11
Flues & Chimneys
Flues and Chimneys
Guidance for sizing flues and chimneys is given in
CIBSE and HVCA guides and also HovalTechnical
Data Sheets. Flues should conform to the relevant
British Standards, Codes of Practice etc.
General Guidelines
• Individual chimneys and flues should be used
whenever possible for multi-boiler plants so that
combustion conditions are not disturbed by the
operation of other boilers. Consideration should be
given to the condition of one boiler at low fire with
regard to low temperature and condensation
problems.
• Specialist flues are available for difficult situations
such as a low level flue adjacent to a tall building.
In this case a vertical balanced flue can be helpful.
Refer to Hoval for further details.
Fan Dilution Systems - General Guidelines
In circumstances where the provision of a conventional or
natural draught flue is impracticable because of the location
of the boiler plant, a fan dilution system may be used.
Dilution fan capacity (m
3
s) =
9.7 x boiler rating (kW)
at normal
3600 x 0.8
temperature and
pressure (N.T.P)
To adjust for the normal diluted outlet temperature of 50
o
C,
multiply the N.T.P. figure by 1.1154.
The dilution air intake should be independent of the
boilerhouse combustion and ventilation air supply.
Velocities in the fan intake and exit ducts should not exceed
7.5l/s to avoid ductwork resonance noise.
Air flow switch to be interlocked with burner controls.
Local Authority approval of height of the diluted flue outlet
is generally required.
Attention is also drawn to the need to include the fan and its
proving switches as in the normal routine boiler maintenance
programme.
Chimney sizes and height will need to take
into account the following:
1) Clean Air Act.
2) Local Regulations.
3) Environmental Protection Act 1990.
4) Considerations to adjacent buildings.
5) Transmission of noise, particularly low
frequency sound pressure levels in
quiet periods of the day or night from
natural gas fired plant if dwellings are
very close.
6) Flue gases discharged to the
atmosphere which contribute to health
dangers should be minimised by
maintaining the burner and boiler to
ensure correct combustion and high
efficiency to reduce running time to a
minimum and to maintain low levels of
CO
2
(Greenhouse effect); CO and NOx
( d a n g e r o u s g a s e s ) . C u r r e n t
regulations for NOx limits (1993) are
maximum 260 mg/kWh (148ppm at 0%
O
2
). Burners supplied with Hoval
boilers fully meet this requirement.
• Adequate doors should be provided in flues and
chimneys for cleaning and inspection purposes.
Square tees must not be used for creating a drain
point at a flue change of direction as this creates
difficulties in firing the burner.
• Flues should be supported independently to prevent
undue weight and forces due to expansion being
transmitted to the boiler outlet connection.
• Where high gas outlet velocities are required from
the chimney, a coned outlet can be used. The
additional loss of draught due to this should be taken
into account in the chimney design.
• Steps should be taken in designing to prevent or
minimise condensation forming by using double skin
and/or insulated flue systems ad suitably insulated
brick stack or double skin steel chimney. This helps
prevent condensation from gas fired plants.
• Header cross sectional areas should take account of
the quantity of gas flowing at each intersection. Allow
for condensation points in chimney and flue.
Condensation should be drained at the base of the
vertical run. Horizontal runs should drain away from
the boiler.
• Where flues from two or more boilers join the header
the gas streams should be flowing in the same
direction at the point of intersection with the header.
• If two flues must enter into one common stack they
they should be positioned to cause the least
disturbance to the other gas stream (i.e. not opposite
to each other).
• In deciding the flue run, the chimney height buoyancy
should provide
a negative pressure condition at
the boiler flue outlet of 15 pascals.
• Flues should not be less in diameter than the boiler
outlet connection size.
• Use of short runs of flues and with the minimum of
large radius bends without horizontal runs before
entering the main chimney at a 45
o
are recommended
and will cause the least resistance.
IMPORTANT
A condensate drain point is provided in the
boiler flue outlet box and this should be piped
to drain via a drain trap to prevent flue gases
escaping. No isolating valve should be fitted
in this pipework.
August 2014
