3.21 Combustion appliances – air for combustion

Mandatory Standard

Standard 3.21

Every building must be designed and constructed in such a way that each fixed combustion appliance installation receives air for combustion and operation of the chimney so that the health of persons within the building is not threatened by the build-up of dangerous gases as a result of incomplete combustion.

3.21.0 Introduction

All combustion appliances need ventilation to supply them with oxygen for combustion. This air, which must be replaced from outside the dwelling, generally comes from the room in which the combustion appliance is located although many appliances are now located in specially constructed cupboards or appliance compartments. Ventilation of these cupboards or appliance compartments is essential to ensure proper combustion. Ventilation is also needed to ensure the proper operation of flues, or in the case of flueless appliances, to ensure the products of combustion are safely dispersed to the outside air.

Failure to provide adequate replacement air to a room can result in the accumulation of poisonous carbon monoxide fumes.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

3.21.1 Supply of air for combustion generally

A room containing an open-flued appliance may need permanently open air vents. An open-flued appliance needs to receive a certain amount of air from outside dependant upon its type and rating. Infiltration through the building fabric may be sufficient but above certain appliance ratings permanent openings are necessary.

Ventilators for combustion should be located so that occupants are not provoked into sealing them against draughts and noise. Discomfort from draughts can be avoided by placing vents close to appliances e.g. floor vents, by drawing air from intermediate spaces such as hallways or by ensuring good mixing of incoming air. Air vents should not be located within a fireplace recess except on the basis of specialist advice. Noise attenuated ventilators may be needed in certain circumstances.

Appliance compartments that enclose open-flued appliances should be provided with vents large enough to admit all the air required by the appliance for combustion and proper flue operation, whether the compartment draws air from the room or directly from outside.

The installation of a mechanical extract system should be checked against the recommendations in clause 3.17.8.

3.21.2 Supply of air for combustion to solid fuel appliances

A solid fuel appliance installed in a room or space should have a supply of air for combustion by way of permanent ventilation either direct to the open air or to an adjoining space (including a sub-floor space) that is itself permanent ventilated direct to the open air. An air supply should be provided in accordance with the following table:

Table 3.16. Supply of air for combustion

Type of appliance Minimum ventilation opening size [2]
Open appliance without a throat [1] a permanent air entry opening or openings with a total free area of 50% of the cross-sectional area of the flue.
Open appliance with a throat [1] a permanent air entry opening or openings with a total free area of 50% of the throat opening area.
Any other solid fuel appliance a permanent air entry opening or openings with a total free area of 550mm2 for each kW of combustion appliance rated output more than 5kW. (A combustion appliance with an output rating of not more than 5kW has no minimum requirement, unless stated by the appliance manufacturer).

Additional information:

  1. THROAT means the contracted part of the flue lying between the fireplace opening and the main flue.

  2. Where a draught stabiliser is fitted to a solid fuel appliance, or to a chimney or flue-pipe in the same room as a solid fuel appliance, additional ventilation opening should be provided with a free area of at least 300mm2/kW of solid fuel appliance rated output.

  3. Nominal fire size is related to the free opening width at the front of the fireplace opening.

3.21.3 Supply of air for combustion to oil-firing appliances

An oil-firing appliance installed in a room or space should have a supply of air for combustion by way of permanent ventilation either direct to the open air or to an adjoining space which is itself permanently ventilated direct to the open air. This also includes a sub-floor space. However this may not be necessary if it is a room-sealed appliance. An air supply should be provided in accordance with the recommendations in BS 5410: Part 1: 1997 or OFTEC Technical Book 3.

3.21.4 Supply of air for combustion to gas-fired appliances

A gas-fired appliance installed in a room or space should have a supply of air for combustion. An air supply should be provided in accordance with the following recommendations:

  1. BS 5871-3: 2005, for a decorative fuel-effect gas appliance

  2. BS 5871-2: 2005, for an inset live fuel-effect gas appliance

  3. BS 5440-2: 2000, for any other gas-fired appliance.

3.21.5 Flue-less gas heating appliances

Flue-less gas heating appliances obtain the necessary air for combustion and disperse the products of combustion from and to the room or space within which they are located. As condensation could occur when flue-less appliances are used as the only means of heating a room or space then not withstanding BS 5440-2:2000, the appliance standard BS 5871-4: 2007 provides additional installation and ventilation guidance for independent flue-less gas fires, convector heaters and heating stoves with a heat input of not more than 6kW in a domestic building or a commercial building.