Cold taps run hot if rising main off...?

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I had a plumber here a few weeks ago, sorting out a leaking close-coupled loo. Turns out numpty PO (owned a building company apparently, god help clients if what we see here is anything to go on) did the piping, then put the loo pan in place, THEN realised the tank didn't quite line up with the inlet, so sat the tank over a bit.

I shut down the stoptap under the sink, whacked on the kitchen hot mixer tap, waited for it to stop before we did anything. Bath/basin colds were on as well, not even a drip.

Loo tank off, all was OK until I noticed there seemed to be a tiny bit of pressure in the loo inlet feed pipe. Stuck a finger in the thing. The kitchen tap was dripping a little, so maybe the stoptap isn't fully closing down.

Thought nothing much of it, until plumber turned on a hot tap to fill a bowl, to fill the cistern and check the flush valve etc for leaks. When he did, the hot tap ran fine. So did the bath cold tap. Which also then ran hot.

When I took my finger out of the loo feed pipe, it was a merry little fountain for a few seconds..of warm water. Turn off the basin hot tap, the rest stopped.

Some headscratching later,and I said "just fix the loo. Everything else is usually fine, so .."..so we did. Isolator on the loo feed, new piping and a flexi, all sorted out.

Turned the stoptap back on...all cold taps and loo inlet now running cold, as usual. Hot is fine still.

Plumber was mildly baffled.

Any thoughts on here? It's a circa 1950s wimpey no-fines, ex council, tank in loft, hot tank in airing cupboard, back boiler still in fine form..just been butchered a bit by the PO..I assume it's an indirect system..but then why would the loo feed and cold taps shut off with the rising main? From what I've read, that's not usual..?
 
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Actually, I'm darn sure the cold taps on the bath/basin/cold feed to the loo aren't tanked...the pressure in them compared to the hot taps is extremely noticeable.

If it sheds any light at all, i've been reading about cross-feeds in mixer situations, and we DO have a mixer..a monoblock bristan kitchen tap..one of the "left for hot, right for cold, up for on, down for off" jobs...or this may be a red herring. All other taps in the place are individual/mono-body with 2 seperate taps.
 
Can't have been a very experienced plumber if he was baffled by a common occurrence. Caused by a mixer tap that is not biflow and doesn't have check valves fitted or the check valves are faulty. It's more common when the hot is disconnected and the customer turns on the non water regs compliant kitchen sink mixer and drowns you in the airing cupboard. This is why I always put temporary blanks on pipes if it's feasible to do so.
 

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