Flue and plume

1 Why a plume appears

Worcester's Greenstar i ErP guide says the flue terminal "will, at times, give out a plume of water vapour" and calls this normal¹. Glow-worm's Ultracom 2 cxi guide says a plume of condensation in cool weather comes from the high efficiency and low flue gas temperature, and is not a fault². The Main Eco Compact guide says the cooled flue gases may look like a cloud of steam⁶. See condensing boiler.

2 Warning signs that are not plume

HSE lists different signs of incomplete burning: soot or yellow or brown staining, yellow or orange flames instead of blue, a pilot light that keeps blowing out, and extra condensation on the inside of windows⁷. Those point to a possible carbon monoxide problem rather than ordinary vapour. If a CO alarm sounds or poisoning is suspected, get everyone outside into fresh air first.

Older flues can also be a material question. HSE says asbestos may be in domestic buildings built or refurbished before 2000, and its examples include asbestos cement flues⁸. Leave them alone and tell a contractor before work starts⁸.

3 Where a terminal may sit

In England, Approved Document J gives minimum distances. These are the figures for a fan-assisted, room-sealed boiler (a fanned balanced flue), shown so a homeowner can read an installer's reasoning. They are not a checklist for moving anything yourself⁴.

The other rows of that table and the maker's own instructions also apply, and other countries have their own documents. Worcester's oil flue kit instructions include a plume management system, one example of a kit an installer can use to take the discharge away from a wall⁹.

4 Flues hidden in voids

Approved Document J says a flue routed through a void needs access points so that its length, joints, support and fall back to the boiler can be seen, both at installation and at each service¹⁰. Its diagram guidance shows hatches at least 300mm square, with no joint more than 1.5m from the edge of one¹⁰. If a concealed flue has no visible hatch, ask the engineer at the next boiler service.