Stumped by hot water turning cold on LOW flow

11 Jan 2015
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United Kingdom
Hoping for help on a problem that has me stumped.

In the last week or so, we have started experiencing a new problem where our hot water runs cold. I think I've ruled out all the obvious things, and we are not talking about the usual combi boiler problems - we don't have a combi and have a cylinder full of hot water!

Briefly, here's our system: typical gas fired condensing boiler, about 10 years old, heating an adjacent vented hot water cylinder. Hot water is tee'd off the vent pipe at the top in the usual way, then goes directly to a booster pump before feeding the rest of the house. Cold water is also boosted by the same pump, separately from the hot. All has worked basically fine for the 7 years we have lived here. Have always had ample amounts of hot water, throughout the house, at 55-60C, until the last week or so.

Current symptoms:
- ensuite bathroom bath won't run hot by itself, but if I turn on the basin tap at the same time to increase total flow, then the bath runs hot! Same is true of the ensuite shower. Note the flow rate from bath and shower is always what I would normally expect - it's just lukewarm. Also note the ensuite is one of the furthest rooms from the HW cylinder, and the flow rate through the bath tap has never been very strong.
- Main bathroom: this is the first T off the HW pipe approx 3 feet after the booster pump. The bath HW tap (not a mixer) runs fully hot at max flow, but turn it down below 50% flow and it goes lukewarm! This is even though the booster pump is still running ( I can hear it) and the HW cylinder still has hot water in it. Turn the tap back up to full and the water is fully hot again!!
- Kitchen: hot water here runs fully hot at full flow (non mixer tap) but then after a while goes lukewarm and stays that way. I can still get hot water out of the bathroom tap though, straight afterwards!

Things I have investigated and ruled out:
- The boiler: it's working normally and is heating the cylinder;
- The cylinder: the gauge on the vent pipe (and my hands) say it's at 55-60C, and so are the outlet pipes;
- The pump runs as normal when any tap is turned on, and the flow rates seem normal too.

So, I'm flummoxed. If I have HW in the tank, how can I be getting lukewarm water out of some of the (non-mixer) taps? It's almost as if I have a cold supply connected into the HW system downstream of the booster pump, which is pushing cold into the system when the flow is restricted and there is more back pressure on the pump. But how could that be? And why is it happening now when we haven't changed anything??

Thanks for reading if you have got this far. Any ideas or advice welcome!
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Check that the end of the vent pipe over the large cold water tank isn't into below water level.

I suspect exactly what you have brushed aside!

That you some cold leaking into the hot!

Thats usually from a mixer tap or shower of some kind.

Can be difficult to find though!

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I suspect exactly what you have brushed aside!

That you some cold leaking into the hot!

Thats usually from a mixer tap or shower of some kind.

Can be difficult to find though!

Thanks Tony. Interesting to have someone else think it might explain the symptoms. I had wondered about flow across a mixer, but never having had one apart It didn't feel to me like there would be enough flow from cold to hot side to notice. Could there be?

And how might I identify which mixer is the culprit (if any)? I have 3 thermostatic mixer valves (2 x shower plus 1 x bath), and one simple mixer tap on a basin ( could that have the same fault?)

Thanks again,

Sounds like a mixer passing, you need to isolate the cold mains to the mixers and test the hot again.
Just to wrap this thread up for any future readers. The culprit was a thermostatic shower mixer that was passing from the cold to the hot side. I dismantled it, descaler it and fitted new o-rings ( as best I recall) and that fixed the problem.

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