Hearth conditions

All Class I appliances must have a constructional hearth at least 125mm thick. This hearth must extend the full depth of the recess and project a minimum of 310mm in front of the face of the recess and at least 150mm on either side of it.

Any Class I appliance not set in a recess must stand on or over a constructional hearth at least 840mm2 x 125mm thick.

Timber or combustible material must not be placed under a constructional hearth within a vertical distance of 255mm from the hearth surface, unless there is a 50mm clear air space between it and the base of the hearth. Timber supporting a floor at the edge of a hearth is excluded from this.

The following Class II appliances are not required to discharge into a flue; a gas cooker, a room-sealed appliance with a special intake and outlet terminal, a gas heater in a drying cabinet or airing cupboard.

A Class II appliance must be placed over a non-combustible hearth at least 15mm thick. It must extend at least 125mm beyond the back and sides of the appliance and extend forward at least 230mm horizontally in front of any frame.

If the flame of the appliance is more than 230mm above the floor, then a non-combustible hearth is not required.

The appliance must be isolated at the back, top and sides from any combustible part of the building, except the floor or hearth, by a shield of non-combustible material at least 25mm thick and separated by an air space at least 75mm wide.

Some makes of boiler incorporate a drip tray at the base of the flue. Water which collects quickly evaporates as a result of the local heat from the boiler.


The main descriptions used in connection with chimneys, flues and appliances are as follows:

Appliance a heat-producing appliance (including a cooker) which is designed to burn either solid fuel, oil or gas.

Chimney includes any part of the structure of a building forming any part of a flue other than a flue pipe.

Flue pipe a pipe forming a flue, which does not include a pipe built as a lining into either a chimney or an appliance ventilation duct.

Class I appliance a solid-fuel or oil-burning appliance having in either case rating not exceeding 37.6kW/h (150,000 Btu/h).

Class II appliance a gas appliance having rating not exceeding 37.6k W.

Class distinctions

Any chimney serving a Class I appliance should be lined with either one of the following materials: Rebated or socketed clay flue linings; rebated or socketed flue linings made from kiln-burnt aggregate and high-alumina cement; glazed clay pipes and fittings which comply with an approved standard; or round or square-section pipe.

On solid-fuel boilers, the first lm-l-5m of pipe should be of vitreous enamel. A flexible liner may be used on oil-fired appliances, together with one of the above materials, or it may be used on its own.

These linings must be jointed and pointed with high-alumina cement mortar and fireclay cement. The linings must be built into the chimney with their socket ends uppermost.

A chimney serving a Class II appliance may be lined with any of the following materials: Round or square-sectioned pipe; stainless-steel flexible flue lining, on its own or with other materials; acid-resistant tiles embedded in and pointed with high-alumina cement mortar; or glazed clay pipes and fittings which are jointed and pointed with high-alumina cement mortar.

These linings must also be built into the chimney with the sockets uppermost.

10. November 2011 by admin
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