Hot water too hot?? Help!!

22 Oct 2014
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West Midlands
United Kingdom
Hello - we've recently moved house and are having a few issues with the hot water system in the new house.Any advice or suggestions would be very gratefully received!!

By way of background, the central heating is served by a Potterton Suprima condensing boiler (about 4 years old we understand) which is (conveniently?!) located in the garage. We have a traditional central heating system with a hot water cylinder in the airing cupboard which is fed by a cold water tank in the loft. The water pressure is also improved by a Salamander shower pump.

The problem we have is with the temperature of the hot water. It is far too hot, possibly dangerously so. We checked out the thermostat on the side of the hot water cylinder and, sure enough, it was set close to 70 degrees. We reduced this to around 55 degrees but the taps in the bathroom, en suite and kitchen seem unaffected - the water is still crazy-hot even at 3 in the morning (despite the hot water being set to come on between 5.30 and 7am, 6-6.15pm and 7-7.10pm only).

Strangely, though, both showers (which are fed from the same system and not electric showers) are much much cooler than previously (now too cool!!) as is the bath (which has a single mixer tap controlled using the same controls as the shower over - three dials - one to select bath / shower, one for flow and one for temperature).

We then turned the thermostat on the cylinder right down - the numbers only go as low as 50 but I'm guessing it would be somewhere in the region of 35-40. This did reduce the temp of the taps to be bearable, but still very hot - much hotter than I'd expect if the thermostat was working correctly. The shower and bath were, unsurprisingly, however, too cold to be comfortably used.

It seems that the showers and bath (which are served by the pump) are correctly reflecting the temperature of the thermostat on the cylinder, but the taps both upstairs and downstairs (which aren't served by the pump) are providing water which is much hotter. How can this be - assuming it's all coming from the same supply?!!?

The timer seems to be working fine and it doesn't seem to be the case that the hot water is permanently "stuck" on - at 8am this morning I drained out all the hot water and have left it all morning - it hasn't heated up again since and so is still running cold. The Central heating has been on at certain times this morning independently so this isn't affecting matters.

We have an Emerson heater but this is definitely off. The cylinder thermostat is correctly located about a third of the way up the cylinder and in contact with its exterior wall.

Any ideas what might be the issue? This is my first post so please be gentle!!

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Does the hot water cylinder have an electric immersion heater? (Cable from wall switch to the top - possibly side - of the cylinder.) Is it switched on?
I don't think that David ( above ) bothered to read the last three paragraphs of the OP !

It seems to be a wiring fault at first sight.

But you have not explained you occupation. Have you bought it? Or renting it?


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Sentenced to a slap on the wrist and an unconditional discharge!

Had you not pleased guilty so promptly, then it could have been five years in a notorious South African jail.
Yes, we've bought the property so it's our problem to solve sadly!!
I'm an office worker and although I understand the basics of how the system works this is well outside my comfort zone!!
You suggest a wiring fault Agile - do you mean the electric wiring of the thermostat?
Interestingly when I turned the thermostat on the cylinder back up to 50 degrees, the boiler fired up so it seems that this is working - of a fashion - but obviously doesn't explain why the water is getting so hot. Obviously now it's set back to 50 degrees the tap water is caustic again.
I'm all out of ideas ... I will happily call a professional in to take a look but would love to have some idea of what it might be to avoid a cowboy taking me for a complete ride - as we're new to the area and don't know anyone local we're going to have to rely on pot luck from the yellow pages rather than a tried and tested or recommended tradesman!
I suppose you have the boiler running your central heating.

It is possible that a fault is allowing some of the heat destined for the radiators to leak into the cylinder circuit.

Turn off your CH for a day, and see if the hot water stops overheating.

Use a thermometer to see how close the tapwater is to the 50C you have set on the cylinder stat.
Thanks John, I'll try that. My only reservation is that if heat was leaking from the central heating into the hot water, wouldn't this have generated hot water after I emptied the cylinder of all of the hot water? Instead, between 8am (after I'd emptied the cylinder) and midday - with the hot water switched off but heating switched on - there was definitely no hot water produced - the hot taps were all running cold.
'mmm, yes, it probably would, unless the valve is not so much leaking, as jamming in the wrong position.

Look for a 3-port valve (or possibly two 2-port valves) like pictured here

The valve should shut off supply to the cylinder if HW is set to off, and open it if HW is set to On and the cylinder stat is calling for heat.

The other arm of the valve does the same for CH.

If water is passing along the wrong arm when not wanted, it will have the result you describe. Feel the two arms to see how hot each is.
As suggested earlier, check the water temperature, I thought my water was far too hot so checked it with a cheap cooking thermometer. Varies between 60-70 degrees C, AND Unbearable to touch!!

From my understanding the water should be kept at around 60 degrees to ensure bacteria are killed off so it maybe that the temp is OK.
Also I suspect the water temperature from the mixer units is cooler as they are designed to prevent scalding so will be premixing the hot with some cold water. They may have numbers indicating temperature on them but I doubt they are accurate.
Agreed - it had crossed my mind that the thermostatic shower cartridges might be the cause of the problem - when the cylinder thermostat is set to a suitable temperature the showers are simply too cold. Only when turned to their highest temperature are they even bearable, which can't be right.

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