Ventilation rates for chimneys and flues should be entered only when they are unrestricted and suitable for use. A restricted chimney is treated as permanently blocked. Permanent restrictions include brickwork, plasterwork or timber panelling fixed in place. Temporary restrictions are ignored.
The specified ventilation rate for chimneys, flues and flueless appliances includes an allowance for the associated permanent vent for air supply, so this vent should not be entered separately1.
If a vent on a permanently blocked/restricted chimney/flue can be closed to less than 3,500 mm2 open area, then it is not considered a chimney/flue but rather is ignored. If the chimney/flue is being ignored, any permanent vent on the external fabric of the room associated with the chimney/flue is counted under the normal background ventilation section as per Section 2 here.
If a fireplace has a hearth, it is considered to be capable of supporting a fire, even if there is no grate present. However, where a hearth is in place, and includes a gas supply pipe, with no gas room heater installed in the hearth, it is not considered capable of supporting a gas or solid fuel fire in its current state. The rules around identification of the chimney opening apply subject to this section, regardless of whether the fireplace is capable of supporting a fire or if the fire is not considered as a secondary heater subject to rules in Primary and secondary heating systems
A chimney is defined as a vertical duct for combustion gases of diameter 200 mm or more (or a rectangular duct of equivalent size). Vertical ducts with diameter less than 200 mm should be counted as flues. The following are also counted as flues:
Ventilation rates should be entered only for open flues and should not be included for room sealed (for example balanced flue) boilers or room heaters. A room heater is considered to be room sealed if it does not have any air exchange with the room.
When a dwelling has a chimney or chimneys entered in DEAP, the dwelling will generally have secondary space heating entered in the DEAP assessment. This secondary space heater may or may not be the open fire associated with the chimney, subject to guidance in section 4 here. The following are sample scenarios where the open fire would not be considered as secondary heating:
Apart from these cases, it is likely that a dwelling with one or more chimneys would have an open fire selected as secondary space heating. In cases where the open fire is not considered the primary or secondary space heating system in DEAP, the chimney associated with the open fire will still contribute 40 m3/h to the ventilation of the dwelling in DEAP. For that reason, the chimney associated with an open fire must always be accounted for in the Ventilation section in DEAP unless the chimney is permanently blocked.
An open flue is (usually) a vertical duct up to 200mm in diameter. It is considered as open because it takes the air required for combustion from the room in which the appliance is situated. Independent oil/gas boilers typically no longer use open flues although they can be seen on an old boiler (30+ years) in an existing dwelling.
Example
Many modern stoves use an open flue. In the stove shown, combustion air is drawn into the stove from the room through a controllable vent. The combustion products – CO2, water vapour, smoke particles and other gases – are removed through the vertical duct – the open flue - rising from the top of the stove. If the stove is in a fireplace, the flue will continue upwards through the chimney, but an open-flued stove can be located elsewhere in a dwelling with the flue passing up through the roof to expel the combustion products to the outside.
An open fire would normally have an associated chimney in DEAP. A chimney is specified if the open diameter of the flue is 200mm or more.
Modern boilers tend to have room-sealed flues. This means that the combustion air is taken from outside the dwelling (and the combustion products are expelled to outside the dwelling) so that the boiler is, in effect, sealed off from the air in the room in which it is located. This reduces the ventilation heat loss in the dwelling.
A balanced flue is an example of a room-sealed flue. In a balanced flue the intake and exhaust ducts are concentric as shown.
To ensure that there was a plentiful supply of air for combustion early balanced flues (from ca. 1980) tended to be quite large (up to 30 cm in diameter) with a rectangular cross-section. Modern balanced flues use a fan to ensure a continuous supply of combustion air. As a result, the diameter of the balanced flue in a modern boiler has reduced: it is usually less than 150 mm in diameter and the flue has a concentric circular cross-section as shown.
Balanced flues are increasingly common on stoves as they are necessary in houses with good air tightness levels and/or balanced whole house mechanical ventilation systems. In either situation, drawing air from the dwelling for combustion would impact on airtightness, so a balanced flue is used.
Note: Permanent air supply vents in the same room as chimneys, open flues or fixed flueless appliances should not be counted in the DEAP ventilation section. The chimney, flue or fixed flueless appliance are reflected in the room by room survey. This air supply is accounted for when the chimney, flue or flueless appliance is entered in DEAP. However, intermittent fans and permanent air vents in the same room as a balanced or room sealed flued appliance must be accounted for as normal, subject to the rules in Vents and Fans.