The symptoms are very distinctive, so I'm hoping someone might be able to work out what's wrong.

When the system is cold, I get DHW in the usual way. However, if I run a hot bath, and then try to top it up a few minutes later, it runs cold. After a longer period of cooling down, it works again. The overheat thermostat doesn't trip.

I've replaced the flow sensor turbine, and bedded in the thermistors with paste, looked at the pcb, which looks clean. Any ideas, please?

bellx15, May 2016

Thanks for that. Actually, though, the boiler doesn't fire up when I try to top up my bath!

bellx15, May 2016

I think it's basic physics - to begin with the water entering the boiler is at a temperature similar to the house - the boiler can pass on to it enough heat energy to satisfy your hot water need.

When you come to top-up - the relatively warmer water has passed through the boiler's heat exchanger - colder water from the lower depths outside passes through but at the rate of flow your tap demands there is not enough time to warm it sufficiently.

I suspect that when you come to top-up that if you reduced the flow to a trickle then it will be warmer - the temperature dependent on how much the tap is opened.

But this is conjecture - I just guess.

Others may have better ideas.

I have a hot water storage tank and do not suffer this problem

Good Luck..., May 2016