Combi Boiler hot water problem

[someone here]

I have a Worcester/Bosch 24CDi combi boiler, fitted in 2000 it has been trouble free ever since. This week I have a problem with the hot water supply. Cehtral Heating is fine. Boiler is fitted upstairs in the old airing cupboard. The boiler feeds hot water taps in the bathroom sink and bath (same floor) It also feeds a hot water pipe into the washing machine (downstairs) All three of these work correctly, turn on the tap and the boiler fires up, you get hot water.

The problem is with the kitchen sink. Turn on the tap and nothing! Boiler doesn't light. No hot water.

Where do I start looking? Thermistor? Sensor?

Thanks

Dave

[Ed Sirett]

Is the flow rate to the kitchen sink tap less than the others? What is the flow rate in litres/min ? The boiler should fire certainly with 4 litres/minute flowing.

If the boiler is becoming insensitive to DHW flow then the culprit is almost certain to the be the DHW flow diaphragm on the end of the diverter valve.

[someone here]

Flow downstairs is higher than upstairs, not able to measure precisely but is in excess of the 9l/min for the boiler.

Even if it is only one out of four outlets?

That's where I'll start the hunt then. Thanks

Dave