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2.4 BS6644
– 70kW to 1.8MW
Open Flued Type B and Room sealed Type C Appliances Installed within an Enclosure (Natural
ventilation requirements direct to Outside Air.)
Flue type
Ventilation direct to outside air
Ventilation to an internal space
Open flue Type B
5 cm²
10 cm²
Not permitted
Not permitted
Room sealed Type C
5 cm²
5 cm²
10 cm²
10 cm²
Open Flued Appliances Installed within a Boiler Room (Natural ventilation requirements direct to
Outside Air.)
Open flue Type B
Ventilation direct to outside air
Low Level (Free Area / kW)
4 cm²
High Level (Free Area / kW)
2 cm²
Balanced Flued Appliances Installed within a Boiler Room (Natural ventilation requirements direct to
Outside Air.)
Room sealed Type C
Ventilation direct to outside air
Low Level (Free Area / kW)
2 cm²
High Level (Free Area / kW)
2 cm²
Room sealed appliances with an Maximum Operating Pressure not exceeding 100 mbar in ventilated
occupied spaces with an air change rate exceeding 0.5 air changes per hour do not require additional
ventilation. If the air changes rate is below 0.5 per hour, 2 cm
2
kW
-1
ventilation at high and at low level
shall be required.
Open Flued Appliances Installed with a Draught Diverter (Mechanical ventilation flow rate requirements
direct to Outside Air.)
System Type & Operational Time Grille Location
Ventilation direct to outside air
Low Level Inlet (m3/h / kW)
2.8
High Level Extract (m3/h / kW)
(Difference Between Inlet and Extract Air)
2.07 +/- 0.18
Open Flued Appliances Installed without a Draught Diverter (With or without stabilizers) (Mechanical
ventilation flow rate requirements direct to Outside Air.)
System Type & Operational Time Grille Location
Ventilation direct to outside air
Low Level Inlet (m3/h / kW)
2.6
High Level Extract (m3/h / kW)
(Difference Between Inlet and Extract Air)
1.35 +/- 0.18
Where high level discharge openings are not mechanically assisted, the free area must be calculated as
detailed above. All air inlet and extract fans must be fitted with automatic controls (interlocks) causing
safety shut-down or lockout of the installed gas burning appliances in the event of an inlet or extract air
flow failure.