MHS Boilers Ultramax 115, Operating & Maintenance Manual, Page: 21 / 46

MHS Boilers Ltd ©
011204 21
5.3.5 The Flue system
5.3.5.1 General
The flue system must be installed by a recognised installer in accordance with the applicable
national regulations.
The flue pipe connection is at the back of the appliance and has been designed for direct connection
to a corrosion resisting flue pipe.
When the boiler functions as a high capacity boiler, the high efficiency can lead to the formation
of condensation in the chimney.
The condensate drain must never become blocked!
Direct connection to brick built stacks is not permitted because the combustion efficiency of the
boiler is › 83%.
The following table lists all the flue gas data for all types.
Type
Flue gas temperature
at full capacity
Flue gas quantity at full
capacity
Maximum permissible
chimney resistance
° C
m
3
/h kg/s mbar
73
≈ 135
130 0.033 1.8
90 ≈ 135 161 0.040 1.8
115 ≈ 135 214 0.053 1.0
146 ≈ 135 261 0.065 1.0
192 ≈ 135 345 0.086 1.0
241 ≈ 135 430 0.106 1.0
287
≈ 135
513 0.127 1.0
Table 5 Flue gas data
Capacity 100%
Flow temperature 80
°
C
Return temperature 60
°
C.
5.3.5.2 Chimney
Chimney length
Because the appliance is fitted with a “premix burner” with a fan, an over-pressure is built up in
the unit. This overpressure is sufficient to overcome the resistance of the water cooled burner, heat
exchangers air intake and chimney up to a maximum of 1.8 mbar (types 73 and 90) or 1.0 mbar (types
115 to 287) external to the boiler.
The back-pressure outside the unit depends on:
a The resistance of the flue pipe
b The degree of cooling of the burner system
c The resistance of the discharge system.