HPM
INDUSTRY
WATCH
“Unsafe combustion can result, with
raised levels of carbon monoxide in
the flue gases – and in some instances
boilers can shut down”
What to do with a shared flue
Neil Macdonald, technical manager at the Heating and Hotwater Industry Council, discusses
high-efficiency boilers and shared flue ducts.
Around 350,000 homes in the UK
have gas appliances, often central
heating boilers, connected
to shared flue systems – the
most common being the SE-duct and
U-duct systems found in older multistorey
dwellings. These systems allow a
number of appliances on different storeys
to receive their air supply from, and
discharge their combustion products to, a
common duct.
This is usually built into the building
during construction, although in recent
years, more modern communal flue
systems (CFS) have become increasingly
commonplace in the UK, and differ in
that they are designed for room-sealed
appliances only (typically types C4 or C8).
SE-ducts and U-ducts work due to the
“flue flow” effect, whereby hotter, more
buoyant combustion products are drawn
to the centre of the duct as they rise,
“pulling” an envelope of fresh air up the
duct wall, from which the appliance takes
its combustion air supply.
At the time of their construction,
both flue systems required careful
dimensioning to ensure the correct flue
flow pattern and were designed on the
basis of relatively high temperature flue
gases, from the standard-efficiency gas
appliances in use at that time.
LOWER FLUE GAS TEMPERATURES
Modern condensing boilers have much
lower flue gas temperatures, with less
buoyancy and a much higher level of
condensed water vapour. The use of
a condensing boiler, change to the
appliance type/heat input, and the use of
a fan flued product to replace a balanced
flue, may result in a poor flue flow effect
within the shared duct.
This can result in combustion
products, including water vapour, being
taken into those appliances sharing the
flue along with the previously “clean”
combustion air supply.
Unsafe combustion can result, with
raised levels of carbon monoxide in the
flue gases – and in some instances boilers
can shut down.
These ducts were also not designed
to cope with high levels of condensation
from boiler flue gases, or with the mildly
corrosive nature of the condensed water
vapour. As a consequence, degradation
of the flue duct walls may occur, unless
the duct has been specifically adapted by
a competent person to accept the use of
condensing appliances, for example, by
lining, use of exhauster fans, installation
of a shared system chimney, or other
treatment.
As such, a condensing boiler shall not
be fitted onto a standard shunt duct, SEduct
or U-duct system, and only appliances
which are designated by the manufacturer
as suitable for the application shall
be used on either SE-duct or U-duct
installations (C2 appliances).
INVALIDATED APPLIANCE
WARRANTY
Failure to follow this could invalidate
any appliance warranty, and lead to an
unsafe situation. Specific guidance on the
types of appliance allowed on shared flue
systems; restrictions on their heat input;
how the appliance should be installed
and commissioned etc. are given in the
pre-eminent standard on this topic - IGEM
UP-17.
Shared flue ducts should have an
annual inspection to ensure that their
condition remains suitable, and it is
a legal requirement under Gas Safety
(Installation and Use) Regulations 1998
(with 2018 amendments) for a landlord to
confirm that the common duct has been
inspected and has a valid certificate.
The customer and/or engineer should
obtain a copy of this certificate from
the managing agent, landlord or owner
before any new or replacement appliance
is connected to a shared flue.
This certificate should also be checked
on the annual service visit and at any
maintenance visits. In the absence of a
certificate the requirement of annual
flue inspection should be highlighted to
the customer and documented on any
paperwork – and, where required, the
Gas Industry Unsafe Situations Procedure
should be followed.
So, what is the solution then? Well,
if you are asked to replace an existing
appliance in a property which has a
shared flue you could:
• Install an equivalent appliance,
specifically approved for SE-duct/U-duct
installation (see TB 123, or consult the
appliance manufacturer for advice)
• Explore whether the installation of
a condensing boiler fitted within the
individual dwelling, and with a flue
terminating to outside air elsewhere,
for example, external wall of building,
would (a) be acceptable to the property
owner/landlord/customer and (b)
be technically and economically
feasible, given the property’s
construction method, room layout
etc. The decommissioned boiler(s)
must be removed and the redundant
connection(s) to the shared flue must be
sealed in the required way
• Suggest that the householder(s) and/
or landlord might be able to organise
the co-ordinated replacement of all
boilers in the building, together with
the installation of an approved chimney
system serving all appliances. This would
require a thorough survey of the shared
flue system by a specialist company, and
agreement from all residents and/or the
landlord of the property.
28 Heating & Plumbing Monthly | JUNE 2018 | www.hpmmag.com
/www.hpmmag.com
