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Thread: EvoHome - Boiler running when no demand

  1. #1
    Automated Home Jr Member
    Join Date
    Nov 2019

    Default EvoHome - Boiler running when no demand


    I have a situation where late in the evening (10pm as HW goes off and no Heat demand) the boiler continues to run and will run overnight unless I stop it. If I turn HW relay on/off it then stops it. This is an intermittent issue but probably 50%.

    - Worcestor Bosch CDI 30 boiler with integrated pump.
    - Y Plan config with 3 port valve. Recently replaced the head -

    System appears to be wired correctly as per

    Things already checked
    - 3 port valve was old and a little "sticky". Replaced it with new head as per above.
    - We had a Sundial Wiring Centre which heating engineer has seen issues with previously so has been replaced by a standard Drayton wiring box.

    When I look at the voltage on the feed going back to the boiler there seems to be a floating voltage (around 12v) after the Heating demand has gone. e.g.
    - With HW or CH demand feed = 240v
    - When HW only has been on and then switched off feed = 0v
    - When CH only has been on and then switched off feed = about 12v. There is a small capacitor (0.1uF) already installed across the boiler feed/neutral and if I remove this the floating voltage moves from 12v to 60v.

    This article here talks about a snubber capacitor - https://ensupport.getconnected.honey...language=en_US

    I presume the 12v is still enough to drive the boiler to continue to run the pump?
    Has anyone else experienced this type of issue? Is it a case of getting a larger capacitor and can anyone advise size?


  2. #2
    Automated Home Legend
    Join Date
    Sep 2014


    Could one of the switches in your zone valves be faulty ? (Leakage due to burnt contacts maybe ?)

    I assume from your diagram you don't have a dedicated boiler relay but just have the two switches from the zone valves wired in parallel to switch the boiler and that the boiler is expecting 240v on the control input ?

    When you say there is "about 12v", do you mean AC or DC ? Some boilers have an option to use low voltage switching on the control input so you'd want to be sure the boiler is configured to be expecting 240v. In this mode you should see zero volts when both zone valve switches are open, not 12v or 60v...

    It also seems a bit odd to me that there would be a 0.1uF capacitor across an input that is expecting switched 240v. Why ? Has this been installed by someone in the wiring centre or is it on the boiler's PCB ?
    Last edited by DBMandrake; 19th January 2022 at 09:42 AM.

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