Technical data

8
4.3 flue
systems
4.3 flue systems
Midmat HT boilers are fully condensing, fully modulating appliances and it is recommended that each boiler
is flued independently, however, it is possible to connect multiple boiler flues together providing no part of
the common flue pipe is horizontal. A suitable flue must be connected to the boiler and routed to
discharge above the roof of the building.
Where it is required that the boiler should be room sealed, an air supply duct must be connected to
the appliance to take air directly from outside the building via suitable fittings.
Under all circumstances, plant rooms must be ventilated in accordance with BS 6644:1991, ventilated
for combustion air and cooling or cooling only dependent on flue type.
The combustion system includes a high pressure fan which is able to overcome a flue or a flue duct plus
air duct total resistance of 200Pa.
Extended horizontal flue duct routes are possible but there must be at least a 3° fall on the flue duct
back towards the boiler, to facilitate condensate removal. (3° = 5.0cm of fall per metre of duct).
Flue ducts must be gas and watertight and resistant to flue gas condensate and suitable for a working
temperature of not less than 120°C. Flue termination position must meet the requirements of BS
6644:1991 and the Clean Air Act Memorandum on Chimney Heights.
Note: The Clean Air Act Memorandum prevents the use of balanced flue appliances discharging at low
level where the total heat input to the plant room exceeds 150kW Gross.
4.4 ventilation requirements
Where the appliance is room sealed, use guidance given in BS 5440:2/2000 for ventilation provision.
Where the appliance takes air for combustion from the boiler house, an adequate supply of fresh air for
combustion and ventilation of the boiler house must be supplied in accordance with BS 6644:1991,
or IGE/UP/10.
The air supply should be achieved using either:
a. Natural ventilation with low level and high level openings.
b. Using a fan to supply air to a low level opening with natural discharge through a high level opening.
c. Using a fan to supply air to a low level opening and a fan to extract air at a high level opening.
Where natural ventilation is used, suitable permanent openings connected directly to the outside air
must be provided. The openings should be fitted with grilles that cannot be easily blocked or flooded or
permit the entry of any foreign bodies.
The free area of the grilles should be as follows:-
Low Level (inlet) 540cm
2
plus 4.5cm
2
per kW in excess of 60kW Gross of total rated input.
High Level (outlet) 270cm
2
plus 2.25cm
2
per kW in excess of 60kW Gross of total rated input.
Note: Total rated input is the sum of the heat input of all gas appliances in the boiler room.
This guidance is taken from BS 6644:1991, alternative guidance is given in IGE/UP/10.