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Thread: Evohome CH zones only trigger the boiler if the ‘central’ zone is also on

  1. #1
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    Post Evohome CH zones only trigger the boiler if the ‘central’ zone is also on

    I have an Evohome with HW and CH systems in a S-plan configuration. It works except for this one issue that I hope someone has solved.

    The HW works fine.
    The CH triggers when the Kitchen demands heat, but all the other rooms do not independently trigger the boiler, i.e. only get heat IF the Kitchen also needs heat.
    The Kitchen is set up without any HR92 (just a standard TRV) and gets its temperature sensor from the Evohome controller unit (in the kitchen).
    All other rooms are set up with an HR92. All binding and signal strengths are fine.

    Zone configurations:
    Kitchen: Heating type = Zone valve; sensor = Evotouch sensor
    Other Rooms: Heating type = Radiator valve; sensor = remote sensor (i.e. HR92)

    The system summary shows that the demand % for the boiler control is the maximum of any zone (e.g. 100% if living room is 100% even if kitchen says 0%), but despite this the boiler/pump only goes on if the kitchen also demands the heat.

    Advice is requested on how to set up the configuration so that any room can demand heating when needed, independent of the kitchen. Is this even possible?

    I have a BD91 wireless relay for HW and a BD91 for the CH. These were wired into the legacy boiler system exactly as per the previous “non-smart” timer.
    The CH system has a single motor controlled zone valve.

    At the moment my workaround is to set the target temperature in the kitchen higher than the kitchen radiator/TRV setting can achieve so that the boiler/pump is never “off” during the set period, so if the HR92s in other rooms need heat then they can get it, but this means the kitchen is sometimes unnecessarily warm. Also sometimes if the oven is on / sun coming in etc. then the kitchen ‘overheats’ and switches off the boiler as it doesn’t need heat, which switches off all the other rooms.

    Thanks for any advice.

  2. #2
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    Quote Originally Posted by Robok View Post
    The Kitchen is set up without any HR92 (just a standard TRV) and gets its temperature sensor from the Evohome controller unit (in the kitchen).

    [...]

    Kitchen: Heating type = Zone valve; sensor = Evotouch sensor
    This configuration will not work and this is why your system is not working properly. You can't have a zone with no actuators unless the Evohome is in "single zone mode" where it emulates a traditional thermostat.

    As soon as you have multiple zones every zone must have an actuator to function. In other words you either need to add an actuator such as an HR92 to the Kitchen or forego trying to control the Kitchen with Evohome.

    I suspect you're confused about the difference between a heating zone-valve in the stored hot water configuration, and a zone configured with heating type "zone valve" - they're not the same thing! Easy mistake to make as the documentation is not very clear on this.

    When you set up an S-Plan system and you have both a heating zone valve and a hot water zone valve as it sounds like you have, you bind these in the stored hot water section. When it asks if you have one two port valve or two two port or one mid position valve you would choose the latter, then bind the hot water sensor, hot water zone valve BDR91 and heating zone valve BDR91.

    When there is hot water reheat demand the hot water BDR91 turns on. When any heating zone calls for heat the heating zone valve BDR91 turns on. This fires your boiler and opens the path to all radiators. (But only radiators whose individual HR92's are open will flow and get hot of course as there must be a complete loop)

    What you've done is bound the heating BDR91 specifically to the Kitchen zone - this type of heating zone is designed for use if you had a zone valve controlling a single zone in the house, not for controlling the zone valve that supplies all zones in the house as is usually the case in an S-Plan system.

    I'm guessing that when you set up stored hot water you told it you only had a hot water zone valve and did not have a heating zone valve, otherwise you would have ended up double binding the BDR91, and this is not supported either.

    So to solve your problem you first need to decide whether you want to control the Kitchen - if you do, get another HR92 for the kitchen radiator and fit it. If not, delete the Kitchen zone entirely, as you can't control it without an actuator.

    Then you would want to set up the stored hot water config again - change stored hot water to none, and long press both BDR91's for >15 seconds to clear the bindings.

    Then go back in to stored hot water and follow the steps making sure that you choose the option to have two 2 port zone valves, and that you bind sensor and both BDR91's within the stored hot water section.

    If you have added an HR92 for the Kitchen change the zone type for the kitchen to radiator controller and bind the HR92. (Or re-add the zone if you had deleted it while waiting for an HR92 to arrive)

    That should fix your issues, and the heating zone valve should activate for any zone calling for heat.

    A quick summary of all the different functions you can bind a BDR91 to -

    * Heating zone valve in the stored hot water configuration - turns on when any heating zone calls for heat, except for the "electric heat" zone type.
    * Hot water zone valve in the stored hot water configuration - turns on only when hot water reheat is required.
    * Boiler control relay - turns on when any heating zone calls for heat (except electric heat zones) or there is hot water demand. (Logical OR of heating and hot water zone valve relays, in most circumstances except hot water overrun)
    * Heating type "zone valve" within the configuration of an individual zone - only turns on when that zone calls for heat, not any other zones.
    * Heating type "electric heat" within the configuration of an individual zone - only turns on when that zone calls for heat, not any other zones, additionally does not trigger the system wide heating zone valve relay or boiler control relay when the zone requires heat, intended for zones that don't require hot water to heat eg electric elements.
    Last edited by DBMandrake; 9th January 2019 at 09:18 PM.

  3. #3
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    Many thanks for the advice, I will try this at the weekend. It all makes sense, you have guessed my set-up (and hence probable evohome errors) correctly so I hope this will solve it. I'll report back soon!

  4. #4
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    Thanks DBMandrake. This has solved the issues. All zones now independently drive the heating demand; I moved an HR92 to the Kitchen and re-bound it. So far so good.
    You should write an Idiot's guide to Evohome...!

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