HHIC and its members have been made aware of an increasing number of incorrect and unsafe installations when older boilers or water heaters connected to shared flues have been replaced. Problems mainly occur either through incorrect appliance selection or through incorrect connection of the appliance to the flue system.

Incorrect installation can lead to an unsafe situation for the householder, to invalidation of the appliance warranty and to the matter being dealt with under the Gas Industry Unsafe Situation Procedure. This may result in the appliance being turned off, or even disconnected – leaving a customer with no heating or hot water supply.

This article aims to clarify the current requirements for high-efficiency (condensing) boilers in such a situation.

NOTE: while this article relates to boilers and water heaters, other appliance types may also have specific requirements in relation to shared flues. Guidance from the appliance manufacturer and/or the documents referenced below should therefore be followed.

What are shared flue ducts (SE-ducts and U-ducts)

As indicated in the sketch diagrams below, in this type of flue the appliances take air for combustion from, and discharge products to, the same duct.

This is possible due to the  “flue flow” effect, where 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. In SE-duct system the air inlet to the duct is at low level and the outlet is at high level, while the U-duct has both inlet and outlet at high level, however the flow dynamics are essentially the same.

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.

What’s the problem?

Modern condensing boilers have much lower flue gas temperatures, with less buoyancy and a much higher level of condensed water vapour – which is now commonly seen as a “plume” from individual balanced flue terminals in colder weather. Also, modern combi-boilers, which may replace old water heaters, have a much higher input rating than was envisaged when these shared ducts were designed.

The use of a condensing boiler, or a change to the appliance type/heat input, 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.

Other problems can occur as a result of the increased levels of condensation within a shared flue duct. These ducts were 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 (e.g. by lining, use of exhauster fans, installation of a shared system chimney, or other treatment).

There is also evidence of some confusion regarding boiler specification for shared duct (SE duct and U-duct) installations, resulting in the installation of boilers which are designed for modern “C4” flue systems. These systems have separate air inlet and flue ducts, which can be shared by multiple appliances.

Where can I find information?

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. is given in BS 5440-1, IGEM Standard UP-17 and Gas Safe Register TB 123 (see References section below). These documents also cover the need for inspection of the flue, how the flue duct wall should be drilled, the projection dimensions of the flue spigot, the need for labelling of the appliance and other requirements for safe installation.

Two key requirements of IGEM UP-17 and BS 5440-1 with regard to boilers are:

 “A condensing appliance shall not be fitted on a shunt-duct, SE-duct or U-duct system” [IGEM UP-17, Clause 6.1.4]

 “Only appliances which are designated by the manufacturer as suitable for the application shall be used on either SE-duct or U-duct installations.” [BS 5440-1:2008 Clause 12.4.1]

NOTE: Gas Safe Register have issued Technical Bulletin (TB) 123 (8^th January 2014) which includes a list (not exhaustive) of appliances currently available and suitable for installation on these systems.

A further consideration is that all shared flue ducts should have an annual inspection to ensure that their condition remains suitable. It is a legal requirement under Gas Safety (Installation and Use) Regulations 1998 (see box below and References) for a landlord to confirm that the 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.

NOTE: A tenant is responsible for the safety and maintenance of their own appliances connected to an SE/U duct system; however the landlord/managing agent will have responsibility for SE/U duct under other legal duties. Leaflet number INDG285 (available from HSE) is intended for Landlords and details their duties

What’s the solution?

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 conventional wall-mounted balanced-flue boiler in the individual dwelling 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. This could allow a condensing boiler to be installed without utilising the shared duct. If feasible, a co-ordinated programme of multiple boiler replacements throughout the property could also be considered.

[NB 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.

[NB 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.]

NOTE: before the installation of any non-condensing appliance the requirements of the Domestic Building Services Compliance Guide should be followed to ensure compliance with Part L of Building Regulations. The Guide to the Condensing Boiler Assessment Procedure for Dwellings details the required exemption criteria.