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Thread: OpenTherm control behaviour

  1. #11
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    Quote Originally Posted by StephenC View Post
    Interesting - I get that as well. With Opentherm connected, the pump in my Viessmann 111-W storage combi runs for a long time when it finally does ramp down the flow temp.

    I have a towel rail that stays warm just as you say (equivalent to a auto bypass pipe) with no other radiators with HR92 warm at all.

    It's almost as if one zone/HR92 is calling for a minimal 1%-10% heat, but not opening the valve enough to actually let any water through!

    Didn't ever see this with BR91 Relay that i know of. Could this be radiator balance issues, even slight?
    It's probably related to the pin calibration of your TRV valve bodies, and the "minimum on time" setting which you get with a BDR91 but not with the OpenTherm bridge. I have seen similar symptoms (all radiators cold but the occasional cycling on of the boiler) on rare occasions even on a BDR91.

    What I believe happens is this:

    The HR92 has a certain range of pin travel when it opens and closes the valve. When you fit it to the valve it does a calibration where it pushes the pin down hard until a certain amount of force is reached and this is calibrated as the fully closed position. (0% on the reported valve position in option 10 in the menu) It then opens the valve a certain number of turns of the gear inside the HR92 (a greater number of turns in full stroke 1) or until it hits the limit stop. This gets calibrated as 100%.

    This percentage is a proportional indication of how far the pin is pushed down or released with 0% being pushed right down. The HR92 sends this valve position percentage to the controller as a "heat demand" percentage which the controller then uses to determine whether there is sufficient demand to warrant firing the boiler, and if so by how much. (TPI duty cycle using a BDR91 or flow temperature using OpenTherm)

    Here's the underlying issue - the HR92 doesn't really know for sure at what point of pin travel the valve actually starts letting water flow. In the fully closed position the rubber washer in the valve will be compressed. As the HR92 unwinds and lets the pin move out quite a lot of travel will happen where the washer is decompressing but still sealing the valve, therefore no water will flow.

    From my observations it seems to be typical (at least on MY valves) that the pin will reach about a 30-40% reported opening (based on total calibrated pin travel) before any water starts to flow at all, and typically will be fully flowing by around 80%. Because of this the controller seems to be programmed to ignore heat demands of less than about 40% and won't fire the boiler until at least one zone exceeds this.

    However what happens if due to valve pin dimensions, washer thickness, valve stickiness etc your valve doesn't let water start flowing until "60% open" ? It could very well be that the heat demand is sufficient to fire the boiler intermittently while the valve is not actually letting any water flow. And the system has no real way of knowing this. As far as it is concerned the HR92 is open enough that there must be some flow through that radiator, but there isn't.

    If you use a BDR91 there is an additional factor - the minimum on time setting. By default its 1 minute (and can't be set any lower) and for the default 10 minute cycles it means that even if the controller thinks a valve is open enough to warrant firing the boiler, it won't do so until the required output from the boiler is at least 10%, and would thus exceed the minimum on time. So this would tend to avoid any brief firings where it might think a valve was slightly flowing when it actually wasn't.

    With OpenTherm I don't think there is any minimum on time or minimum demand, so it would tend to try to fire the boiler with a very low flow temperature. (30 ??) This would bring the boiler on for the short time needed to get the flow up to 30, and probably leave the pump running a lot.

    What can you do about this if it is firing a lot when radiators are not flowing ?

    The first thing I would do is remove each HR92, unwind the black wheel and refit the HR92 to allow it to re-calibrate itself. Then give it a few days for the valves to fine tune their calibration.

    If that doesn't help, next time you see the boiler intermittently firing when all radiators are cold I would nip around each HR92 and go into the settings, look at option 10 (valve position) and record what percentage it believes the valve is open. If you have a figure significantly higher than 40% and the boiler is heating up and the radiator is stone cold I would be suspicious of that valve body.

    You could try enabling full stroke 1 mode in the HR92 settings, which increases the amount of torque to close the valve and also increases the number of turns of the motor between closed and open. The disadvantage of this mode is the batteries don't last as long and there is more tendency to get temperature overshoots, however on a couple of my valves I've been forced to use full stroke 1 mode.

    The valve could also be sticky (causing hysteresis in the opening point depending on whether the valve is closing or opening) or it may not be 100% compatible with the HR92's due to for example a pin length that is a bit longer than average. Or if you are using an adaptor the adaptor might not be sitting right.
    Last edited by DBMandrake; 14th April 2017 at 03:14 PM.

  2. #12
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    Cheers for this. I've literally just gone through my schedule and changed the low point from 16 to 5 degrees. I'm going to see if this stops the shuttling.

    I was guessing that the efficiency was going off times etc etc. But your right, it could just be something for the end user to cooo over! Like I've just been doing.

    I see how it goes with the reduced temps and report back. Maybe the low point just wasn't low enough.

  3. #13
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    That's loads of information on the valves too! Will definitely have a look at that. Is there a way to force them to do their calibration again? I've literally had a whole new heating system, boiler, tank, rads and pipe work, so it shouldn't be sticking valves. I fitted all new Honeywell Valencia valves at the point of instal.

  4. #14
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    Just to be clear, my second post was replying to StephenC's problem of the boiler firing while the radiators stay cold - that's not what's happening to you is it ?

    I got the impression that your boiler is firing when you think it shouldn't be but that the radiators are slightly warm ?

  5. #15
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    A week in, I have noticed that the HR92s cycle a lot less frequently under OT control than was the case with the BDR. My boiler seems to 'slow' down as zones reach set temperatures whilst still maintaining a high flow temperature. Only when all the zones are at their set temperatures do I see the temperature on the boiler fall back to a steady condensing flow temperature. (This was described this to me as Evohome OT treating heating as a single zone - presumably, doing all the sums before communicating with the boiler ). Last night, the boiler was ticking over at 54/44 for a while. I have not heard the boiler cycling when the house is warmer than the set temperatures, as was the case with my old boiler under BDR control.

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    Sorry, yes I did get that, just thought it would be another thing to try.

    TBH it's difficult to tell! We have installed cast rads, so they stay hot for quite a while. This morning while the boiler was firing, the room that had its temperature near its set point, had a cold rad, and the inlet pipe was also cold. I didn't get chance to check the others.

    In the mean time reducing the set point has seemed to have stopped the boiler firing.

  7. #17
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    Quote Originally Posted by kimber.kimber View Post
    That's loads of information on the valves too! Will definitely have a look at that. Is there a way to force them to do their calibration again? I've literally had a whole new heating system, boiler, tank, rads and pipe work, so it shouldn't be sticking valves. I fitted all new Honeywell Valencia valves at the point of instal.
    Take them off the valve body; turn the adapter fully anti-clockwise and replace the HR92. You will then see CYCL as the valve motors open and closed.

  8. #18
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    Well with the set point dropped, the boiler hasn't been firing at all. So looks like that may have got it. I probably need to adjust the low set point to a temperature just below the rooms natural resting temp. We've only been in the house since a week before Christmas, so we're still learning its intricacies!

    Even with a little bit of fiddling, it's still going to be better that what we're taking out...

    This for heating... (no thermostat fitted at all)
    IMG_6908.jpg

    And this for hot water... (gas and on 24/7)
    IMG_6909.jpg

    Think the gas man might be round to check I've not been fiddling the meter when we're all up and running!!

  9. #19
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    In case anyone else finds this and wants to read, here's an article by Honeywell about TPI...

    http://www.honeywelluk.com/documents...0explained.pdf

  10. #20
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    Quote Originally Posted by kimber.kimber View Post
    In case anyone else finds this and wants to read, here's an article by Honeywell about TPI...

    http://www.honeywelluk.com/documents...0explained.pdf
    I bet you can't find a description of Honeywell Adaptive Fuzzy Logic.

    Other than:

    A fuzzy control system is a control system based on fuzzy logic—a mathematical system that analyzes analog input values in terms of logical variables that take on continuous values between 0 and 1, in contrast to classical or digital logic, which operates on discrete values of either 1 or 0 (true or false, respectively)
    Last edited by HenGus; 14th April 2017 at 05:52 PM.

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