Chimney flue

A chimney flue is the internal passage that carries combustion gases from a stove, fireplace or boiler up to the open air. In Slovak practice it is called dymovod, and it is the part of the chimney that decides whether an appliance draws cleanly or smokes into the room. For a new airtight house or an old chimney being reused, the flue is where most of the risk sits.

What is a chimney flue?

The flue is the empty channel inside a chimney through which hot gases and water vapour leave the appliance. The chimney is the whole structure, while the flue is the passage it houses. One chimney can contain one flue or several, and a flue can be lined or unlined.

The gases lose heat as they travel, so the walls, shape and length of the flue all change how the appliance behaves.

How is a chimney flue different from the chimney body and the chimney liner?

The flue is the space itself. The chimney body is the outer shell around it, and the chimney liner is the lining that may be installed inside the body to form the flue. The chimney head is the top section where the flue exits above the roof.

Element	What it is	Role for the flue
Chimney flue	The empty passage	Carries the gases; its size and shape set the draught
Chimney body	Outer masonry or prefabricated shell	Carries the structure and keeps out weather
Chimney liner	Pipe or lining inside the body	Makes the flue smooth, sealed and heat-resistant
Chimney head	Top section above roof level	Releases the gases where wind can disturb them

How must the flue size match the appliance?

The flue cross-section must suit the appliance, and the manufacturer's installation data states the flue conditions it needs. A flue that is too large lets the gases cool quickly, so draught weakens and condensate forms. A flue that is too small restricts the flow, and smoke gets pushed back into the room.

Check the appliance datasheet against the flue that will actually be used, not against the size of an old opening or the existing outlet on the roof. Round passages are preferred because they give smoother flow and are easier to sweep. Sharp offsets, long horizontal runs and irregular internal shapes all cost draught and collect soot.

How does draught depend on flue height and temperature?

Natural draught is the pressure difference that pulls gases up the flue. It comes from the density difference between hot gases inside and cooler air outside, so nothing moves the gases mechanically and any weakness in the flue shows up directly in how the appliance burns. A taller, warmer flue draws more strongly, but height alone is not enough.

Factor	Effect on draught	Planning note
Flue height	More height adds more pull	Height is set by the roof and the building form
Gas temperature	Hotter gases rise more strongly	A cold start gives weak draught until the flue warms
Bends and roughness	Friction and turbulence slow the flow	Keep the run short, straight and smooth
Cross-section	Too large cools the gases; too small restricts them	Match the appliance datasheet
Wind at the outlet	Gusts and downdraught can push gases back	Check the outlet position against the ridge and nearby obstructions
Room air supply	A depressurised room starves the flue	Covered in the airtight house section

Why do old unlined flues cause condensation and soot fires?

An old unlined flue is usually a masonry void with rough faces, open mortar joints and cracks. Wood smoke carries water vapour and tar compounds that condense wherever the flue wall is cooler than the gas. In a cold unlined flue the condensate is acidic and soaks into the brickwork, while the tarry deposits, called kreozot in Slovak, build up on the rough surfaces where they can ignite.

Aspect	Old unlined flue	Relined flue
Condensation	Likely in cold sections	Reduced by a warmer, smoother passage
Soot and tar	Collects on rough masonry	Smooth surface is easier to sweep
Gas leakage	Possible through cracks and joints	Sealed liner keeps gases inside
Fit with a modern appliance	Often unknown or oversized	Sized for the chosen appliance

Leakage is the other risk. Gaps in old mortar let carbon monoxide and smoke pass into the roof space or the walls. A relining, usually with a stainless steel liner, is the common remedy because it creates a sealed passage matched to the appliance rather than to the original opening.

How does an airtight house affect a wood stove flue?

A new airtight house gives a heating appliance little outside air to burn with, and the flue depends on that air. Extract fans, a kitchen hood or the exhaust side of a ventilation unit can lower the pressure in the room, which reverses the flue and sends smoke into the living space. This is called backdraught.

The fix is a dedicated combustion air supply to the appliance, placed and sized by the designer, plus a check that the mechanical ventilation does not depressurise the room where the stove stands. The building's airtightness decides how much air leaks in on its own, so the appliance should be chosen before the envelope is finished. Room-sealed appliances, which take their air and discharge gases through a dedicated two-pipe system, avoid much of this problem and are worth comparing with an open fireplace in the design brief.

Why does a chimney sweep inspect the flue before first use?

A chimney sweep (kominár) inspects the flue before the first firing to confirm that it is continuous, clean, sound and suitable for the appliance it will serve. The sweep looks for cracks, offsets, loose liner joints, blockages and soot, and checks that the flue draws once the appliance is running.

The cheapest point to find a problem is before the stove is connected, because a flue fixed afterwards often means opening walls or moving the appliance. Ask for the result in writing, including the flue's condition and the appliance it was checked against.

What are the common misconceptions about chimney flues?

The most common misconception is that a bigger flue always draws better. An oversized flue cools the gases, weakens draught and invites condensation. The second is that a chimney which worked for a gas boiler will work for a wood stove. Wood produces more soot, more moisture and more tar, so the flue has to handle more than the old appliance did.

A third is that smoke in the room always means a blocked flue, when a cold start or a missing air supply is just as likely. A fourth is that a liner only fixes fire risk, when it also restores draught and seals against gas leakage.