Flue Temperature Probe Ntc And Safety Thermal Fuse; Function; Removal; Checks - Biasi Activ A 25C Service Manual

20 Flue temperature probe NTC and
Safety thermal fuse
20.1

Function

The Flue temperature probe NTC A in Fig. 20.1 and
Fig. 20.2 senses the temperature of the combustion
products that flow through the condensing heat ex-
changer.
Fig. 20.1
If the temperature of the combustion products circuit
reaches the limit temperature, the Flue temperature
probe NTC reduces the gas flow rate to the burner. The
temperature of the combustion products should de-
crease to a safe value temperature.
In the case that the temperature of the combustion
products reaches a potentially dangerous value, it
stops the boiler operation (lock ---out).
This allows the use of plastic materials for the flue outlet
pipes and bends.
The use of kits different from the original isn't however
allowed, since the flue pipes are integral parts of the
boiler.
A
Fig. 20.2
42
A
B
20.2

Removal

Warning: isolate the boiler from the mains
electricity supply before removing any
covering or component.
1 Remove all the case panels.
Disconnect the connector B from the Flue tem-
2
perature probe NTC by pressing the plastic hook
placed on the side of the connector.
3 Unscrew and remove the flue temperature probe
A (Fig. 20.2) from the condensing heat ex-
changer.
4 Assemble the Flue temperature probe NTC carry-
ing out the removal operations in reverse se-
quence.
20.3

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 temperature
probe NTC and no overheat intervention should
occur.
Temperature---resistance relationship
n
1 Remove the Flue temperature probe NTC (see
section 20.2).
2 Measure the Flue temperature probe NTC electri-
cal resistance at the ambient temperature and
check it according to the graph in Fig. 20.3
Ω
12500
12000
11500
11000
10500
10000
9500
9000
8500
8000
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Fig. 20.3
˚C