Why would this be? (Plumbing)

So when you turn our water off from outside in the street and run the cold tap, the system empty’s of cold water then runs hot water until the (combi) boiler is emptied.

?

Gremlins playing mind games.

you may have a cross over where the hot and cold circuits are mixing ie a faulty mixer valve ?

Water in the hot pipes between your boiler and taps is pressurised and above the height of the tap through which you are draining the system. It’s not going to stay that way through magic. It will flow to lower pressure. Is your boiler heating it as it flows backwards through it, or were you using hot water so that you had freshly heated water in your hot pipes? I usually try to turn my boiler off before shutting off the water, just in case bad things might happen.

(You might also have a preheat on your boiler which keeps a small quantity of heated water on hand in case you want it – I think many have disabled this in these days of raised gas prices though.)

Shouldn’t there be a nrv on there to the regs to stop back flow ? (Often removed when you have piss poor flow as can restrict)

Awesome thanks.  I checked the pre heat was still off – we have had it serviced recently.  The water will be off for a short while today while the toilet gets plumbed in, so I’ll check the boiler / switch it off.

Im pretty confident then it’s the reverse flow issue, in my head somehow there was a separate hot and cold circuit, but how could there be.   I’m not surprised there isn’t an NRV either, the former owner was a DIY enthusiast and we’ve discovered “not quite right/straight” issues over the years as we’ve decorated…