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Thread: OpenTherm control behaviour

  1. #141
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    When I initially contacted Ideal, they said "The set temperature for the flow on the front of the boiler is over ridden by the open-therm controls which will increase or decrease the flow temp due to room temperature etc.". However, when I went back, this is what I described;

    "The main issue I am seeing is that without temperature control of the boiler, the boiler is running at 80 degrees at initial load, which is then causing overshoots in room temperatures and can make it uncomfortable. It does throttle back to 50 degrees when load is satisfied, but it takes a while. Is this what you would expect? Is there any downside to the boiler running at it's maximum operating temp for periods of time? I should point out that I do not have the hot water enabled yet, as I am still completing the switch over.

    I was under the impression that the previous generation boiler has the ability to control the heat curves and therefore the flow temperature. It not only disables the knob, but also the installer menu."

    So previously I was seeing that the boiler was ignoring any temperature setting and heading straight to the maximum, whenever there was any demand for heat. Our system is a little funny in that we have cast iron radiators (Edwardian house) which ended up getting roasting hot, and making rooms uncomfortable. The cast rads are great as they hang on to the heat for longer, but when they get hot, they are HOT! If this is too hot, it all gets a little sweaty!

    It turned out that Honeywell supplied the PCB to Ideal, and when they fed back what I was seeing, Honeywell decided it wasn't right and adjusted the board. Whether this is right or not unfortunately I don't know! I haven't yet been in the cupboard to see if when there is hot water demand the temperature is ignored. And I haven’t tried changing the relationship between the front knob and the installer max. That will be the next thing to trial.

    I must admit I’ve been pretty impressed with how Ideal have reacted. From a company that had a bad reputation for quality a few years back, they have been helpful, fed back well and replaced the part FOC.

    Regardless of how well the OT is working, since we moved into the house, we’ve cut out gas bill in half… In January last year, gas was touching 11/day! At this rate the system will pay for itself in a few years.
    Last edited by kimber.kimber; 29th November 2017 at 10:57 AM.

  2. #142
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    I am pleased that you have a solution but I confess that I am none the wiser as to how Evohome/OT should actually work. I confess that I was surprised that I had lost manual CH max boiler flow temperature control. That said, I can see the logic behind letting the boiler and Evohome via OT set the max flow temperature as required. It means that my radiators get up to temperature well within the first hour and then the boiler flow temperature reduces markedly. Presumably, for those with boilers where max flow manual temperature control still works, there is a need to look ahead at forecast OATs and set the max flow temperature accordingly. As I type, my boiler TSet Calculated flow temperature is 47C with a much lower return temperature. That said, I know that if I increase a zone temperature from 15 to, say, 20C, then the TSet Calculated will steadily increase towards the TSet Max (70C) set in the boiler profile. As I say, all this seems to be very logical.

  3. #143
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    I know what you mean about being none the wiser. I guess, yes the logic systems should sort it all out for you.

    The one thing with the Ideal is the TSet and TSet Max temps you talk about I can't see. I have a dial on the front that can control the flow temp, and there is an installer flow temp.

    Tonight, I'm going to have a play with adjusting the installer flow temp, and seeing what effect that has, especially if the dial is set at a higher temperature. I'll feedback when I've played around a bit more.

  4. #144
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    Quote Originally Posted by kimber.kimber View Post
    I know what you mean about being none the wiser. I guess, yes the logic systems should sort it all out for you.

    The one thing with the Ideal is the TSet and TSet Max temps you talk about I can't see. I have a dial on the front that can control the flow temp, and there is an installer flow temp.

    Tonight, I'm going to have a play with adjusting the installer flow temp, and seeing what effect that has, especially if the dial is set at a higher temperature. I'll feedback when I've played around a bit more.
    The 'installer flow temperature' on the Atag is what Atag calls the P0 parameter:

    1. CH Tmax: 80C; Gradient: 5; Heating line 24
    2. CH Tmax: 70C; Gradient: 5; Heating line 19
    3. CH Tmax: 60C; Gradient: 4; Heating line 15
    4. CH Tmax: 50C; Gradient: 3; Heating line 11

    My installer went for Line 2 because he felt that the boiler was slightly bigger than needed for CH. We went for a 24kW boiler rather than an 18kW to ensure that we got rapid HW heating. No matter whether it is CH or HW heating that is being demanded, the boiler calculated max flow temperature will not go above 70C. In truth, I have seen the actual flow temperature 1 - 2 degrees higher.

  5. #145
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    @HenGus - Are you just reading the temps from the boiler display or anything more fancy?

  6. #146
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    Quote Originally Posted by fergie View Post
    @HenGus - Are you just reading the temps from the boiler display or anything more fancy?
    I wish. From the actual boiler display.

  7. #147
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    Quote Originally Posted by HenGus View Post
    The 'installer flow temperature' on the Atag is what Atag calls the P0 parameter:

    1. CH Tmax: 80C; Gradient: 5; Heating line 24
    2. CH Tmax: 70C; Gradient: 5; Heating line 19
    3. CH Tmax: 60C; Gradient: 4; Heating line 15
    4. CH Tmax: 50C; Gradient: 3; Heating line 11
    This is what I don't get with the Ideal. I literally get the dial on the front, and then the installer set temp on the electronic menu. No option to change or even any mention of gradients etc.

    We have a 32Kw boiler, which is perfectly matched to the radiator outputs allowing additional for the stored water heating. One thing I have been thinking though is that its a little odd with Evohome, as one of the advantages is not having to heat all of the rooms at once. This means that at any point, you're not really using the boiler efficiently. It's one downside of the bigger Vogue's that the modulation ration isn't brilliant 6.4-32kW @70 degs, 6.8-34.3kW @40 degs, but as some of the rads in the house are monsters (Lounge 3.15kW, kitchen 2.4kW), hopefully it won't come into play too much.

    It is slight odd though as the Ideal website states that the Vogue has a modulation ration of up to 7:1, but all the models listed, I can only see 5:1 - unless I'm calculating something wrong...
    Last edited by kimber.kimber; 29th November 2017 at 02:24 PM.

  8. #148
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    So home tonight, and the wife is out, so I’ve tucked myself in the airing cupboard for a bit of a play!

    It would seem on the ideal, all the set temp does in the menu is to control the maximum that the knob can be turned up to! Also, with the flow temp sat at 63degs, when I got in none of the rooms were up to temp. Was set at this to try and get it into condensing mode this morning.

    Efficiency setting is way down at 25% again, meaning it’s not been in condensing mode much.

    I’ve knocked the flow temp up to 70degs to see if I can get the zones up to temp.

    TBH I’m getting a bit frustrated by it all! I don’t actually know if OT is giving me any benefit. I’ll stick my head back in later and see if the flow temp drops off.

  9. #149
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    The real issue here is the fact that ANY zone less that 1.5C less that set point will kick the boiler into full. The OT Bridge then sets the requested flow temperature to 90C. That then makes the boiler ramp up to whatever the max might be. OT really works well when the house is constantly heated and the boiler just needs to tick along. Zones coming on at various times and then rooms cooling down etc all play havoc with the current implementation of OT. I now have some fairly complex scripts that watch the heat demand from every zone and also the aggregated heat demand and then vary the boiler's knob accordingly to clip the OT demand at a more sensible level. Seems to work but I shouldn't have to do that. I am effectively now doing what my OT Bridge should have been doing in the first place. I can fully understand the 90C demand for HW but when it comes to CH, I think a more gradual ramp up of temperature should have been allowed for or a wider proportionate band.

  10. #150
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    Quote Originally Posted by bruce_miranda View Post
    The real issue here is the fact that ANY zone less that 1.5C less that set point will kick the boiler into full. The OT Bridge then sets the requested flow temperature to 90C. That then makes the boiler ramp up to whatever the max might be. OT really works well when the house is constantly heated and the boiler just needs to tick along. Zones coming on at various times and then rooms cooling down etc all play havoc with the current implementation of OT. I now have some fairly complex scripts that watch the heat demand from every zone and also the aggregated heat demand and then vary the boiler's knob accordingly to clip the OT demand at a more sensible level. Seems to work but I shouldn't have to do that. I am effectively now doing what my OT Bridge should have been doing in the first place. I can fully understand the 90C demand for HW but when it comes to CH, I think a more gradual ramp up of temperature should have been allowed for or a wider proportionate band.

    Agreed, 'flooring the pedal' until it gets to the 1.5c proportional band is an unnecessary approach and isn't what my Remeha iSense controller does.

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