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Discuss Hot water is ok but only runs when two or more taps are on in the UK Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Hello, I'd love some advice on my system, I've had a couple of plumbers come in that seemed to specialise in gas systems and seemed perplexed by my all-electric systems and the issues I'm having. My system is all electric heating and hot water system, in a flat on the 4th floor. The mains pressure has already been verified by the building plumber to be good (not sure the value, but pressure out of kitchen tap is strong, and overall pressure seems good).

The problem is that, although the water is heating up fine and the hot water cylinder gets filled, I can often only get hot water at the tap if I run another tap at the same time. If I need hot water at the shower I need to keep the sink running as well. If I run the shower and the tap (set to hot) then the shower will get hot but the tap will be stone cold, and the shower will keep working hot even if I turn the sink tap down to a trickle. It seems like the hot and cold water pressures are not balanced, and perhaps need recalibration, but I'm not sure where. The tun dish seems to be activating occasionally – I've never managed to catch it to figure out when, but I need to keep a container under it because there is enough splashing that a small container fills up over the course of a week (e.g. around 300ml over a week).

The heating seems to work fine when it's needed, but even when it's in action the hot water functions as above.

The systems consists of an electric boiler (Heatrae Sadia), a 300L accumulator (GWS), a pump to fill the accumulator (GWS) and a large hot water cylinder (there's also a Harvey MiniMax water softener).

The problem is there about 70% of the time, but sometimes it works exactly as it should, so it feels like the hot water pressure is just not quite enough at some points and the cold water pressure therefore prevents it coming out at all.

Does anyone have any suggestions about how I could resolve this?
 
It sounds as though you have an unvented cylinder setup and would engage with a G3 registered engineer to investigate further and service the cylinder too. I would check for crossover of a mixer valve, a failed TMV (thermostatic mixing valve), and I believe the unvented cylinder has a blending valve?
 
Thanks for the suggestions, I've attached a couple of photos here in case it's useful. I don't think I have a TMV anywhere, but I'll go searching. Is there a way that I can text the mixer valves my self? I didn't know the cylinder has a blending valve, that certainly seems possible. If I have to get a G3 plumber in, it sounds like it could be quite a big job to drain the whole system and then systematically track down which valve has failed, am I right? Or are there relatively straightforward ways to do that? I'm asking because if it's a big job then one option is just to live with it until the system needs a major service and get it all done in one go..... I've already spent so much on it... Thanks in advance.
 

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