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Thread: OpenTherm control behaviour

  1. #151
    Automated Home Legend paulockenden's Avatar
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    Didn’t someone come to the conclusion that the proportional band is even narrower when using OT?

  2. #152
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    Frustrating as you buy something based on all the spec saying it’s better and more efficient, and turns out it’s not that brilliant!

    Makes me wonder if I’d be better off ditching the OT all together?

  3. #153
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    Quote Originally Posted by paulockenden View Post
    Didn’t someone come to the conclusion that the proportional band is even narrower when using OT?
    Probably hearsay.

    Actually measuring the width of the proportional band with any degree of certainty or accuracy is more complicated than it appears, and certainly couldn't be done without direct access to the heat demand figures sent from HR92's and that sent to the OpenTherm bridge, and full precision un-rounded, unbiased temperature readings captured using something like a Domoticz.

    Has anyone done this or are they just eyeballing the rounded, biased temperature displayed by Evohome itself and comparing that to the boiler flow temperature ? If so, good luck with that. The half degree biasing of the temperature reading towards the set-point alone would make such a test erroneous.
    Last edited by DBMandrake; 29th November 2017 at 11:50 PM.

  4. #154
    Automated Home Legend paulockenden's Avatar
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    Quote Originally Posted by DBMandrake View Post
    Probably hearsay.
    I think it was Richard who said he'd managed to measure it using an OpenTherm monitor.

    Unless I've remembered incorrectly.

    P.

  5. #155
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    Quote Originally Posted by kimber.kimber View Post
    Frustrating as you buy something based on all the spec saying it’s better and more efficient, and turns out it’s not that brilliant!

    Makes me wonder if I’d be better off ditching the OT all together?
    It is better than the ALL Off or ALL On Boiler Relay. But is it better by a country mile, NO.

  6. #156
    Automated Home Legend paulockenden's Avatar
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    I think it's the "It could have been so much better" aspects that people find most disappointing.

    But an OT Bridge which just takes the same parameter that cycles the BDR91 and converts it to a demand figures instead also could gave been so much better too. There's a HUGE difference between morning startup where lots of zones suddenly call for heat, and a schedule change during the day where just one room needs a two degree boost. To fire the boiler full-pelt for the latter is just asking for problems.

  7. #157
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    Quote Originally Posted by paulockenden View Post
    I think it's the "It could have been so much better" aspects that people find most disappointing.

    But an OT Bridge which just takes the same parameter that cycles the BDR91 and converts it to a demand figures instead also could gave been so much better too. There's a HUGE difference between morning startup where lots of zones suddenly call for heat, and a schedule change during the day where just one room needs a two degree boost. To fire the boiler full-pelt for the latter is just asking for problems.
    OT most certainly works well within the parameters set by the boiler and the control. That said, I am now pretty well convinced that once all the zones are up to temperature then there is little value in setting Economy as my gas usage shows that 2 hours of savings on Economy will more than be offset by the increased usage when Economy is turned off. The said, I am talking low gas usage here with 19 radiators/12 zones all at set temperatures of 15C and above with a constant state usage of 0.4C3Ms/hour. The boiler calculated flow temperature seems to vary from 20C (no demand for heat) to 30 to 57C (with demand for heat with zones in their set target temperature range) to 70C (initial start up and Economy up to Normal operation).

    I agree that the HR92 range does seem to be a bit too narrow, and I can see merit in having Economy as user-defined temperature reduction. In my situation, a 2C reduction might give better economy that a 3C reduction.

  8. #158
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    So my current schedule turns the heating down to 5degs when we don’t need the heating system on. As the house is old, uninsulated (to the most part) and with lots of single glazing, the heat disappears pretty quickly and rooms can drop to 10degs or slightly less over night. This does mean that heating works pretty hard the get the rooms back up to temp. Do you think I’d be better off having the set back at a higher temp. My understanding at this though is that the flow temp is lower, so I don’t know if it would actually get the radiators warm.

  9. #159
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    Quote Originally Posted by paulockenden View Post
    There's a HUGE difference between morning startup where lots of zones suddenly call for heat, and a schedule change during the day where just one room needs a two degree boost. To fire the boiler full-pelt for the latter is just asking for problems.
    And if you have a large boiler that is sized to heat the entire house at once to a comfortable temperature, that single zone dropping below the proportionate band with the boiler being driven at full causes all sort of problems. Ask me!

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    Quote Originally Posted by bruce_miranda View Post
    And if you have a large boiler that is sized to heat the entire house at once to a comfortable temperature, that single zone dropping below the proportionate band with the boiler being driven at full causes all sort of problems. Ask me!
    There does seem to be an issue here. I agree that it makes sense to demand a max flow temperature at start up but doing so at other times really doesn't take into account such things as people turning up HR92s in their bedroom. A wider temperature band might solve this problem but I suspect that users might notice the higher zone temperature variation. I am fortunate that no one fiddles with the HR92s.

    All set temperatures were reached in my home just after 8am this morning with a flow max temperature of 70C. The boiler is now 'ticking along' with a flow temperature of 47C and return temperature of 39C. Gas usage in the first hour was 1.4CM3s but only 0.4CM3s in the second. I am therefore just letting the system do its own thing.

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