One for the boffins, coz I'm stumped!

[Sid Viscous]

Right then, today I changed a basin tap fed by an unvented cylinder. It was fitted with isolators so I didn't touch anything on the supply side. When I'd done I tried the tap, cold fine, hot ran cold. Tried the bath, that was fine. Tried another hot tap, that too was fine. Went back to the basin, still running cold! I thought it could be back feeding somehow so I closed the cold iso - still cold? Left the basin running & tried the bath again. The bath ran hot, then I noticed the basin was now running hot. Turned off the bath & the basin went cold. Went & turned on hot in the kitchen, fine, went back to the basin which was now running hot again. Turned off kitchen tap, basin went cold again. Next I tried one of the showers. That didn't run hot until I opened another hot tap!
This seems to be happening throughout house, you have to run 2 or more outlets to get any hot water. There's no balanced cold supply or any sign of a TMV anywhere (unless it's under a floor somewhere, which would be a dirty trick!). The house has a master bathroom, 2 en suites, kitchen, utility & cloakroom. When you do get hot water to run, there can be as much as 7-8C difference in temp between any 2 of the outlets.
The whole thing just fried my head! Your thoughts please.

 

[gasmanrob]

That bath tap a mixer tap?

 

[gasmanrob]

Of not I'm putting money on one of the taps or showers being thermostatic with no check valves

 

[plumberpete]

isolate both sides of the new tap and see if everything else is ok again.must be a faulty mixer.

 

[Ryanmuz01]

Has to be a faulty tap, if it was ok previously, take the tails or flexi off the hot iso valve, put a new flexi on and run into a bucket, see if it stays hot then

 

[drip-dry]

Go with thermostatic shower with faulty non return valve on hot side allowing cold to back flow

 

[Sid Viscous]

Like it, like it! I'll check that out. Seeing as everything's concealed & tiled in, would I get away with putting an in-line PRV on the cold main (before getting on me hoss & riding into the sunset in me Stetson hat)?

 

[SJB]

Was it deffo all working fin before the tap change? I've known it before to be called out to a problem when the customer knows full well there's something else wrong so that they can feign ignorance so you think it's your fault.

 

[Sid Viscous]

Know what you mean SJB, we can't be sure of these things unless you rock up saying 'Morning Mrs, do you mind if I try every outlet in your house & run the heating up before I touch anything?'

 

[gasmanrob]

Like it, like it! I'll check that out. Seeing as everything's concealed & tiled in, would I get away with putting an in-line PRV on the cold main (before getting on me hoss & riding into the sunset in me Stetson hat)?

No once you open the tap pressure becomes irrelevant to a degree as your just allowing water to move freely.