chimney
- Damp patches or peeling paint on the wall where the chimney runs through the house
- Rust, scorch marks or melted plastic on top of the water heater around the draft hood, the marks of exhaust spilling back
- A carbon monoxide alarm that sounds when the water heater fires: leave the house and call from outside
How it works
We record the input of every appliance staying on the chimney, the flue height from the connector to the cap, and whether the chimney is inside the house or on an outside wall. A draft test at the water heater's hood shows whether it is spilling today.
The liner diameter comes from the B149.1 table for that input and height. It goes down from the roof, connects to the appliance vent connector at the base, and is sealed at the top with a plate and cap so rain and cold air stay out of the old flue.
With the house closed up and the dryer and exhaust fans running, we fire the appliance and confirm the draft hood pulls air in instead of spilling. You get the liner size and the appliance inputs it was sized for in writing.
Common questions
Do I need a liner if my furnace and water heater both still use the chimney?
Possibly not. A chimney that has vented the same two natural-draft appliances for years without spillage or masonry damage may be within the tables as it stands. The liner question comes up when the appliance mix changes: a furnace leaves the flue, a smaller boiler goes in, or an inspection finds cracked tile. We check the inputs against the table before recommending anything.
Can the liner go in on the same day as the new furnace?
Usually. On a two-storey house with a walkable roof the liner is a few hours of work and is done while the furnace crew is on site, so the water heater is never left venting alone into the oversized flue. Steep roofs, very tall chimneys and winter ice can push it to a separate visit, and we say so on the quote.
Why aluminium and not stainless steel?
Aluminium is the standard liner material for natural-draft gas appliances: it handles their exhaust temperature and costs less. It is not rated for oil, wood or condensing appliances, whose exhaust is hotter, dirtier or far more acidic. If a wood stove or oil appliance shares the chimney, that flue is stainless-liner work for a WETT-certified installer, not part of our job.
My water heater is a rental. Who pays for the liner?
The liner is part of your house, so it is normally the homeowner's cost even when the tank belongs to a rental company. That is worth knowing before a furnace replacement: some owners use the moment to end the rental and buy a power-vent tank they own. Our water heater rental buyout page explains how that comparison works.
Is a spilling water heater dangerous?
Yes. Spillage puts combustion products, which can include carbon monoxide, into the basement instead of up the flue. Ontario requires a working carbon monoxide alarm near sleeping areas in any home with a fuel-burning appliance. If the alarm sounds, leave and call 911 from outside. For a non-urgent check of a water heater you suspect is spilling, book a draft test.