Faulty water cylinder thermostat?

stem · 20 Oct 2019

Thanks appreciate the info

I often get confused with Vaillant, as they seem to use the same name for all of their products so 'EcoTech Plus' will no doubt come as a vented and un-vented, heat only and a combi boiler.

In the case of the OP though, If the 'optional accessories' were fitted and the dial does control the temperature of the water in the cylinder, then surely there wouldn't need to be a thermostat on the cylinder, as one would interfere with the operation of the other.

On the other hand, if the accessories aren't fitted, then the the central heating dial would be the sole means of adjusting the boiler temperature, so OP have you tried turning up the heating dial and seeing if the hot water gets any hotter.

Harry Bloomfield · 20 Oct 2019

stem said:
So what is this dial actually controlling?

Nothing at all, only the lower dial controls the boiler output temperature in your present control configuration.

Harry Bloomfield · 20 Oct 2019

Hugh Jaleak said:
I believe, if using full Vaillant controls, it allows remote adjustment of cylinder temperature, i.e. rather than adjust the cylinder stat, the user can adjust HW temperature at the boiler. With the OP's setup it may not be connected to anything so will have no effect.

No, the two temperature controls allow two independent boiler output temperatures - one when the boiler is heating for central heating, the other when heating for the hot water, but only when a Vaillant ebus control system is fitted.

The boiler output temperature always has to be higher than the cylinder stat setting, otherwise the cylinder stat will never be satisfied and switch off/ a constant call for HW.

stem · 20 Oct 2019

Harry Bloomfield said:
the two temperature controls allow two independent boiler output temperatures - one when the boiler is heating for central heating, the other when heating for the hot water,

With this type of installation, if the central heating dial was set to 50 degrees to facilitate 'condensing' and the central heating was running, yet at the same time the hot water dial was set to 70 degrees and the hot water was calling for heat, what output temperature would the boiler be operating at?

Harry Bloomfield · 20 Oct 2019

stem said:
With this type of installation, if the central heating dial was set to 50 degrees to facilitate 'condensing' and the central heating was running, yet at the same time the hot water dial was set to 70 degrees and the hot water was calling for heat, what output temperature would the boiler be operating at?

The higher setting, 70C - but the boiler with Vaillant controls will only try to satisfy one demand at a time. That assumes your question was in regards to when fitted with Vaillant controls?

Mojo12345 · 20 Oct 2019

Ok so I went in loft and checked the pipes out the two pipes going in to the green hot water cylinder were warm to hot, but the pipe coming out the top of the cylinder and bending around was not very warm, this was after the hot water has been on for half an hour also I checked the thermostat on the hot water cylinder and it clicked at 35, I have also turned both dials on the boiler to maximum I’ve added some pictures

Mojo12345 · 20 Oct 2019

I have also noticed that I have 2 cold storage tanks the blue one which is high up on the wooden platform seems to be doing nothing the outlets have been capped off this is strange because I thought the blue cold water tank supply’s the hot water cylinder???, also the black one on the loft floor seems to fill up when the cold bathroom tap is on full, but the blue one in the platform does nothing, but that makes me wonder how is the hot water tank being supplied with water?

Harry Bloomfield · 20 Oct 2019

The smaller black tank appears to be just sat on the plasterboard ceiling, so at risk of making its way through the plasterboard.

It seems to me that someone has done a partial, unfinished conversion from an open vented system, to a combi. It really needs a thorough survey by someone who knows what they are looking at, to tell you what's going on..

sbgasman · 20 Oct 2019

Harry Bloomfield said:
Its your boiler output temperature which is too low to heat the water. Boiler output temperature has to be higher than the cylinder temperature, or no heat will be transferred and the boiler will futilely burn lots of gas trying.

where will the heat go?

Harry Bloomfield · 20 Oct 2019

sbgasman said:
where will the heat go?

I don't follow?

The boiler will cycle on and off, maintaining its output temperature at what ever it is set to achieve, while ever the cylinder demands heat - which will never be met.

The boiler continually cycling is wastfull of gas and can cause lots of wear and tear.

sbgasman · 20 Oct 2019

Harry Bloomfield said:
I don't follow?

The boiler will cycle on and off, maintaining its output temperature at what ever it is set to achieve, while ever the cylinder demands heat - which will never be met.

The boiler continually cycling is wastfull of gas and can cause lots of wear and tear.

if the boiler does not sense an adequate flow of water it will fault and not fire , you must be getting confused will old cast iron boilers, to the op air trapped in cylinder coil or faulty h/w zone valve are the most likely faults , turn on the h/w and see how far the heat goes up the flow pipe.

Harry Bloomfield · 20 Oct 2019

sbgasman said:
if the boiler does not sense an adequate flow of water it will fault and not fire , you must be getting confused will old cast iron boilers, to the op air trapped in cylinder coil or faulty h/w zone valve are the most likely faults , turn on the h/w and see how far the heat goes up the flow pipe.

I'm not getting confused - ignoring the OP's particular problem, why would there not be an adequate flow? The pump would be circulating water through the boiler, so the boiler would cycle continually on its output temperature sensor, or if the boiler had anti-cycling fitted, would cycle on that.

sbgasman · 20 Oct 2019

Harry Bloomfield said:
Its your boiler output temperature which is too low to heat the water. Boiler output temperature has to be higher than the cylinder temperature, or no heat will be transferred and the boiler will futilely burn lots of gas trying.

so you are saying no heat will transfer , why ?
burning gas will produce heat ,burning lots of gas will produce lots of heat , where does it go ?
btw in the picture the ops boiler stat is set at around 70 degrees .

Harry Bloomfield · 20 Oct 2019

sbgasman said:
so you are saying no heat will transfer , why ?
burning gas will produce heat ,burning lots of gas will produce lots of heat , where does it go ?
btw in the picture the ops boiler stat is set at around 70 degrees .

Yes, the OP's boiler is set at 70C and if his cylinder stat were set at 60C all would work fine. If he were to set the boiler at 60C and the cylinder stat at 70C, then the boiler would just cycle endlessly wasting gas to achieve nothing useful.

How can heat from something (the circulated boiler water), (normally) be transferred to something which is the same temperature (the cylinder)?

Where will the boiler heat go? It would just be wasted warming the pipes up, as it cannot be transferred into the cylinder..

Mojo12345 · 20 Oct 2019

It’s me the OP I set the boiler to 70 and the hot water cylinder only clicks at around 35 it never clicks at 60

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Faulty water cylinder thermostat?

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