Flues and chimneys:

Warm air boilers installed before around 2005 generally have an “open flue”. An open flue is a vertical or near-vertical tube that takes the boiler fumes to outside by natural convection. If a warm air boiler fails an inspection or safety check, it is as likely to be from a fault with the flue tube as from a fault with the boiler itself. 

The open flue tube is usually about 5” in diameter, leads from the top of the boiler to outside, and is usually made from thin galvanised steel sheet or cement-encapsulated asbestos. 

There is nothing wrong with this type of flue if in good condition, but not all are, and checking the condition of this flue tube is one of the mandatory checks we have to do when working on a gas appliance. A proportion of gas technicians are very wary of this type of flue. Some will have a very negative attitude, and a very few can be more focused on finding fault with the flue in order to condemn the boiler, than to fix or service it.

 

As these installations grow older, the condition of the thin steel parts of the flue (sometimes the whole flue) can deteriorate or sections get damaged, especially in the loft. The very top, where the flue connects to the ridge terminal and passes flue gas to outside, is a common point of failure. The flue pipe simply becomes disconnected and discharges partly into the loft. This is usually revealed during the “smoke test”, where a smoke pellet is ignited in the boiler to test the flue for leaks. All the smoke should escape to outside up the flue and none should visible leak into the house or loft space.

When the flue tube becomes detached from the ridge terminal in the loft or attic, or the tube itself leaks flue gas into the roof space via a joint poorly made or coming apart, this is considered dangerous and must be repaired immediately and the boiler disconnected from the gas supply to prevent its use until a repair has been completed.

Getting a flue and/or ridge terminal repaired when the flue tube leaks or has become detached is usually awkward and difficult due to poor access - loft spaces often don’t have solid flooring to support a step ladder. Its a dark and dirty environment to work in, and sometimes involves outside roof work so few gas engineers are willing to do it, me included. Consequently when a warm air boiler fails a flue inspection/test, the near impossibility of getting the flue repaired leads to the boiler itself having to be scrapped and replaced. A different approach when faced with a leaking flue is to replace the flue system in its entirety. There are firms specialising in fitting modern twin-wall flue systems which are designed for for gas fires, Agas, Rayburn kitchen ranges etc. These modern twin wall systems are perfectly suitable for vintage, open flue warm air boilers.

When the expense/difficulty of actually finding anyone to work on a 40-year-old leaking flue or ridge terminal triggers a decision to replace an older warm air boiler, a new high efficiency condensing warm air boiler (such as one from the Johnson and Starley “WarmCair” range) can be fitted to retain warm air heating, or alternatively a combi boiler can be fitted with conventional radiators. The combi boiler then also provides gas hot water at mains pressure so showers etc work well. Finally, a combi boiler can be fitted and connected to a “heat interface” device which converts the radiator output of the combi boiler to warm air, which can then be connected to your existing warm air ducts and vents so your house is still heated with warm air. 

 Although I don’t install new boilers I’ve written about these options further  on my Mike the Boilerman site. Eventually I’ll transfer that content to here! See this link (opens in new page or tab) 

https://www.miketheboilerman.com/warm-air-heating.html

Edit to add, here are a few links to flue companies that might be useful:

SFL Chimneys

https://www.sflchimneys.com/

MI Flues

https://mi-flues.com/

Flue Direct

https://www.fluedirect.co.uk/

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