Stumped: hot water from cold tap for 10 seconds only

[Dannno]

I'm completely stumped on this one and so are the plumbers who installed the system. Would love any Sherlock-like ideas!

Background:
We replaced an oil-fired combi boiler with a new Grant 21 vortex oil combi boiler - relocating it to the floor above. At the same time, we also changed a bathroom into a kitchen which added some new copper and plastic pipework for taps and some new copper pipework for the boiler. All of the pipes are separate and the cold pipe is insulated away from the hot pipes.

However - when ever the heating is on overnight, in the morning any cold tap will run cool for 5 seconds, red hot for 10 seconds and then run as you'd expect from then on. Only the first tap used each day does this, but every tap we've tried first has done it. If the heating is on through the day without the taps being used, the same thing happens.

We're 95% sure that no pipes are touching (some are hidden in newly plastered walls but really confident that they're not touching), the cold pipe into the boiler is completely cold and I'm sure it can't be crossed pipes as the water does run cold. But 10 seconds of hot on full flow is a lot of water and I've no idea how it's heating up??

any ideas? Thanks so much?
Dannno

 

[CombustionGuy]

Cold pipe on heating pipe or a mixer tap / shower passing

 

[Dannno]

Thanks for the reply - I'm really sure it's not a cold pipe touching the heating pipe - we've checked everywhere and the hidden pipes are almost impossible to be touching (it also seems like it would need such a massive amount of contact to transfer that amount of heat to heat so much water) - and we haven't changed the mixer tap/shower.

What do you mean by shower passing?

Thanks for the help

 

[picasso]

Check the installer hasnt wired the hot water pump and the central heating pump to run at the same time.