presenting high head losses.
At each restart after the period of slow
ignition, the boiler will set itself for about
1 minute at the minimum modulation
pressure, and will then move to the hea-
ting pressure value set.
When the connecting link is inserted,
both the programmed technical pause
and the period of operation at minimum
pressure in the startup phase will be
cancelled. In this case, the times ela-
psing between turning off and subse-
quent re-ignition will depend on a tempe-
rature difference of 5°C detected by
the C.H. sensor (SM).
ATTENTION: It is essential that the opera-
tions described above be carried out by
authorized technical staff.
3.2 TEMPERATURE SENSOR
The “METRÒ” boiler is equipped with sen-
sor for detecting heating temperature.
The sensor acts as a limit stat, switching-
off the burner when the temperature mea-
sured is higher than 85°C; the reset tem-
perature is set at 80°C.
When C.H. sensor (SM) has tripped, the
boiler will not function for either service.
Table 3 shows the resistance values (Ω)
that are obtained on the sensor as the
temperature varies.
3.3 ELECTRONIC IGNITION
Ignition and flame detection is controlled by
an electrode located on the burner.
These guarantee maximum safety with
intervention times, for accidental switching
off or gas failure, of within one second.
3.3.1 Operating cycle
Rotate the rotary switch to summer or win-
ter, the red led should light up.
Burner ignition normally takes place within
10 seconds.
However, it is possible for igni-
tion failures to occur, with consequent
activation of signal indicating that the con-
trol box has “locked out”.
– Gas failure
The control box runs through the cycle
normally sending electric power to the
ignition electrode.
The electrode continues spark discharge
for a maximum of 10 sec.
If the burner does not ignite, the control
box “locks out”.
This may occur upon first ignition or
after long periods of boiler lay-off when
there is air in the pipes.
It may be caused by the gas cock being
closed or by one of the valve coils having
a break in the winding, so that the valve
cannot open.
– Ignition electrode fails to spark
In the boiler, only the gas to the burner is
seen to open. After 10 sec. the control
box “locks out”.
This may be due to a break in the wire of
the electrode or to the wire not properly
fastened to the electric terminal of the
ignition transformer.
The electrode is earthed or very worm
and needs to be substituted.
The control box is defective.
When there is a sudden voltage failure, the
burner shuts out immediately; when power
supply returns, the boiler will start up again
automatically.
3.4 “25 OF” SMOKE
SAFETY DEVICE
This is a safety device against possible
smoke emission into the ambience (15 fig.
3). The safety device goes into action by
blocking operation of the gas valve when the
return of the smoke into the ambience is
continuous and in quantities that might con-
stitute a danger. To restart the boiler reac-
tivate the button of the thermostat.
Should the boiler continue to “lock out”, it will
be necessary to make a careful check on
the flue pipe, making all the necessary modi-
fications and amendments so that it can
work properly.
Check for proper functioning after all work on
the device. If you must replace a part, use only
original Sime spare parts.
NOTE: The device must not be turned off.
3.5 “25 BF” SMOKE
PRESSURE SWITCH
The pressure switch with fixed settings 4.5-
6 mm H
2
O
is able to guarantee the boiler
operation even with air intake and smoke
outlet pipes at the maximum limit of the
length allowed (10 fig. 3).
Impurities and possible formations of
condensate, which are more likely in cold
periods of the year, could lead the pres-
sure switch not to work and the boiler
fail to start.
3.6 NO WATER FLOW
SAFETY DEVICE
The boiler has a flow switch (8 fig. 6) which
is tripped when it does not detect water
flow in the primary circuit (>400 l/h), pre-
venting the burner from functioning.
To start the burner again, check the pres-
sure in the system and make sure that
the pump and the flow switch are working
properly.
3.7 SYSTEM AVAILABLE HEAD
The head available for the heating plant is
shown as a function of the flow in graph in
fig. 20.
3.8 TIME PROGRAMMER
(optional)
The control panel allows a time program-
mer to be utilised for the management of
the heating, supplied on request in the kit
81
TABLE 3
Temperature (°C) Resistance (Ω)
20 12.090
30 8.313
40 5.828
50 4.161
60 3.021
70 2.229
80 1.669
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