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1.3 Combi General Requirements
The boiler will have a DHW priority when both domestic hot water (DHW) and central
heating (CH) are selected. So if the flow switch is closed or the heat store has not
been satisfied the entire output of the boiler is directed to DHW before the boiler will
switch over to CH. When fully cold it can take up to 20 minutes for the heat store of
a 90,000 Btu/h combi to be satisfied, and slightly longer for a 70,000 Btu/h combi.
After a draw-off of 120L at 24L/min, with an average temperature rise of 32ºC,
the thermal store of a 90,000 Btu/h combi has a recovery time of approximately
7 minutes. A 70,000 Btu/h combi will take slightly longer to recover.
Note: If HW has not been selected no hot water can be produced even if the heat
store is up to temperature.
1.3.1 Pump Overrun
Where there is a build up of excess heat in the boiler primary heat exchanger
and the central heating has not been selected then the pump overrun thermostat
will operate. The excess heat will then be pumped into the heat store. Once the
temperature has fallen in the boiler and the pump overrun stat is satisfied, then the
hot water pump will stop.
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