One for the boffins, coz I'm stumped!

Sid Viscous · Mar 31, 2015

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 · Mar 31, 2015

That bath tap a mixer tap?

gasmanrob · Mar 31, 2015

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

plumberpete · Mar 31, 2015

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

Ryanmuz01 · Mar 31, 2015

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 · Mar 31, 2015

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

Sid Viscous · Mar 31, 2015

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 · Mar 31, 2015

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 · Mar 31, 2015

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 · Mar 31, 2015

Sid Viscous said:
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.

johnnyboyoy · Mar 31, 2015

Any chance this is a combi masquerading as a system boiler, serving the heating, coil, and some outlets? Just that the hot water sounds like a diaphragm on its way out on a pressure differential valve. Needs 2 outlets to get the pin to push out enough.

Sid Viscous · Mar 31, 2015

It's deffo a heat only, 100%

Sid Viscous · Mar 31, 2015

Why would faulty or missing check valves on a shower cause a problem if the shower is not in use? As I said in the OP, this happens when any combination of outlets is opened including mixers & non mixers

SJB · Mar 31, 2015

Cold always pressing against the faulty check valve, when the hot pressure drops because an outlet elsewhere gets opened the check valve totally gives out

diamondgas · Mar 31, 2015

If possible turn off the cold mains supply to the unvented cylinder ... If you still get a some supply through the hot tap/s then I'd suspect a faulty TMV/dual mixer valve somewhere is passing? It could account for cold at the hot taps?

Sid Viscous · Mar 31, 2015

SJB said:
Cold always pressing against the faulty check valve, when the hot pressure drops because an outlet elsewhere gets opened the check valve totally gives out

I get how that could happen if the shower were open & in use but not if it's turned off, surely that's the same as if a tap is closed, no water moving anywhere?

gasmanrob · Mar 31, 2015

No one side mixes the water and the other diverts the water to the shower head. The side that blends isn't actually shut off

Sid Viscous · Mar 31, 2015

Got it, nice one! So if I isolate each pair of outlets one at a time, that should get me to the fault? Unless of course, there's no check valves on anything, in which case I'd be p*****g in the wind!

R.Oak · Mar 31, 2015

diamondgas said:
If possible turn off the cold mains supply to the unvented cylinder ... If you still get a some supply through the hot tap/s then I'd suspect a faulty TMV/dual mixer valve somewhere is passing? It could account for cold at the hot taps?

this.

Sid Viscous · Apr 1, 2015

Isolator on the composite valve is seized solid. I'll have a go at freezing it.
Just another thought, mains pressure is 4.75 bar. If something was passing back, wouldn't that cause the T&P relief to open?

gasmanrob · Apr 1, 2015

T and p will be 6 bar

Sid Viscous · Apr 1, 2015

Expansion's at 3 bar

diamondgas · Apr 1, 2015

here's a link to a PDF on TMV's ... http://www.horne.co.uk/resources/HowDoesAThermostaticMixingValveWork_09_02_2011_14_32_03.pdf

Basically if you get debris on the cold seat it'll pass to the hot side. If the cold mains hasn't a Pressure Reducing Valve fitted after main stopcock then there's a chance that the hot and cold will be a different pressures (Cold higher), Pressure reducing valve on hot water prior to cylinder reduces the mains pressure accross the hot water circuit. The TMV valve will then pass cold through it when any hot tap is opened!

Does that make sense?

Sid Viscous · Apr 1, 2015

It does Diamond. Thanks for the link

tommy1712 · Apr 4, 2015

Got to be a faulty mixer I guess sounds like a head scratcher.

diamondgas · Apr 4, 2015

Easy checked if you have access to the pipes at he mixer and a helper ... Run the tap till you can feel hot water supplied to the mixer. Turn tap off then ask custard/helper to open another hot tap. If the mixer is passing the hot supply pipe will go cold!

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