24
Hoval STU Wood Pellet Boiler
Flues and Chimneys
Guidance for sizing flues and chimneys is given in CIBSE
and HVCA guides and also Hoval Technical Data Sheets.
Flues should conform to the relevant British Standards,
Codes of Practice etc.
General Guidelines
• Individual boiler chimneys and flues should be used.
• The chimney/flue design should encompass the minimum number
of bends - any bends should be swept - avoid excessively long
horizontal runs and if possible have the flue enter the main chimney
riser at an angle of 45
o
to minimise the flue duct resistance.
• Flues should not be less in diameter than the boiler outlet
connection size.
• The flue design will be such that the chimney height should
provide sufficient buoyancy to give a
negative pressure condition
of between 10 and 30 Pa at the boiler flue outlet.
The chimney
and flue should, under normal firing conditions, provide a draught at
the boiler outlet.
• Common flue headers must be avoided.
• Steps should be taken to prevent or minimise condensation
forming within the flue by using twin walled insulated systems.
• Adequate doors should be provided in flues and chimneys for
cleaning and inspection purposes. This is particularly important on
any horizontal section. The flue should include some pressure relief
and / or draught stabiliser as Appropriate.
• Flues should be supported independently to prevent any undue
weight and forces due to expansion being transmitted to the boiler
outlet connection.
• Fuel Quality Requirement. The design of the combustion system
is dependant on using the correct quality wood pellet fuel. The fuel
MUST NOT contain halogenated organic compounds or heavy metals
as a result of treatment with wood preservatives or coatings.
•
Note:
Flue gas temperature at high fire may approach 200
o
C
and the flue system should be able to safely operate under such
conditions (further advice can be found at www.heatas.co.uk).
Chimney sizes and height will need to take into account
the following:
1) Clean Air Act.
2) Local Regulations.
3) Considerations to adjacent buildings.
4) The discharge to atmosphere of the products of
combustion that may contribute to health dangers and
be such to minimise the impact of the same. When
used with the correct quality fuel, the STU meets the
requirements of Class 3 of EN 303-5 in this respect.
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 (plastic fittings not suitable).
