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Shared ducts pose challenges

Since the massive growth of the high efficiency condensing boiler market, the requirement for standard efficiency (SE) boilers has dramatically declined. Inevitably this is having an impact on the availability of quality SE boilers for the estimated 1.5 per cent of homes working with SE ducts.

SE boilers are installed in properties where more than one unit is connected to the flueing system, such as flats and apartments where the ducting system has to be shared due to the layout of dwellings in the building.

When replacing an existing boiler on a shared flue arrangement, complete like-for-like is not essential. For example, it is possible to replace a gas water heater with a combination boiler.

High efficiency

At the moment, the installation of condensing high efficiency boilers is not recommended on these shared systems, mainly because the flue systems and material used on such installations are not generally suitable for condensing. In addition, when the system was originally designed, the relevant calculations would not have been made to work with high efficiency boilers meaning that replacing them with such equipment is not possible.

Referring to BS 5440-1:2008 will provide general guidance on the design, installation and maintenance of shared flue systems. It is important to consult this standard as well as the relevant instructions from both the boiler and flue manufacturer’s instructions and specifications.

When undertaking a replacement project, there are a number of areas for careful consideration. There are simple steps that can be taken such as making sure there is no debris or rubble entering the duct during the removal of the existing boiler.

It is also important to ensure that any redundant openings into the duct are sealed and that the duct is not left open for any prolonged period of time, as it is possible that combustion waste products could escape and create a hazard.

When installing a replacement boiler, the air inlet of the flue terminal must be fitted flush with the surface of the inner wall of the duct. Also, the exhaust outlet of the flue terminal must project past the surface of the inner wall of the duct by the correct distance.

Any new boiler must be commissioned in accordance with the manufacturer’s instructions and it may be necessary to commission several appliances simultaneously. Ensuring equipment is correctly labelled is vital to identify that it is connected to a shared flue and the details of the person responsible for the system should be included.

Only approved natural gas appliances can be used on a shared flue. Fan assisted appliances must not be fitted back to back with others on the same duct, which must also be checked to ensure the cross sectional area is of sufficient size.

Alister Maclachlan is technical director of Vokèra