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Hi folks. First post on this forum. I'm a gas safe plumber/engineer and came across a hot water problem in a property with a type of direct hot water cylinder I've not encountered before. I know someone on here is going to wonder what type of plumber i am for not understanding this system, but there's always something you haven't come across.

Anyway, to explain the system, there's a cube shaped, insulated hot water cylinder with 2 side entry immersion heaters and a header tank. The header tank is all within the cube shaped cylinder body. It seems to hold approximately 5 litres of water and has the normal side entry inlet with float valve and a bottom entry outlet, as well as an overflow that leaves the cylinder body and then runs out of the property. The header tank also has what looks like a vent entering it, which is coming from within the main compartment of the cylinder.

The cold water mains enters at two points at the top of the square cylinder body in 15mm pipe. One to feed the header tank, and one for something else (in the case that the header tank is not supplying the water to be drawn at the taps, then I presume this second cold supply entering the cylinder is the water to be heated and drawn at the taps).

There are also two hot water pipes exiting the top of the cylinder body, also in 15mm pipe, which connect together and then T off into the hot inlet of a thermostatic mixing valve (the cold side of this mixing valve is supplied from the same cold water mains pipe that supplies the cylinder). The outlet of the mixing valve then obviously supplies the hot water to the property.

As for the immersion heaters, one is labelled "mains" and is constantly live, which means this is not an economy 7 system. And the other is labelled "boost" and it has a timer wired between its power switch and the immersion heater.

Now for my 2 questions (if you're still reading. Lol). Firstly, can someone please explain what type of system this is? Because i don't know why there is a header tank as well as another cold water feed entering the top of the cylinder body, plus i don't understand why there are two hot water pipes leaving the top of the cylinder body (even though they're connected together and then T'd off)....I've tried googling this, by the way.
And secondly if, when I open the hot water tap, i get hot water for 10-15 seconds before it goes cold again (although not as cold as the cold water mains, so there is still a little hot water coming through), and I've tried adjusting the thermostatic mixing valve, am I Still looking at the obvious diagnosis of a dodgy mixing valve? I ask this just to be certain before changing anything, since I don't fully understand the whole system...better to be in the know....cos you never know.

Many thanks if you read all of that and much appreesh for any helpful responses.

Cheers, Baz.
 
Any idea on age? Local councils used to fit a lot of them with Bermuda 401's etc, could be scaled up to burgery!
 
Thanks for the responses....you got it in one. It is exactly as in the picture in the Elson manual. So this is obviously a thermal store then? It's got a label on the front with maintenance records. I remember reading that the blending valve was changed in 2004 and there we at least 3 other records before that, for which i can't remember the dates. So it's probably at least 15 years old. So.....now to see how to descale this thing, if at all possible.
 
OK, before you say anything, i just opened the manual again and i realise the first thing it says is "open vented thermal store".
 
Please log back in. You're showing up as an unregistered account. We usually delete these posts and close the thread.
 
Sorry I didn't realise i wasn't logged in. Not been able to post on a forum without logging in before.

Anyway....as an update, now that I understand the system and have discussed this with the builder who went in before me, I think I know what happened....the main immersion heater sprang a pinhole leak which tripped it's thermostat and the overheat limit stat. This also fried the limit stat according to the builder, who simply removed the limit stat altogether and replaced the immersion with a normal incoloy, 11" immersion heater (the one he removed is 14"). Now according to the manual, the main immersion heater should be set at 85 degrees C (or 75 degrees minimum). However the new immersion heater is currently set at the standard 60 degrees C. And the max temp setting on this immersion heater's thermostat if I remember correctly, is 70 or maybe 75 degrees C. So I'm hoping I can simply get the correct thermostat for this new immersion heater, plus a new overheat limit stat and that'll fix the problem. Worse case, i may have to replace the new immersion with one suitable to run at 85 degrees (if this one isn't for any reason) and able to fit the correct thermostat (at which point I'll service the mixing valve as well). Or I may have to replace the mixing valve if that doesn't cure it.

The reason I'm inclined to rule out the possibility of limescale causing this, is because the hot water was perfectly fine until the original main immersion heater failed. If limescale had caused this, there would have been a gradual decline in hot water supply over time, which according to the customer isn't the case.
 
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As an update to this issue for any future readers who this may help....the elson thermostat fixed the issue....but not without the elson immersion. Because the elson stat is 11" whereas a standard immersion only fits a 7" long stat. So putting the elson stat into a standard immersion meant it stuck out by 4 inches.

So i bought the elson immersion heater, replaced the bottom immersion with it, set its thermostat to 85 degrees and that solved the problem.

As a side note; heatrea sadia (who deal with the customer support for these cylinders) told me the immersions are only sold in packs of 2 and do not include the thermostats; which have to be bought separately. But Plumbase supplied me with a single immersion, which included the thermostat. And they even refunded me for the separate thermostat i bought with the immersion since that actually wasn't needed...thanks Plumbase. Lol.
 
Ps. I did also install a new limit stat as required, by the way :)
 
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