My niece has a problem with her combi boiler. It heats the radiators fine,
but does not fire up when she opens a hot water tap.

The boiler is:

Worcester Bosch Thermo-Technique 24CDI Gas Combi Model AOF.

She only moved into her flat a few weeks ago and is not sure how old the
boiler is. She says it looks quite new (I haven't seen it yet, but guess I
soon will :-)

I assume there is some device (flow switch?) that senses the demand for hot
water and turns the boiler on. I guess there is also some 'overheat' cutout
device.

Can anyone suggest what the problem might be?

TIA
--
Mike
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