Does a Gas Furnace Chimney Need Cleaning?

What gas exhaust leaves behind

Natural gas burns to carbon dioxide and water vapor, with very little soot when the burner is adjusted correctly. In a warm, correctly sized flue, the vapor leaves the chimney as a faint plume you can see on a cold morning. In a cool or oversized masonry flue it condenses on the walls before it gets out, and that condensate is mildly acidic.

Season after season, the acid softens mortar joints and flakes the glazed inner face off clay tile. On an unlined flue it eats into the brick and the old parging. None of this looks like the black, crusty buildup people picture in a dirty chimney, which is why a gas flue can be in poor shape while looking clean from the basement.

Debris from above and below

Everything that comes loose inside the flue drops to the base, the same spot where the vent pipe from the furnace or boiler enters. Material from outside joins it if the cap is missing, rusted through or torn. Over time the pile can rise to the level of the thimble opening and start to narrow the exhaust path.

• Flakes of clay tile and crumbled mortar
• Nests, leaves and animal debris from an open or damaged top
• Rust scale from an old cap, damper or vent connector
• Standing water from condensation or rain
• Light soot, when a burner has been running out of adjustment

Why a blocked gas flue is easy to miss

Gas exhaust is invisible and nearly odorless. An oil flue that starts to block usually advertises it with soot and smoke smells; a gas flue can be partly blocked for weeks with no obvious sign. The exhaust simply spills back at the draft hood of the water heater or the draft diverter of the furnace and mixes into the basement air.

Many furnaces, boilers and water heaters have a safety switch that shuts the burner off when exhaust spills at the draft hood or the vent is blocked. If your heat or hot water keeps cutting out and the heating technician finds a tripped spill switch, the chimney is a prime suspect. Resetting it again and again without finding the cause leaves the real problem in place.

Carbon monoxide is dangerous, which is why every home with fuel-burning equipment needs working CO alarms on each level and near the bedrooms. If an alarm sounds, or people in the house feel unwell and you suspect the heating, get outside before anything else, then call 911 or the utility from there.

Clues you can check in the basement

A homeowner can do a useful look around without tools or ladders. Check with the heat running and again with it off, and look closely at the pipe where it joins the chimney and at the top of the water heater. These signs point to condensation or poor draft:

• White, chalky crust or rust streaks on the vent connector joints
• Water dripping from the connector or pooling on the basement floor below it
• Discoloration or scorch marks around the draft hood on the water heater
• Warped or melted plastic parts on top of the water heater
• Tile pieces or wet grit behind the cleanout door
• A stale, damp or sharp smell near the appliances when they fire

What the yearly service involves

The visit begins with the appliances: what vents into the flue, how the connector is pitched and joined, and whether it is rusted or loose. The technician then opens the cleanout, removes what has collected and looks up the flue with a light. From the roof, the cap, crown and flue outlet are checked, and the flue is examined for eroded tile, open joints and anything lodged inside.

Brushing is done where there is soot or loose scale on the walls; in a healthy gas flue it may be light. Last, the draft is checked with the appliances firing to confirm the exhaust is leaving the house. You get a plain account of what was found, including anything that should not wait until next season.

How often a gas flue needs attention

Under NFPA 211, chimneys and vents burning any fuel get an inspection every year, and a cleaning whenever the inspection finds deposits. For most gas flues that means one visit a year, ideally before the heating season so problems are found while the house is still mild. A flue serving both a boiler and a water heater works year round and deserves that visit without fail.

Some events call for a look outside the usual schedule: a new furnace, boiler or water heater connected to the flue, a CO alarm that sounded, a storm that damaged the cap, or water appearing at the base. Each changes how the flue behaves, and none of them shows from the basement alone.

Shared flues after an equipment change

Many houses vent a gas furnace or boiler and a gas water heater into the same flue, which works when the flue was sized for both. Replace the furnace with a high-efficiency model that vents through the side wall, and the water heater is left alone in a passage built for far more exhaust. Its small, cool exhaust condenses heavily and drafts weakly.

The same thing happens in reverse when a water heater is replaced with a tankless or power-vented model and the boiler stays on the chimney. Any change in what vents into a flue changes how much heat travels up it, so the flue size is rechecked whenever the lineup changes.

If your heating equipment was replaced in the last few seasons and the water heater still vents into the chimney, ask for the flue size to be checked against the water heater on its own.

When cleaning is no longer enough

Cleaning removes what has fallen, but it does not restore eroded tile or shrink an oversized flue. When the lining is breaking down, when water keeps returning to the base, or when the flue is far too large for what now vents into it, the fix is a liner matched to the furnace, boiler or water heater and installed to its venting instructions. A stainless liner protects the masonry from condensate, keeps exhaust warmer and gives a steadier draft.

The options, from a simple cleaning and cap replacement to a full liner, are written up with the work involved and the price in a free written estimate, letting you decide with everything in front of you.