Hot water from cold taps

[kduck]

Our house was built in 2000, and we've lived here from new. It has an unvented hot water system. Recently we've noticed warm or even hot water coming out of the cold taps for a minute when they are first turned on. This is a new fault, so it isn't caused by hot and cold pipes running too close. There are no header tanks, so it's not that they are heating up. I wondered whether the non-return valve on the cold feed had failed and hot water was expanding back into the street, but I've felt the feed pipe when the tap is run and it remains stone cold.

Any ideas please?

 

[cross thread]

Possibly one of your mixer taps/shower is passing through

 

[kduck]

Possibly one of your mixer taps/shower is passing through

Interesting idea, but wouldn't that run hot continuously? It does run cold after a minute.

 

[kduck]

I've just found this old thread, which does sound exactly like the same problem, except I don't have a Megaflo cylinder. It's a Santon Premier TP210B. I've downloaded the installation guide for this cylinder and can't see any mention of maintaining an air bubble. Does anyone know if it does have/need one?
https://www.diynot.com/diy/threads/warm-water-from-cold-taps.508083/