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Thread: Evohome + Opentherm v Evohome + Boiler Outside Weather Compensation

  1. #21
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    Isn't it Honeywell that invented the OpenTherm spec in the first place, then released it for other manufacturers to use ?

    The fact that a compatibility list is needed at all is a bit humorous in that case. If it was a Honeywell spec, what are the boiler manufacturers excuses for it not working properly with a flagship Honeywell product ?

  2. #22
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    Interesting stuff.

    I'm suitabley convinced that the Honeywell Open Therm bridge will do its job properly.

    Fairly confident that my Vogue boiler will too given it has 2 connections labelled 'OpenTherm', although not sure on the detail as there is no real mention of Open Therm in the manual whereas there is a lot of detail regarding the affect of adding an outside temperature sensor. For example the manual explains that the HW heating flow temperature can be set independently of the CH flow if the outside temperature sensor is fitted (and will override the CH flow temp if there is a simultaneous HW and CH requirement). Hopefully the same would apply with Open Therm (bearing in mind I do not have the Evohome Hot water kit) and that HW heating flow call from the zone valve would override any Open Therm flow temperature adjustments made from a CH perspective.

    Think I will go with it and will report back my findings.

    I also ordered the outside temperature sensor yesterday as it is quite cheap.

    Not sure however whether the outside sensor and Open Therm will work together, override each other, cause general chaos or be the best of everything! Anyone got experience in this respect?

  3. #23
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    Quote Originally Posted by scut0 View Post

    Fairly confident that my Vogue boiler will too given it has 2 connections labelled 'OpenTherm', although not sure on the detail as there is no real mention of Open Therm in the manual whereas there is a lot of detail regarding the affect of adding an outside temperature sensor. For example the manual explains that the HW heating flow temperature can be set independently of the CH flow if the outside temperature sensor is fitted (and will override the CH flow temp if there is a simultaneous HW and CH requirement). Hopefully the same would apply with Open Therm (bearing in mind I do not have the Evohome Hot water kit) and that HW heating flow call from the zone valve would override any Open Therm flow temperature adjustments made from a CH perspective.

    Think I will go with it and will report back my findings.

    I also ordered the outside temperature sensor yesterday as it is quite cheap.

    Not sure however whether the outside sensor and Open Therm will work together, override each other, cause general chaos or be the best of everything! Anyone got experience in this respect?
    If it helps, I have a Remeha Avanta 39c working with the OT interface quite happily. Initially I left the external compensator connected but on reading more on the internet decided that it wasn't necessary.

    The thrust of the argument being that the Evohome only needs to know how much heat the room is losing and how far it is from the setpoint in order to judge the best time to target burner rate. I always forund the compensation curve needed adjustment when working with an older Honeywell CM727 & relay partially due to the external sensor not being on a truly North face of the property.

    I have tweaked the Avanta to limit the max flow temp to around 70 degrees as I initially found times that the Evohome was running at flow temps up to 85 just in order to minimise the room warm up times, which clearly took it well outside the condensing mode but I guess then it would have remained at a much lower flow temp to maintain the room temperatures.

    For sure the OT will help with the Evohome Optimisation and all but eliminate overshoots from my experience.
    Last edited by Chestnut 2000; 11th March 2016 at 06:29 PM.

  4. #24
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    Had the OT module running for a couple of days now, and the trends that I produce seem to suggest that it has made quite a difference. To be fair, Evohome was always doing a good job of maintaining target, but the latest trends seem to show very little deviation from set point on most rooms. When the house is warm it is modulating down nicely, so boiler would appear to be spending more time in its condensing mode. Overall, really impressed with the way this has all worked together.

  5. #25
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    Quote Originally Posted by DBMandrake View Post
    Isn't it Honeywell that invented the OpenTherm spec in the first place, then released it for other manufacturers to use ?

    The fact that a compatibility list is needed at all is a bit humorous in that case. If it was a Honeywell spec, what are the boiler manufacturers excuses for it not working properly with a flagship Honeywell product ?
    Indeed there is no need for a compatibility list per se.

    But many boiler manufacturers keep the fact that their appliances are OT as a kind of a secret.

    Some of the tech teams are not even aware of what OT is.

    A very bizzare situation
    I work for Resideo, posts are personal and my own views.

  6. #26
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    Quote Originally Posted by StuartG View Post
    Had the OT module running for a couple of days now, and the trends that I produce seem to suggest that it has made quite a difference. To be fair, Evohome was always doing a good job of maintaining target, but the latest trends seem to show very little deviation from set point on most rooms. When the house is warm it is modulating down nicely, so boiler would appear to be spending more time in its condensing mode. Overall, really impressed with the way this has all worked together.
    Great feedback it's a really worthwhile and notice able improvement isn't it.

  7. #27
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    Quote Originally Posted by StuartG View Post
    Had the OT module running for a couple of days now, and the trends that I produce seem to suggest that it has made quite a difference. To be fair, Evohome was always doing a good job of maintaining target, but the latest trends seem to show very little deviation from set point on most rooms. When the house is warm it is modulating down nicely, so boiler would appear to be spending more time in its condensing mode. Overall, really impressed with the way this has all worked together.
    I have been looking at the evohome for some time now after an electrician friend of mine installed it in his house. I have a Vaillant ecotec boiler with VRC470 weather compensator, which, according to most forums/sites, seem to be incompatible with the evohome. I also have spoken to a couple of heating engineers who have also said I need to rip out the vaillant controls in order to get the evohome to work.

    I am very interested to to hear that you have this working with the evohome and the ecotherm module. Can anyone give me some details on what needs to be done in order to get the 2 systems integrated ? Should the VRC470 unit go into the boiler housing instead of being used a control panel downstairs? Also where is the best place to get hold of the VR33 openthem module?

  8. #28
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    Our installer got me the VR33 module, for fitting in to a EcoTec Plus 637 system boiler. The instructions in the box were in Dutch, so I assume it was just a bog standard Vaillant module.

    Fitting was simply a case of opening the electronics area, attaching it to the relevant connector, and replacing one jumper. The old cabling from the BDR91 was disconnected, and a new two core run from the VR33 connector to the Evohome OpenTherm bridge.

    Everything powered up first time, and the boiler registered the presence of the eBus controller (it's one of the diagnostic parameters) so no issues with the boiler not recognising it, or being in some way programmed to "ignore" it.

    It was then just a case of binding the OpenTherm bridge to the Evohome Controller in place of the BDR91, which was very simple.

    Unlike the BDR91 the bridge just blinks green every ten seconds or so to say it is active.

    I didn't do any other reconfiguration - it all "just worked".

    What I'm seeing now is very tight temperature control of all zones. I'm using the PHP API to log every 15 minutes. Zones look like they're often fluctuating +/- 0.5 C. I don't data log the set point request to the boiler, but every now and then I go and look, and have seen the bridge request set points of 90C when the house is cold with multiple heat demands, falling to low fifties when the house has warmed and it's just maintaining. Right now I have a day off so a couple of rooms are active, and I'm seeing 59C requested.

    At some point I'm going to try and compare "before" and "after" trends on the rooms. Just looking at it I would say it is holding set point even better than it was. That said, the BDR91 was getting +/- 1 C much of the time so Evohome was never "bad". My gut feeling from the trends is also that the system is responding more quickly in the warm up phase.

    Optimisation (I have both optimum start and stop enabled) is looking good.

    I didn't have weather compensation on the system, so cannot say anything about that.

    Overall, I would say the system certainly seems to be doing what it should, and the fact I am seeing low remote set point temperatures when I spot check is what I was hoping. In terms of energy saving, I'll wait and see, but my whole install was more because it was a "responsible" thing to do than purely financial.

    Hope that helps.
    Last edited by StuartG; 22nd March 2016 at 01:30 PM. Reason: Typo!

  9. #29
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    Any idea if the external temperature sensor data is used by the evohome to pass on to the OT bridge and then on to the boiler? I may be one of the few that has an external temperature sensor on my evohome set up.

  10. #30
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    Quote Originally Posted by bruce_miranda View Post
    Any idea if the external temperature sensor data is used by the evohome to pass on to the OT bridge and then on to the boiler? I may be one of the few that has an external temperature sensor on my evohome set up.
    Pretty sure if you search back in other threads you'll find that Rameses or top brake have said that the external temperature sensor is not used by the Evohome algorithm at all - Eg its reading has no impact on the behaviour of things such as optimum start or control of boiler flow temperature.

    The temperature reading is purely informational on the screen, and as other have noted the ability to even bind an outdoor temperature sensor was removed in the wifi version of the controller.

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