Combi boiler hot water flow rate

[Tony57]

I have a vokera HE combi boiler.
It heats the flat and provides hot water. There is sufficient hot water for the sink, basin and shower but when I run a bath the water is hot to start with but then cools down.
I have had the heat exchange replaced.
There are 4 water pipes running into the boiler, 2x22mm and 2x15mm
My gas engineer doesn't know what's wrong and suggests the combi boiler may not be sufficiently powerful for the flat but other flat owners do not have the same problem

 

[ReganAndCarter]

Maybe its the tap. What sort is it?

To tell if it's tap or boiler, run the bath hot tap while holding the hot water pipe underneath the boiler. If the pipe stays hot while the tap runs cool it may point to the tap being the culprit.

 

[Doggit]

Replace your gas engineer next; he should have contacted Vokeras tech support if he wasn't sure of the problem. What's the boiler model though. It may be a faulty thermistor that's not controlling the hot water accurately, or possibly the boilers PCB. If you have a preheat function on the boiler, then it could be that the flow rate is too high, hence it comes out jot to start with, and then cools. Check how long it takes to fill a jug, and that'll determine the flow rate through the taps.