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Thread: Evohome on a Y plan system - should i add a boiler demand relay?

  1. #1
    Automated Home Jr Member
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    Default Evohome on a Y plan system - should i add a boiler demand relay?

    Hello,

    We moved to a new house and I replaced the faulty CH/HW time controller with an Evohome setup and HR92 on most radiators. The system is a Y plan and i have two BDR91 relays controlling the zone valve as i also have the hot water kit. I currently have it configured with no boiler demand relay. Once the system is up to temperature i get a lot of short 'on' cycles, sometimes so short the boiler is only on for a few seconds. I understand why this happening, but wonder if it would be better if I added a boiler demand relay so i can set the max. number of cycles and minimum on time?

    Anyone done this?


    thanks,
    Richard.

  2. #2
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    Page 43 of the Evohome Installation Guide might be of help.

  3. #3
    Automated Home Legend top brake's Avatar
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    sounds like there may be a binding error so recommend that you contact Honeywell support
    I work for Resideo, posts are personal and my own views.

  4. #4
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    Quote Originally Posted by richardjh View Post
    ...........if I added a boiler demand relay so i can set the max. number of cycles and minimum on time?
    You don't need a boiler demand relay to be able to adjust the cycle rate. I use my Evohome to control zone valves with no boiler BDR. Each of my zone valves is controlled by its own BDR and I have set the cycle rate to 3 per hour. As I understand it, that sets the period for which each BDR gives a heat demand. For '3 per hour' that means 20 minute periods. If the zone is calling for, say, 10% heat demand, then the BDR will be off for 18 minutes and on for 2 minutes. I have valves that take about a minute to open and a minute to close so anything more than '3 per hour' cycle rate causes my valves to be spinning constantly which is a bit pointless. Even at '3 per hour' I still get valves closing and then reopening one minute later, which has no material effect on the zone, given that I have some large heavy cast iron radiators. I assume that is for a 95% heat demand in the zone.

    Try setting your cycle rate to 3 per hour and see if you get fewer short cycles.

  5. #5
    Automated Home Legend paulockenden's Avatar
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    You shouldn’t see those settings unless you've bound a boiler relay.

  6. #6
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    Quote Originally Posted by paulockenden View Post
    You shouldn’t see those settings unless you've bound a boiler relay.
    You can see those settings when a boiler relay isn't bound and altering them seems to apply them to the heating relay. My parents have the same set up (Without the boiler relay) and you can alter the cycle rates and minimum on time. And on altering these it appear it adjusted it accordingly.

  7. #7
    Automated Home Legend paulockenden's Avatar
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    The only time I could see those settings was when my installers messed up and double bound my heating relay as a boiler relay too.

    Once I’d corrected the configuration and removed the boiler relay binding those parameters went away.

  8. #8
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    I can confirm that if you have any BDR91 within the CH heating system you get those settings. I use OT but one of my UFH zones uses a BDR91, I can see those settings. But I have no idea if those settings affect the BDR91 of the UFH zone or not. Haven't tried.

  9. #9
    Automated Home Legend paulockenden's Avatar
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    How odd. You don’t see them on a conventional Y or S plan (with no electric controlled zones).

    I might try binding a phantom BDR91 to a fake zone and see if they appear!

  10. #10
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    That is what i thought i would try - either on a fake zone or as a boiler control relay that isn't actually connected to the boiler!

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