How to design in solid fuel

Addressing the challenges of designing solid fuel into the modern built environment, experts from the Solid Fuel Association (SPA) reports on three issues which can leave architects scratching their heads.

The SFA offer expert technical guidance to the consumer and professional alike. Many clients wish to incorporate some form of solid fuel heating within their building project and it falls to the architect to make this a practical reality. Below is a small selection of typical problems encountered by the architect and some possible solutions.

How do I design a chimney into a property that was built without one? The best place to site a new chimney is at the centre of the property so that maximum benefit is obtained from the heat that is produced and stored within the chimney. However, chimneys can be added onto an external wall or even a party wall.

The chimney itself can comprise ordinary standard density block work with a sectional clay or concrete liner inside, prefabricated casing blocks with concrete or pumice aggregate lining systems inside or, the least expensive option, a prefabricated twin walled and insulated metal chimney system.

A frequent and fundamental error made by the builder is to merely provide a blockwork shell in the full expectation that the addition of a flexible stainless steel liner will create a chimney suitable for a solid fuel appliance. Unfortunately, this is not acceptable under the Building Regulations. Since the introduction of the 1965 Building Regulations, all chimneys are required to incorporate a proper primary lining such as clay or concrete. It is also important to add an insulating fill of clay or vermiculite granules between liners and block work to keep the chimney as warm as possible and prevent tar and soot deposits building up on the cold walls of the flue. An ordinary concrete fill is not an effective insulator.

Where a new chimney has been built without a liner, this must be rectified. Sometimes it is possible to lower the liner sections down the chimney but it is likely that the chimney will have to be demolished and rebuilt with the liners added as the block work is erected.

With regard to flexible liners, these are classed as temporary and can only be used to repair an existing chimney.

Can I use an external air supply duct kit in lieu of ventilation into the room? Approved Document J calls for sufficient combustion air to be provided into any room containing a solid fuel appliance. A popular misconception (unfortunately reinforced by some manufacturers’ spurious claims), is that where an external air supply kit is employed, the requirement for combustion air has been met and there is no need to provide any additional ventilation. This fails to take into account two important facts; firstly, these ducts and the appliances themselves cannot be classed as room sealed and secondly, the appliance door must necessarily be open during refuelling, which means that unless a minimum velocity of air can be maintained across the mouth of the stove, there is a risk of fume emission into the room.

In a conventionally built dwelling, enough adventitious air may be available into the room to prevent a serious incident occurring. However, as homes become more airtight, the risk of a partial vacuum occurring increases. The SFA would always advise that the correct level of ventilation according to Approved Document J is provided into the room in every case.

Can I site a solid fuel appliance in the same room as a mechanical extract fan? The short answer to that question is “no”. However, there are ways of avoiding the issues or mitigating the effects of extraction systems.

A mechanical extractor will create a depression or partial vacuum within the room it is situated in. This will affect the performance of any chimney which opens into the room and relies on the air pressure being greater at its base than at the terminal to create a natural draught.

One simple solution is to use a recirculating fan, which typically employs a charcoal grease filter. The air is purified by the filter and returned to the room so there is no net extraction of air from the room and consequently no depression created. The requirements under Part F of the Building Regulations for a minimum number of air exchanges per hour could be met by the appliance flue itself acting as a passive stack ventilator.

Where mechanical extraction is unavoidable, there are ways of mitigating the effects. The size of the ventilator can be increased or its position moved to be as close as possible to the appliance. The effects of extraction on an appliance will also be less marked in a room with a larger volume.

Before the installation can be signed off, the installer will need to prove that the flue system is capable of working efficiently with the extractor on full. This is done by carrying out a standard spillage test.