28
6.5 BOILER OPERATION
Domestic hot water supply always takes priority over central heating. If a demand for hot water is required during a
central heating period, the boiler will automatically switch to the hot water mode until the demand is satisfied. This
interruption in the central heating is only when the demand for hot water is present and should not be noticed by
the User.
Central Heating Mode
If there is a call for heat, the pump will start to circulate the central heating water. The fan will run and the premix burner will
light. The burner output then automatically adjusts to suit the system demand; as the temperature of the water in the boiler
approaches that set by the adjustable temperature thermostat, the burner output is reduced. When the set temperature is
reached, the burner is turned off. The fan continues to run for 30 seconds and the pump continues to run for three minutes,
after which the burner can relight if required. If the primary sensor has not registered the preset temperature but the room
thermostat is satisfied the burner is turned off. The fan continues to run for 30 seconds and the pump continues to run for 10
seconds. In this instance there is no delay before the burner will relight.
If there is a demand for DHW during the burner delay, the boiler will operate to provide DHW until the tap is closed; the boiler
will then revert to provide CH if there is a demand.
Note:
If the system pressure is very low, the primary pressure switch will prevent the boiler from operating.
Domestic Hot Water Mode
When a demand for hot water (by opening a hot tap, etc.) is sensed by the flow switch, the pump starts, the fan will run and
the burner lights, increasing immediately to its maximum output. Water in the boiler is then diverted from the central heating
system to the domestic hot water heat exchanger, heating the incoming mains water. The burner output is varied to maintain
the temperature of the hot water as that set by the adjustable temperature selector. When the flow switch senses that hot
water is no longer required the burner is turned off and the boiler returns to the central heating mode.
If the boiler is set for hot water only (Summer ) - The burner is turned off, the fan will continue to run for 30 seconds and
the pump will stop immediately. In both modes the fan modulates according to the output required.
Frost Thermostat
The boiler incorporates a built in frost thermostat which automatically turns on the boiler if the water in the boiler falls below
4°C, providing the electrical supply is on and the boiler is in standby mode. The boiler will operate until the water temperature
in the boiler reaches 42°C.
Any other pipework outside of the boiler must be protected from the risk of freezing and insulated. Additional protection from
an external frost thermostat and pipe thermostats should be considered.
Boiler frost protection will not prevent damage caused by freezing of pipework outside the boiler casing (i.e. external
pipework, incoming cold mains etc.). Boiler damage caused by freezing external pipework is not covered under warranty.
Pump Cycle
If the electrical supply is on and the boiler has not operated for 24 hours in heating or hot water, the pump will operate
automatically for thirty seconds every 24 hours.
Alpha Evoke 28 and 33 - Commissioning
