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Thread: Boiler with separate HW and CH max and OpenTherm

  1. #51
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    I think we need to remember that Honeywell is not here just to serve the UK. The UK has some of the worst insulated houses in Europe. It is really this that raises the question as to whether external weather compensation in the control system is necessary. If you are building a near passive house in Germany, then external weather compensation is not going to be much needed.

    Unless I'm mistaken I thought I remembered an old Honeywell heating control system (CM5000) or perhaps it was a drayton that had an external temperature sensor . Clearly for some good reason this strategy is dropped for Evohome, as a guess probably based on the increasing efficiency of domestic properties.ey

    Also remember that Opentherm was a standard initially created by Honeywell. Clearly they are not impressed if they have to update their systems to compensate for boiler manufactures not applying the correct/expected implementations.

    https://www.theguardian.com/environm...u-fuel-poverty
    Last edited by rvb99; 26th January 2018 at 02:52 PM.

  2. #52
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    Quote Originally Posted by DBMandrake View Post
    Interesting. I am yet to see a discrepancy between boiler demand and maximum zone demand, and I've kept tabs on it quite a bit.

    This is typically what I see on my controller:IMG_0942.jpg

    The rest of the zones (bar one which is at 5%) are all 0%.

  3. #53
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    Quote Originally Posted by HenGus View Post
    This is typically what I see on my controller:IMG_0942.jpg

    The rest of the zones (bar one which is at 5%) are all 0%.
    I've never seen that happen. Do you have any zones with multiple HR92's ? I don't.

    Perhaps only the "primary" HR92 in a zone has its heat demand reported on the new diagnostics screen ?

    I had assumed that in a multiple HR92 zone the diagnostics screen would show the highest heat demand from the HR92's in that zone but maybe this is not the case and it is always the primary one!

  4. #54
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    No multi zones. The hall sensor is the Evotouch controller.

  5. #55
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    I use a VR33 so my observations are based on the eBUS side. What I have seen is that the Flow requests vary from 10-90. Also there is constant feedback from the boiler to the device to tell it what the flow actually is. e.g. you asked for 90 but I can only go to 65. But the OT appears to be fixed in its way and ignores this. I could quite easily have done the mapping of OT % to flow temp requested, but the OT % implementation on Domoticz is wrong and the OT % is not on the web API yet! We have suspected this for a while but now its evident because the reported Heat Demand on Domoticz and the controller dont match.

  6. #56
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    Quote Originally Posted by bruce_miranda View Post
    I use a VR33 so my observations are based on the eBUS side. What I have seen is that the Flow requests vary from 10-90. Also there is constant feedback from the boiler to the device to tell it what the flow actually is. e.g. you asked for 90 but I can only go to 65. But the OT appears to be fixed in its way and ignores this. I could quite easily have done the mapping of OT % to flow temp requested, but the OT % implementation on Domoticz is wrong and the OT % is not on the web API yet! We have suspected this for a while but now its evident because the reported Heat Demand on Domoticz and the controller dont match.
    The problem, as noted before, is the inconsistency of boiler operation under OT control between manufacturers. As I have posted previously, the max HW and CW temperatures demanded by Evohome OT are limited on the Atag is boiler by the TMax Set temperature in the installation parameters. My boiler max flow temperature is limited to 70C, and I have never seen the TSet Calculated temperature demanded by Evohome rise above this setting. The 'downside' (I have yet to determine whether it actually is a downside) is the OT control renders the manual setting of CH maximum flow temperature inoperative.

  7. #57
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    All zones are up to temperature and this is what OT is demanding:

    IMG_0943.jpg

    I am not bothered about it as the Evohome/Atag/OT combination is working well as far as I can tell.

    I have just noticed that the difference is 14% on both images. Uhm?

  8. #58
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    I've been playing a wee bit with the settings on my Atag iS32 (HenGus has the 24). As HenGus says, if the boiler is set up at the 70deg preset, it never goes above that, but I have found that if I then manually drop the temp it will 'overshoot' by approximately 8 degrees. So for example if I set the boiler temp max at 57, it will generally max out about 65.

    However though, I still saw the heating push it gradually over 65 and back up to 70deg when there was a lot of heating demand, but NOT when there was hot water demand.

    As an experiment, I soft set the boiler at 50deg last night, but woke up to pretty cold house. Basically the Hot water tank only reached aprox 57deg which on my priority system stopped the heating valve from opening as it was constantly trying to heat the tank (set to 60 in Evohome).

    My theory therefore is that with the HW kit, the Evohome is sending a lower temp than it does when demanding CH!

    Any thoughts?

  9. #59
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    This doesn't match what I see. HW creates a 90C demand. Exactly the same as a CH max heat demand. This also matches what the Honeywell guys themselves said.

  10. #60
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    Quote Originally Posted by bruce_miranda View Post
    This doesn't match what I see. HW creates a 90C demand. Exactly the same as a CH max heat demand. This also matches what the Honeywell guys themselves said.
    Fair enough, but something must be different in the other setting requests maybe? But at least the Atag seems to be further capping it (to an extent).

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