New unvented cylinder - immersion possibly not working properly?

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Hi all, i had a new unvented cylinder fitted on Monday and the gas boiler side of things works beautifully, but the immersion heater doesn’t seem to act as I would expect.

It heats the tank to around 40C (measured at the thermostat pocket, but also roughly confirmed by seeing where the cylinder stat clicks when rotated). Then it goes into a cycle of being on for a few mins, then off for a few mins etc, but with little temperature rise.

We have an EV tariff and so get cheap electricity at 5.5p/kWh between 20.30 and 01.30, so heating cylinder this way is much cheaper than using gas at 8.9p/kWh. We are also having solar fitted in a few weeks, so will be planning to use the immersion to heat the water as much as possible, so want to make sure it’s working right.

The immersion is a Thermowatt Alloy 800 3kW, with an RTS-3 thermostat. I have this set to maximum (which should be 65C according to specs).

Some photos:

F2F331B8-4106-4599-B081-BB9F8E58500F.jpeg E1DA5786-BE52-4E76-B34D-065B2AE293EE.jpeg 4D8F4CB1-29F7-454D-9C3E-CFAE2D4CAB4C.jpeg B113B8EB-C48E-4FCB-AE86-E97BD000C537.jpeg

Here is a readout from our smart meter from 20.30 to 22.30, showing the immersion cycling repeatedly, despite cylinder temp only being ~40C:

75733D35-74DA-415A-A8A0-5669864CC105.jpeg

Any thoughts appreciated before I call the installer back. Duff immersion thermostat perhaps?

Thanks!
 
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It states that both control stat & safety stat are adjustable, have you checked the safety stat setting although one would think that this would have to be manually reset if it operated.

Thermowatt RTS-3 181316 (red dial)Dual Function Rod / Stem Thermostat​

Product Code: ST-301

Dual Function Rod / Stem Thermostat from the RTS3 range with adjustable trip and safety cut-out. Both functional and safety cut-out are bi-polar with bimetallic disc. This unit unit is depicted by a red adjustment dial
 
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I can only see one adjustment. There is a small recessed button too, which i believe is to reset the safety cut out, if activated.

I read that as the trip is adjustable, plus there is a safety cut out, no?

What could cause the trip stat to activate at too low a temperature?
 
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You are correct, I misread that, it's "with adjustable trip and safety cut-out" so looks probably, as you said above, like a faulty stat.
 
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Are you sure that's set to maximum? I thought fully clockwise was maximum that looks to be fully anticlockwise which I think is minimum.
 
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If it is clicking at 40 C then it suggests that stat is OK so more likely the heater element breaking down when hot.
 

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