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SECTION 1 - DESIGN PRINCIPLES AND OPERATING SEQUENCE
1.1 PRINCIPLE COMPONENTS
• A fully integrated electronic control board featuring electronic
temperature control, anti-cycle control, pump over-run, self-
diagnostic fault indicator, full air/gas modulation.
• Aluminium heat exchanger.
• Electronic ignition with flame supervision.
• Integral high-head pump.
• Fan.
• Expansion vessel.
• Water pressure switch.
• Flue sensor.
• Pressure gauge.
• Safety valve.
1.2 MODE OF OPERATION (at rest)
When the appliance is at rest and there are no requests for
heating or hot water, the following functions are active:
• frost-protection system – the frost-protection system protects
the appliance against the risk of frost damage both for CH and
DHW. For CH line, if the main temperature falls to 5 °C, the
appliance will function on minimum power until the temperature
on main reaches 35 °C.
Moreover if the DHW temperature falls to 5 °C, the appliance
will function on minimum power until the temperature on main
reaches 55 °C.
• anti-block function – the anti-block function enables the pump
and divertor valve actuator to be energised for short periods,
when the appliance has been inactive for more than 24-hours.
1.3 MODE OF OPERATION (Heating)
When there is a request for heat via the time clock and/or any
external control, the pump and fan are started, the fan speed
will modulate until the correct signal voltage is received at the
control PCB. At this point an ignition sequence is enabled.
Ignition is sensed by the electronic circuit to ensure flame stabil-
ity at the burner. Once successful ignition has been achieved,
the electronic circuitry increases the gas rate to 75% for a
period of 15 minutes. Thereafter, the boiler's output will either
be increase to maximum or modulate to suit the set require-
ment. When the appliance reaches the desired temperature the
burner will shut down and the boiler will perform a three-minute
anti-cycle (timer delay).
When the request for heat has been satisfied the appliance
pump and fan may continue to operate to dissipate any residual
heat within the appliance.
1.4 MODE OF OPERATION (Hot water)
When there is a request for DHW via a hot water outlet or tap,
the pump and fan are started, the fan speed will modulate until
the correct signal voltage is received at the control PCB. At this
point an ignition sequence is enabled.
Ignition is sensed by the electronic circuit to ensure flame stabil-
ity at the burner. Once successful ignition has been achieved,
the electronic circuitry increases the gas rate to maximum or
will modulate output to stabilise the temperature.
In the event of the appliance exceeding the desired temperature
(set point) the burner will shut down until the temperature drops.
When the request for DHW has been satisfied the appliance
pump and fan may continue to operate to dissipate any residual
heat within the appliance.
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1.5 SAFETY DEVICES
When the appliance is in use, safe operation is ensured by:
• a water pressure switch that monitors system water pressure
and will de-activate the pump, fan, and burner should the
system water pressure drop below the rated tolerance;
• fan speed sensor to ensure safe operation of the burner;
• a high limit thermostat that over-rides the temperature control
circuit to prevent or interrupt the operation of the burner;
• flame sensor that will shut down the burner when no flame
signal is detected;
• flue sensor;
• a safety valve which releases excess pressure from the
primary circuit.
Main heat
exchanger
Return
temperature
sensor
Expansion
vessel
Bottom
AAV
Pump
Safety
valve
Diverter
valve
Automatic
by-pass
Drain
valve
CH
Fig. 5
return
Top AAV
Flow temperature
sensor
DHW
Pressure
temperature
switch
sensor
DHW heat
exchanger
non return
regulator
DHW flow
switch
CH
DHW
DHW
flow
outlet
inlet
DHW
valve
Flow
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