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Thread: New unvented cylinder - immersion not behaving as it should

  1. #1
    Automated Home Guru
    Join Date
    Jan 2015

    Default New unvented cylinder - immersion not behaving as it should

    Hi all, i had a new unvented cylinder fitted on Monday and the gas boiler side of things works beautifully, as does Evohome.

    However, the immersion heater doesnít seem to act as I would expect.

    It heats the tank to around 40C (as indicated by Evohome probe, 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 are 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 itís 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:


    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:


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

    Last edited by garmcqui; 25th May 2022 at 09:59 AM.

  2. #2
    Automated Home Sr Member
    Join Date
    Oct 2020


    My only thought is that the local temperature around the immersion is much higher than where you are measuring. As the heat dissipates to the rest of the cylinder so the immersion comes back on.

    I do find that since I moved from vented to unvented (and so a more modern design), my immersion (driven only be solar) is on for more but shorter periods.

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