Thimbles and Connector Pipes

Two short parts with a big job

The connector is the exposed pipe from the appliance's outlet (or its draft hood or draft diverter) to the chimney. It is usually single-wall galvanized or stainless steel, or double-wall Type B vent, depending on the appliance and the manual.

The thimble is the opening in the chimney wall that receives the connector. The Brick Industry Association describes it as the lined opening through the chimney wall that receives the smoke pipe, built into the chimney and made as airtight as possible. Older thimbles are often a short clay sleeve; others are metal.

Model code text has described what the thimble must do: the 2003 International Residential Code, reproduced by Buckley Rumford Fireplaces, required inlets to masonry chimneys to enter from the side, through a thimble that keeps the connector from pulling out of the inlet or extending beyond the wall of the liner.

Following the exhaust from appliance to flue

It helps to look at the connection in three segments.

1. At the applianceA natural-draft water heater has a draft hood on top, and the connector starts there. An oil boiler or furnace usually has a barometric draft control on the connector near the appliance. Many gas furnaces have an inducer fan that pushes exhaust into the connector. Each arrangement comes with its own venting instructions.
2. Along the runThe pipe travels across the basement ceiling or wall toward the chimney. This is where slope, length, joints and support matter.
3. At the chimneyThe pipe passes into the thimble and ends at the inside face of the liner. Here what matters is the seal between pipe and thimble, and how far the pipe goes in.

When two appliances share one flue, their connectors may join with a fitting before the chimney or enter through separate openings. Shared venting is designed as a system, and changing one appliance can affect the other. The orphaned water heater guide walks through the most common case.

Slope, length and joints

Exhaust gases rise. A connector that climbs steadily toward the chimney helps them along, and one that sags or runs downhill gives them a place to cool and stall. One furnace manufacturer's installer guide, for example, says horizontal runs should pitch upward at least 1/4 inch per foot. A short, direct run with few elbows keeps gases warm and moving.

Joints matter just as much. Single-wall connector sections are usually fastened with sheet metal screws, and the run is supported with hangers or straps so its weight is not hanging on the appliance or the thimble. The specific number of screws, the allowed length and the number of elbows come from the appliance manual and the code edition your town enforces. New Jersey's chimney verification form for replacement fuel-fired equipment includes the vent connector's length and rise among the items recorded.

Where the pipe meets the chimney

The end of the connector should sit flush with the inside face of the flue liner. The same Trane installer guide says vent pipe must be inserted into, but not beyond, the inside wall of the chimney.

• Pushed in too far, the pipe sticks into the flue and blocks part of it. It also catches falling debris, and on a shared flue it can interfere with the other appliance.
• Not in far enough, or pulled partway out, the pipe leaves a gap where exhaust can escape into the room.
• A loose fit between pipe and thimble should be sealed with a material suited to the temperature. The Brick Industry Association mentions boiler putty for making the thimble airtight.

The Brick Industry Association's chimney note also describes setting the thimble at least 18 inches below the ceiling, with at least 8 inches of flue liner extending below its lowest point. That space below the inlet is what the cleanout serves; see cleanout doors and ash dumps.

Corrosion: what the pipe is telling you

Connector pipe wears from the inside out. Gas and oil exhaust carries water vapor and acidic compounds. In a warm, correctly sized system they leave the house as vapor. When the gases cool too much, they condense, and the condensate attacks metal and masonry. DuraVent's guide to relining for gas appliances explains how efficient appliances lower flue gas temperature and create condensation zones, and how the acids erode clay tiles, brick and mortar.

Signs worth noticing on a connector:

• white or rust-colored crust at seams, especially near the chimney;
• rust holes, flaking metal or pinholes you can see light through;
• water stains or drips below the pipe or at the thimble;
• soot marks around joints or around the thimble.

None of these tells you the cause on its own. A corroded connector is easily replaced, but the cause may be in the flue, such as an oversized or unlined chimney. The Trane guide states that venting its furnace into an unlined masonry chimney is prohibited and calls for the liner to be inspected for cracks or leaks. If the chimney side of the connection is the problem, see chimney liners.

Clearances: follow the appliance manual

A connector runs hot, and single-wall pipe runs hotter on its surface than double-wall vent. Every appliance manual lists a minimum clearance between its connector and combustible materials such as joists, wood trim, stored boxes or drywall over wood framing. The Trane guide calls for a thimble when a vent connection must pass through a wall or partition of combustible material.

Those distances vary by appliance type, pipe type and the code edition in force, so there is no single number that applies to every basement. What a homeowner can do is keep the area around the pipe clear: no shelving, storage or laundry near or touching the connector, and no new framing or drywall added around it during a basement finish without checking the manual first.

Why this connection is worth checking every year

CPSC's guidance on preventing carbon monoxide poisoning tells homeowners to have vents to furnaces, water heaters, boilers and other fuel-burning heating appliances checked so they are not loose or disconnected, and to have chimneys and flues checked for leakage and blockage. A connector can be bumped during other work, can sag as hangers loosen, or can separate at a joint after years of heating and cooling.

During a flue service we look at the connector run and the thimble along with the chimney, and when the base of the flue is opened we check what has collected there. You get photos with the findings. Appliance burners, controls and combustion settings are a heating technician's work; if we see signs that point to the appliance itself, we will say so. The service itself is described under furnace and boiler flue cleaning.

Questions homeowners ask

Sources

1. Trane gas furnace installer's guide, venting section (PDF)
2. Brick Industry Association, Technical Note 19B: Residential Chimneys, Design and Construction (archived PDF copy)
3. 2003 International Residential Code, Chapter 10, Chimneys and Fireplaces (text reproduced by Buckley Rumford Fireplaces)
4. DuraVent: Relining Chimneys for Gas Appliances (technical guide, PDF)
5. U.S. CPSC: CPSC Urges Seasonal Heating System Inspections to Prevent CO Deaths
6. NJ Department of Community Affairs, Form F370: Chimney Verification for Replacement of Fuel-Fired Equipment (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.