Masonry chimneys, liners, and single wall pipe. Episode sixteen of the Gas Service Animated series.

Many masonry chimneys were built generations ago, when a coal or oil furnace filled them with huge volumes of hot flue gas. Connect a modern gas water heater to that same chimney and the flue is enormous for the job. The small stream of flue gas rises slowly, cools against the cold masonry, and loses its buoyancy. Cool gas means weak draft, and the moisture in the flue gas starts condensing on the walls.

Flue gas condensate is mildly acidic, and masonry hates it. Year after year it soaks into the mortar joints and the clay tile. Mortar washes out, brick faces spall and flake away, and tile liners crack and shed fragments down the flue. Inside the house it can show up as stained or damp walls near the chimney. A chimney can quietly eat itself for years before anyone notices.

That is why the code says inspect before you connect. Before replacing an appliance or connecting a vent connector to a chimney, the passageway must be examined to confirm it is clear and free of obstructions, and it must be cleaned if it ever vented solid or liquid fuel. Chimneys must be lined. The cleanout must stay tightly closed, or be repaired or replaced. And when inspection shows the chimney is not safe for the job, it must be repaired, rebuilt, lined, relined, or replaced before any appliance is attached.

A liner gives flue gas a corrosion resistant path sized for the appliance. The code accepts approved clay flue lining, listed chimney liner systems, or other approved material that resists corrosion and heat. A common repair is a listed corrugated metal liner dropped down the old flue. Remember the sizing rule: corrugated liners in masonry chimneys use the vent tables with the maximum capacity reduced by twenty percent. And when the old flue is unsafe or badly oversized, relining is not optional. It is the fix.

Where the chimney stands matters as much as how big it is. An interior chimney is wrapped in warm house on all sides. An exterior chimney has cold outdoor air against its walls, so flue gas cools much faster. The code gives exterior masonry chimneys their own tables tied to the local winter design temperature, and adds a minimum input check. An exterior chimney can pass the maximum capacity check and still fail the cold weather minimum. When it fails, the usual answer is a listed liner.

Single wall metal pipe is the bare bones option, and the code keeps it on a short leash. In dwellings and residential occupancies it cannot serve as a vent at all — there it is only a connector. Where it is permitted as a vent, it may only run directly from the space where the appliance sits, straight through the roof or an exterior wall to the outdoors. It cannot be buried in concealed spaces or snake through floors. Uninsulated single wall pipe stays out of outdoor use in cold climates. It needs generous clearance to combustible material, and where it passes through a combustible roof or wall it needs a thimble at the point of passage.

Now the failure gallery. A pile of debris at the base of the flue, blocking the connector. Liner sections that pulled apart, dumping flue gas into the chimney cavity. Two appliances jammed into one small tile that was never sized for both. And abandoned openings left unsealed, leaking air and flue gas where they should not. Every one of these hides behind a clean looking chimney face.

So before a chimney carries your appliance, it has to earn it. Verify the passageway with a full visual inspection. Confirm the cleanout closes tightly. Confirm the liner is continuous and intact from the appliance connection to the termination. Size the system from the correct table family for the chimney and connector you actually have. And confirm the termination height above the roof from the termination rules.

Old chimney, modern appliance. Inspect first, line it right, respect the cold exterior wall, and keep single wall pipe out in the open. The printable study sheets in this series carry the table details. Next episode: the common venting masterclass.