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Thread: Zone using T87RF and BDR91 keeps turning on and off even below setpoint

  1. #1
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    Default Zone using T87RF and BDR91 keeps turning on and off even below setpoint

    So my single zone ufh is set up as a separate zone where a T87RF room stat is setup as the sensor and a BDR91 is paired as the actuator. The BDR91 in turn switches a motorised zone valve on, which previously was also firing up the boiler. The ufh zone has it's own flow and return branch from the boiler before the CH motorised zone valve. Because the evohome doesn't deal with S Plan+ it was previously configured as an S plan. So everytime the ufh had heat demand the CH valve also would get opened.
    Today i moved away from using the CH valve controlled by evohome. It's now permanently kept open. Boiler is fired by the Opentherm bridge.
    What I have noticed today, although it may have always happened is the ufh bdr91 keeps turning on an off even if the set point hasn't been reached. I would have expected the zone valve to stay open until set point is reached and then may be at the set point keep going on and off to maintain the set point, or better still let the OT bridge regulate the temperature.

    So why does the bdr91 keep going on and off even below the set point? Is it TPI doing this?

  2. #2
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    Could it be because it's been bound as a Zone Valve? And choosing another device type might make it behave differently.

  3. #3
    Automated Home Legend paulockenden's Avatar
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    Normal for TPI in the proportional band. Otherwise residual heat would cause overshoots.

  4. #4
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    Yep, perfectly normal behaviour for TPI.

    If it stayed on continuously until after the target was passed it would overshoot would it not ?

    The whole point of TPI is to adjust the boiler or zone valve duty cycle to get just the right amount of heat output to hold a steady temperature. Unless your radiators or UFH have to run flat out just to maintain the status quo (which is unlikely) then once rooms are up to temperature and just maintaining, TPI will regularly switch on and off to maintain that balance, even if the temperature is slightly below the set point.

    If it is slightly below it will gradually increase the duty cycle until the temperature reaches the set point, rather than just staying on until the set point is exceeded. (which would cause an overshoot)

    It's a different way of thinking than an old fashioned on/off thermostat with a wide differential.
    Last edited by DBMandrake; 3rd November 2016 at 11:32 PM.

  5. #5
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    The thing is that my underfloor heating zone is a 100sq meter zone. So it takes a while to even get close to the setpoint. So TPI is just delaying the process. Or will the system learn and vary the TPI algorithm.

  6. #6
    Automated Home Legend paulockenden's Avatar
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    The fact it's large is MORE reason to have TPI. There's a large thermal mass there and it'll continue to raise the temperature of the room even after the UFH is off. A bit like having a huge radiator.

    Remember, it's the temperature of the room rather than the temperature of the floor which determines comfort.

    P.

  7. #7
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    Quote Originally Posted by paulockenden View Post
    The fact it's large is MORE reason to have TPI. There's a large thermal mass there and it'll continue to raise the temperature of the room even after the UFH is off. A bit like having a huge radiator.

    Remember, it's the temperature of the room rather than the temperature of the floor which determines comfort.

    P.
    Good point on the large thermal mass. It really is just a REALLY large but low temperature radiator, which is quite problematic to control when you think about it.

    It's like driving a large truck to a destination and wanting to get there as fast as possible - but without considering how long it will take you to slow down when you reach your destination.

    Yeah, if you take the simplistic approach and keep going at 60mph the whole way until you reach your destination technically you'll get there quicker, but you'll keep sailing on past it as you slam on the brakes... better to slow down progressively as you approach the target, that's what TPI and PID control will try to do. (Technically it's the PID control that does this, TPI just gives it one possible means to progressively reduce heat transfer in a nice proportional way)
    Last edited by DBMandrake; 4th November 2016 at 10:48 AM.

  8. #8
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    In my case the issue is also that the UFH pump shuts off everytime the zone valve shuts off. The UFH pump is one of those modern proportional pressure ones that varies the pressure based on resistance. But they also start up slowly. So each time it has to start up, it take a good minute to get to full pressure. Now imagine doing that, several times an hour.
    My main boiler pump also does that but with pump overrun there, it's not so much of a problem.

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