Portrait photo of Mike Bryant, AKA Mike the Boilerman

Mike the Boilerman -

Your Gas Safe Registered boiler 

and central heating repair technician in west Berkshire. Willing to travel :)



I am still carrying out house visits. Full details explained


(Last updated 21st April 2021)

The Potentially Fatal Concealed Flue Duct Problem....


When a boiler burns gas, the 'products of combustion' or 'flue gas' can be harmful to humans so need to be expelled to outside. Originally this done with a vertical chimney or flue pipe similar to a coal fire. Later, a sideways flue ('balanced flue') going directly through an outside wall was developed.

Then about 15 years ago, fans in boilers allowed the use of long thin horizontal tubes for the flue ducts and air supplies. Builders and architects quickly started installing these new modern boilers five, ten or even 15 metres away from the nearest outside wall, with the flue ducts neatly concealed above the plastered ceilings, mostly in town centre luxury flats.

The problem:

If a concealed flue is installed incorrectly and leaks (or develops a leak from, say corrosion), this is generally not be discovered by gas service technicians because occupiers are reluctant to allow inspection holes to be cut in ceilings to check for flue integrity.

Although the vast majority of flues installed above ceilings and behind boxings are correctly installed with no leaks, some are not, and this leads to flue gas leaking into the living space in flats with leaking flues.

Now contrary to popular opinion, boiler flue gas is not dangerous when the boiler is operating correctly as it contains principally carbon dioxide and water vapour, neither of which is a health risk. The problem arises when the boiler develops a fault in it's combustion process which leads to the inclusion of fatally dangerous carbon MONOXIDE (not dioxide) in the products of combustion. The carbon MONOXIDE then leaks into the rooms in a flat with a leaking flue and a number of carbon monoxide poisoning incidents have occurred as a direct result.

The boilers most commonly installed with extended concealed flues are the following:

Range Powermax

Potterton Powermax

Gledhill Gulfstream

Ideal iStor

The solution:

A service engineer working on a boiler is required under The Gas Safety (Installation and Use) Regulations 1998 to check the flue foe correct operation. This cannot be done where the builder has installed it within the fabric of the building with no access.

THe HSE has taken action to stop further carbon monoxide injuries and fatalities from leaking extended and concealed horizontal flues. It has announced that homeowners must have access panels fitted to allow good visual inspection of concealed flues by 1st January 2013. 

If access to visually inspect the whole length of a concealed flue after this date is not available then the gas technician must safety-categorise the appliance "At Risk", turn it OFF, attach a Warning Label and issue the gas user with a written Warning . 

After 1st January 2013 gas technicians (including myself) are likely to decline to service or repair any gas appliance with a concealed flue unless inspection hatches have been fitted which comply with the detailed guidance issued by the HSE in this link.

What doe this mean?

Now that Jan 1st 2013 has passed, all gas appliances with concealed fanned flue systems must have inspection access hatches fitted in accordance with the guidance issued by Gas Safe Register. If there are no hatches, we gas bods must classify the appliance 'AT RISK', label it as such and turn OFF the gas supply until access panels are fitted.

There is more information for consumers about this issue on the Gas Safe Register website here:



***Update, 26th July 2013***

The latest guidance from Gas Safe Register allows use of a carbon monoxide detector to monitor the void space through which a flue runs, provided the CO detector is connected to a boiler interlock which shuts the boiler down if CO is detected. This saves cutting the access hatches.

Cost is likely to be similar to fitting access hatches but disruption will be much reduced, and the aesthetic integrity (!) of your ceilings can be (mostly) preserved. 

The first company to market with such a device is Baxi, with their "NoCO" Carbon Monoxide Detection System. For more details see their website at www.no-co.co.uk.

Demand is phenomenal, and so far Baxi have been unable to supply any of my local merchants with stock, so don't bank on being able to have one of these installed immediately.  I have no doubt other firms are on the brink of bringing competing detectors onto the market though. I'll update again once these become freely available.

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Copyright Michael Bryant 2021

Site first published 16th January 2004

Last updated 26th April 2021

Gas Safe Register 197499, CIPHE registration number 56207

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