Concealed Flue Ducts - What Are these?


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 for 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 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. 

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