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.
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
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: