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Thread: Evohome DHW temperature sensing with CS92

  1. #1
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    Default Evohome DHW temperature sensing with CS92

    There are two temperature sensing "pockets" on my 300L Worcester-Bosch Greenstore unvented cylinder. The installer fitted the CS92 tank stat sensor in the top one (which is about 1/3 down from the top) and the W-B temperature limiting stat in the bottom one (about 1/3 up from the bottom. I haven't challenged the installer yet but I can't believe that this is correct because the tank stat is only controlling the temperature of the top 1/3 of the water ... or have I got an unknown unknown?

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    Quote Originally Posted by DorrisMancer View Post
    There are two temperature sensing "pockets" on my 300L Worcester-Bosch Greenstore unvented cylinder. The installer fitted the CS92 tank stat sensor in the top one (which is about 1/3 down from the top) and the W-B temperature limiting stat in the bottom one (about 1/3 up from the bottom. I haven't challenged the installer yet but I can't believe that this is correct because the tank stat is only controlling the temperature of the top 1/3 of the water ... or have I got an unknown unknown?
    No it needs to be about 1/3 from the bottom.

  3. #3
    Automated Home Ninja Dan_Robinson's Avatar
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    It should be in the lower pocket.

    Installer is a muppet.
    Kind Regards - Dan Robinson (Jennings Heating Ltd)

  4. #4
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    Thanks to both. I was sure that had to be the case but the guy has been good in other ways so I just wanted to check.

    What's the top pocket for?

  5. #5
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    A bit late to reply to this as I have just had a new install with same cylinder - from the Worcester manual, page 10;

    https://tinyurl.com/ycoajdkh

    I have a similar issue - but my electrician stuck CS92 to bottom sensor pocket and did not connect the high limiter sensor at all. So I'm now restrospectively fitting.

    I'm wondering - did you manage to put both sensors into the same pocket at the bottom ?

  6. #6
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    Quote Originally Posted by ojock View Post
    A bit late to reply to this as I have just had a new install with same cylinder - from the Worcester manual, page 10;

    https://tinyurl.com/ycoajdkh

    I have a similar issue - but my electrician stuck CS92 to bottom sensor pocket and did not connect the high limiter sensor at all. So I'm now restrospectively fitting.

    I'm wondering - did you manage to put both sensors into the same pocket at the bottom ?
    I usually manage to get both sensors in the pocket. The easiest way is usually removing the front of your dual stat and threading it through the centre when the two probes go and thread the wire through the wire hole. Sometimes you can be lucky and the stat have two sections for two wires to come through.

  7. #7
    Automated Home Legend paulockenden's Avatar
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    You can buy tiny 10k NTC thermistors which are much easier to fit into the pocket.

    I use one of these: https://www.ebay.co.uk/itm/122775246996

    It's about the same size as a match head, and works perfectly with the CS92.

    P.
    Last edited by paulockenden; 12th July 2018 at 10:48 AM.

  8. #8
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    I put the limiter in the top pocket and the CS92 sensor in the bottom.

  9. #9
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    Thanks all for replies.

    The top pocket suggestion sounds easiest, but I heard the reading might be out and limiter could set off too soon?

    Although I'm guessing there's probably not much in it.

  10. #10
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    Quote Originally Posted by ojock View Post
    Thanks all for replies.

    The top pocket suggestion sounds easiest, but I heard the reading might be out and limiter could set off too soon?

    Although I'm guessing there's probably not much in it.
    A sensor that is trying to detect a temperature drop to control the reheat of the cylinder needs to be about 1/3rd up from the bottom where the heating element is to detect the water at the bottom getting cold when you run the hot tap and cold water enters at the bottom. Such a sensor won't work properly at the top as the cylinder will be almost empty of hot water before it reacts.

    On the other hand the top pocket should be OK for a safety cutout that is not normally triggered, as the top pocket will always give the highest temperature reading of the two locations.

    If the system starts heating well beyond the desired temperature convection in the cylinder during the heating will cause the temperature at the top of the cylinder to be as hot as the temperature further down thus still triggering the safety cutout. (Water coming out the draw off pipe at the top can't be any hotter than what the top sensor is detecting anyway)

    So - CS92 always in the bottom pocket and safety cutout/limit sensor in the top pocket if it won't fit in the bottom pocket.
    Last edited by DBMandrake; 14th July 2018 at 09:21 AM.

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