Hot water and combi boiler

[skier-hughes]

You say the pipe from the boiler is warm, it should be ouch that's hot at around 60/70'.
I'd go back to the boiler not delivering the hot water out properly, so diverter valve again.

 

[Poorprincess]

The pipe directly behind the boiler is cold to begin with then gradually warms as the water flows through it. It's defiantly worse on colder days which i have always put down to the water being extra cold as it comes into the boiler.
The bathroom, taps and bath mixer and separate Mira shower are all brand new.
I think maybe we will ask boiler man round to check the diverter valve as suggested and go from there. We did have someone put a descaler through the boiler 6 months ago and it did improve things slightly. It's just hit and miss, the other day within a couple minutes the basin hot tap was fairly warm but that's unusual. It is warmer if you slow down the rate of the flow.

 

[Me!]

Looking at how combi boiler diverter valves work, and most of them seem to be a pressure balance on a diaphragm that is actuated by suction from a venturi when there is flow to a hot tap like this -
http://www.boilerkingdom.com/wp-content/uploads/2017/09/diverter_valve_operation_diagram.gif

So that looks very likely to be able to get stuck or stiff with a bit of limescale etc., and as you said it did improve when descaler was put through the boiler and is better some days than others, I also vote for a stuck diverter valve being your most likely problem.

 

[Poorprincess]

Just wanted to post an update on my hot water problem.
I finally ran out of patience, just recently it had got worse, 20minute wait for a hot shower!
We have always had our boiler serviced yearly and would always mention our problem to the engineer who would just scratch his head.
Got in touch with a local plumber heating engineer last week, explained the problem and straight away he said it was the heat exchanger.
He came a few days later, fitted a new exchanger and now we have instant hot water!!
I have never been so happy!!