It's a Glow worm Flexicom 24HX on a Y plan system, fully pumped, open. We still have the old inefficient HW cylinder (new, faster reheat cylinder to be fitted next month) so on the plumber's advice we have set the timer so that the boiler works on the HW for half an hour before the CH comes on.
If I come down early when the HW is already on and switch the CH on manually I've noticed that the boiler sometimes seems to be "idling" and doesn't react; the fan is running and there is warmth in the output pipe, the display shows 38-40 but the boiler doesn't fire up to full power. The room stat is calling for heat when this happens. If I turn off HW and CH, I hear the fan slow down and stop and then when I restart HW and CH the boiler fires up within seconds to full power, when it makes a much louder noise.
Any ideas as to why it behaves in this way?
If I come down early when the HW is already on and switch the CH on manually I've noticed that the boiler sometimes seems to be "idling" and doesn't react; the fan is running and there is warmth in the output pipe, the display shows 38-40 but the boiler doesn't fire up to full power. The room stat is calling for heat when this happens. If I turn off HW and CH, I hear the fan slow down and stop and then when I restart HW and CH the boiler fires up within seconds to full power, when it makes a much louder noise.
Any ideas as to why it behaves in this way?