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Thread: Evohome over shoot on UFH zone

  1. #1
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    Default Evohome over shoot on UFH zone

    I've had evohome installed for around 10 days now. For the most part it is working fine except for the Kitchen which is the only room in the house that has UFH. This zone is has a T87RF wireless stat and a BDR91 configured as a boiler relay driving a two port zone valve. It is configured to have a temperature of 20.5 in the morning and evening and sets back to 18.0 during the day and over night. The system has optimized start/stop enabled.

    When it was first installed, we were in the middle of the cold spell and the zone temperature was managed well with very little over shoot (it was typically maintaining the temperature around 21.0). Then on Friday the temperatures warmed up and the zone is now over shooting to around 23.5 every day. Overnight the zone is dropping to around 20.0 so it doesn't need much energy to raise the temperature to 20.5.

    Is this just a case of the system needing to learn, or do I need to do anything to stop this behavior?

    Don

  2. #2
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    I have a netatmo thermostat in the Kitchen. The redline shows the high point and the blue line the low point. The settings for the zone have not changed during this period. image1.jpg

  3. #3
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    Maybe the flow temperature in your UFH is just too high for the low heat loss of the room during warm weather conditions ?

    Have you tried turning the UFH flow temperature down a bit ? Even with normal radiators having the flow temperature too high in warm conditions (especially if you have a well insulated house) can lead to overshoots because the system is not able to modulate down far enough for the small amount of heat required - the minimum amount of controllable heat that it can put into the floor will result in an overshoot, so overshoot becomes inevitable.

  4. #4
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    How come the bdr91 is configured as the boiler relay for the ufh zone. What is firing the boiler then?

  5. #5
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    Quote Originally Posted by bruce_miranda View Post
    How come the bdr91 is configured as the boiler relay for the ufh zone. What is firing the boiler then?
    The boiler is driven off the end-switch on the zone valve.

    Quote Originally Posted by DBMandrake View Post
    Maybe the flow temperature in your UFH is just too high for the low heat loss of the room during warm weather conditions ?
    The flow temperature is at 40 and hasn't been touched since the old system was in. We did use to have some over-shoot with the old system, but nothing near what we have had with evohome in the last few days. The old system was relatively basic as it would only shut off once the set temperature had been reached (hence the over-shoot).

    I'm tempted to not have a constant temperature in the room rather than having a lower temperature during the day and at night. Perhaps this would give evohome a better chance of managing the temperature.

  6. #6
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    I'm wondering if the reason for your overshoot is because the evohome thinks its firing up the boiler but instead it is opening up the valve more often than needed, which in turn is firing up the boiler.

    Do you have the evohome controller or are you using just the T87RF with it's own BDR91?

  7. #7
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    Quote Originally Posted by bruce_miranda View Post
    I'm wondering if the reason for your overshoot is because the evohome thinks its firing up the boiler but instead it is opening up the valve more often than needed, which in turn is firing up the boiler.

    Do you have the evohome controller or are you using just the T87RF with it's own BDR91?
    I do have the evohome controller, but the temperature for the zone is being controlled by the T87RF. For the first week it worked fine, but the last few days have been a nightmare with the overshoot. Today the room is fine, but that's because so much heat was put into the floor yesterday that it's not cooled down enough yet to require heating.

  8. #8
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    What other zones do you have and how do they call for heat? If the boiler relay is tied to the ufh valve?

  9. #9
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    Quote Originally Posted by bruce_miranda View Post
    What other zones do you have and how do they call for heat? If the boiler relay is tied to the ufh valve?
    I'm home now and able to check the settings on evohome and realise I may have been using some of the wrong language. Looking at the settings, the kitchen is configured as a zone valve with a BDR91 driving the relay.

    The other zones are all using HR92's connected up to a separate BDR91. The system is configured such that if the kitchen requires heat, the UFH zone valve is opened and boiler fired. If only radiators are calling for heat then the UFH zone valve remains closed.

    Don

  10. #10
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    I finally think I've solved the overshoot problem - and it was a setup issue on my part. I noticed that the UFH BDR91 was being enabled at times when it shouldn't have been. On digging into this I found out that I hadn't cleared the factory set T87RF binding to the BDR91 - the T87RF had a set temperature of 23 degrees!

    I'm kicking myself for missing this setup step, and taking over a month to find it!

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