Biasi M96.24SM/B Service Manual page 40

21 Flue thermostat
21.1 Function
The Flue thermostat A in Fig. 80 and Fig. 81 senses the
temperature of the combustion products that flow
through the condensing heat exchanger.
A
Fig. 80
In the case that the temperature of the combustion
products reaches a potentially dangerous value, it
stops the boiler operation. It is therefore allowed the
use of plastic materials for the flue outlet pipes and
bends*
The use of kits different from the original isn't how-
ever allowed, since the flue pipes are integral parts
of the boiler.
If the temperature of the combustion products circuit
reaches the limit temperature, the Flue thermostat
opens the electric circuit that supplies the on---off oper-
ators of the gas valve.
Consequently, the full sequence ignition device at-
tempts to light the burner and, at the end, locks the
boiler and lights the lock ---out signal lamp.
The Flue thermostat is a push---button reset type. This
allows to recognise if the lock ---out has been due to its
intervention or to another different cause (flame failure,
main circuit overheating etc.).
The reset push---button is located on the top of the
boiler under a protective cap B in Fig. 81 and can be re-
ached by unscrewing the cap.
The intervention of the Flue thermostat may be
caused by a defective operation of the device but
more probably is due to a poor heat exchange and
therefore to a low efficiency of the boiler.
In this case the cleaness of main and condensing
heat exchangers is recommended.
38
C
Fig. 81
21.2 Checks
Overheat temperature value
n
1 Set the temperature control knobs to their max.
position and run the boiler in d.h.w. and c.h.
2 Allow the boiler to reach its maximum operating
temperature (monitor the temperature gauge on
the instrument panel). The boiler should maintain
a temperature below that of the Flue thermostat
and no overheat intervention should occur.
21.3 Removal
Warning: isolate the boiler from the mains
electricity supply before removing any
covering or component.
1 Remove all the case panels and the sealed
chamber lid.
2 Disconnect the wires from the Flue thermostat.
3 Unscrew and remove the thermostat probe C
(Fig. 81) from the condensing heat exchanger.
4 Unscrew the screws D which hold the Flue ther-
mostat to the frame.
5 Assemble the Flue thermostat carrying out the re-
moval operations in reverse sequence.
Warning: to correctly connect the Flue thermostat,
refer to Fig. 82 and Fig. 69a or Fig. 69b for the cor-
rect wiring.
Fig. 82
D
B
A