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Thread: OpenTherm scheduling to get rooms upto the required temp on time

  1. #1
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    Default OpenTherm scheduling to get rooms upto the required temp on time

    Quick question: If you have the schedule set to be 21C by 1800 then the optimisation comes on in as it should but always by 1800 the room temp is only at 20C and seems to hover there all evening maybe getting to 21C - 3 or so hours later. Increasing the max flow temp allowed on the boiler seems to make no difference as the boiler still modulates down to the same flow temp 1C off the set temp. This seems to be because the OpenTherm reduces the flow temp at 20C assuming that there is still enough heat being put into the house to raise it to 21C. So now I have set it to be 22C by 1800 so that it wont start reducing down until the room temp is up to 21C which is usually by about 1800. Then an hour later at 1900 I then set a schedule period to be 21C for the rest of the evening, this then seems to be the best match for modulation and control of room temp.

    Does anyone else follow this practice or similar?

  2. #2
    Automated Home Legend paulockenden's Avatar
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    Is this Lyric or Evohome?

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    Quote Originally Posted by paulockenden View Post
    Is this Lyric or Evohome?
    Sorry this is lyric, but I imagine Evohome does the same thing as both honeywell and honeywell say the opentherm protocol is standard.

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    Automated Home Legend paulockenden's Avatar
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    Opentherm is the bit that converts a heat demand (percentage) into a flow temperature.

    It sounds like your problem is further upstream - with the heat demand calculation.

    I presume with Lyric that's in the thermostat.

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    Quote Originally Posted by paulockenden View Post
    Opentherm is the bit that converts a heat demand (percentage) into a flow temperature.

    It sounds like your problem is further upstream - with the heat demand calculation.

    I presume with Lyric that's in the thermostat.
    From what I understand though and from my conference call with honeywell is that when the room temp is 1C from the set point the boiler will start to modulate down as it aims to keep the temp within 1C of the required set point as the gap between room temp and required room temp becomes smaller the modulation increases. Increasing the required room temp increases that gap meaning a higher flow temp is required.

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    Automated Home Legend paulockenden's Avatar
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    Yup - Evohome does the same. With that the proportional band is 1.5 degrees. But that's not Opentherm doing this. You get the same thing (ish) with simple on/off switching with a BDR91. Opentherm is simply converting a number between 0 and 200 (the heat demand) into a flow temp.

    But that heat demand requirement is calculated further upstream. It's Evohome it comes from the rad valves, thermostats, etc, and is then marshalled into a single value by the controller. I presume that with Lyric it's the thermostat itself doing this.

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    Quote Originally Posted by paulockenden View Post
    Yup - Evohome does the same. With that the proportional band is 1.5 degrees. But that's not Opentherm doing this. You get the same thing (ish) with simple on/off switching with a BDR91. Opentherm is simply converting a number between 0 and 200 (the heat demand) into a flow temp.

    But that heat demand requirement is calculated further upstream. It's Evohome it comes from the rad valves, thermostats, etc, and is then marshalled into a single value by the controller. I presume that with Lyric it's the thermostat itself doing this.
    So it would seem it (T6 Lyric) is doing what it is supposed to do within this band so if you want to achieve the set point quicker rather than waiting several hours you have to have a higher than required set point for the initial on period to delay the modulation of the boiler. Then set a separate schedule after with the required room temp.

    Opentherm is a different way of controlling and whilst it does what it is supposed to do you have to "fool" it if you want a quicker reaction time.

  8. #8
    Automated Home Legend paulockenden's Avatar
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    If you read through the various threads here, the one thing that people complain about a lot is overshoots, and that's what the system is trying to avoid.

    Imagine getting into your car and trying to get to EXACTLY 30 miles per hour. You wouldn't floor the accelerator and then lift when you got to 30. You'd overshoot massively.

    This is what the proportional band is all about. When the system gets very close to your target temp (close enough to be comfortable, but not quite there) it'll ease off the heat demand and then increase it gradually to reach and then maintain the target temp.

    How well it's able to do this depends on your home (it's fabric, insulation, heating system, etc.).

    I know with Evohome there's a period of learning where it adapts to this, don't know whether Lyric is the same.

    P.

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    Quote Originally Posted by paulockenden View Post
    If you read through the various threads here, the one thing that people complain about a lot is overshoots, and that's what the system is trying to avoid.

    Imagine getting into your car and trying to get to EXACTLY 30 miles per hour. You wouldn't floor the accelerator and then lift when you got to 30. You'd overshoot massively.

    This is what the proportional band is all about. When the system gets very close to your target temp (close enough to be comfortable, but not quite there) it'll ease off the heat demand and then increase it gradually to reach and then maintain the target temp.

    How well it's able to do this depends on your home (it's fabric, insulation, heating system, etc.).

    I know with Evohome there's a period of learning where it adapts to this, don't know whether Lyric is the same.

    P.
    Yeah I understand all those points, I am actually going through all the posts now that I have read (there's a lot & I cant find the one I want) as I remember seeing something similar where someone said they set a higher initial set point to get to the room temp quicker and then lower it after an hour. If the house is warming up from cold then even if the room temp is reporting 20C when were asking for 21C the fabric of the house is still stabilising so to speak so its probably better to set an initial higher set point to benefit from the extra heat injection into the house and then lower after an hour. The chances are when the Lyric is trying to achieve 22C and is reporting its 21C the fabric of the house and room temp will be quite close so at that point it then can start modulating down and then after an hour of running at the higher temp it will then revert to a set point of 21C where it can then tick over nicely.

  10. #10
    Automated Home Legend paulockenden's Avatar
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    Just be careful not to do this while the system is still learning. Give it a few weeks following any install or full reset. That is, assuming Lyric does learn...

    P.

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