Tap mystery

Access the nearest compression joint and pump a few litres of Holy Water through the contraption, that'll fix it.

LOL, never thought of that... ;-)

The message from Mike Halmarack contains these words:

Get a Pope on a Rope?

It's a faith thing. :-)

Aren't you thinking of Mussolini?

The message from Mike Halmarack contains these words:

I get on very well with the street lighting people round here!

Always best I think, in case they single out your specific house.

Twin impeller pump, cold is under more pressure (don't know why) and so flows up the hot pipe. Turning to hot discharges the cold water before the hot water arrives.

Maybe a dodgy pump, the hot side impeller has been partially shredded by cavitation?

Maybe, I suppose.

Well, the pump is about eight months old...

It was a flying guess, but no other suggestions have been offered as yet. It shouldn't happen with 2 identical pumps turning at the same speed. A reduction in the flow rate of one pump would usually cause higher delivery pressure, so one pump couldn't push water against the other. It suggests to me that the performance of the hot side pump has deteriorated compared to that of the cold.

Cavitation (i.e., hot water boils in low pressure inlet; vapour bubbles then implode on entering HP outlet) is a common fault on the hot side, because of the high temperature. Excessive restrictions in the hot pump inlet makes it more likely. It will take bits out of the impeller, which are usually plastic & only rated to 65 degC anyway. See the instruction manual for more info. Cold feed to hot water much further up the cold water tank? Hot pump drawing in air from the top of the HWS storage cylinder (no Surrey, Warix, etc., flange on hot outlet) ?