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Thread: Opentherm with Evohome

  1. #11
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    Quote Originally Posted by sandyman View Post
    best I can say is subjectively so far (having run the same system , full evohome , new condensing boiler, with both control methods) is that so far it subjectively "smoother" with opentherm. I will know more once it gets properly cold as my OT was only fully sorted around april.
    Objectively, I have "loop" monitoring on my gas meter (don't have a smart one yet) and I'll gladly share my consumption data with you in the spring (of course, it may well vary more due to the severity or otherwise of the winter, time spent away from home etc, than anything due to the two control methods ).

    as DBMandrake said, you can get the system installed with one control type (if that helps you with your choice of boiler and installer) then swap it to the other if/when you feel like it - fairly simple change, wiring goes to and from the same place just different terminals on the boiler and a rebind in evohome.
    Thanks. A further question if I may. Am I correct in thinking that with a 'S' plan configuration (2 motorised valves), an open therm connection will control both HW and CH demands on a system boiler? I assume that the CH motorised valve has to be removed/or locked open, and the HW motorised valve remains connected to its BDR and HW sensor? I am just trying to get a feel for the amount of re-wiring involved.

  2. #12
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    more or less yes. If the HW zone or any CH zone in evo is demanding, then the evo controller will tell the OT bridge to call for heat from boiler. If you have a regular BDR controlling the boiler, just replace "OT bridge" with "boiler BDR" in the previous sentence. Hence why the change from one to the other is not that much of a big deal.

    HOWEVER in my case (viessman boiler) the experimental evidence shows that the OT call for heat that results from DHW zone demand, doesn't persuade the boiler to run hot enough to deliver sufficiently hot flow water to recharge the cylinder. whether this is honeywells, viesmanns, or the OT protocols fault is beyond my care/ ability to figure out. to make it work, it needs a parallel connection directly from the HW zone BDR to the HW input on the boiler.

    as for zone valves leave your CH one open (you could replace the head with a "normally open" as I have done , for reasons I shall explain) disconnecting anything wired to it. The HW one, gets wired to the hot water kit BDR, think this is well documented in evohome docs.

    keeping the CH zone valve in the plumbing gives you the option to change the head to a "normally open" as I have - I then use the HW BDR to simultaneously OPEN the HW zone valve and CLOSE the CH zone valve when HW zone calls for heat. Thus recharging the cylinder in no time (250 litres x 10 degrees in 10-15 mins) as it forces the flow exclusively through the cylinder. a.ka. hot water priority.

    honeywell do/did advise to remove CH zone valve from the plumbing as its theoretically unnecessary in a full evo system - but if you want to do a quick/simple flow diversion based on electrical switching , you need it still in there.

  3. #13
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    I have spoken to Viessmann again and they are clear that only the 100 boiler series will work with Opentherm. They sent me a helpful diagram showing a two terminal connection and the removal of a terminal jumper lead. Intergas tell me that they are not aware of any Opentherm/Evohome compatibility issues with their boilers and say that Evohome have visited to test the HW bridge. That said, other forums are reporting Opentherm/Intergas issues with multi HR92s.

    @Sandyman. Do you have a 'S' or 'Y' configuration? If HW is on a BDR, then presumably HW heat Demand over-rides the Opentherm CH requirement (i.e., max boiler temp) until such time as HW has reached sensor temp - or am I misreading this part of the Opentherm/HW BDR equation?

    Sorry, to be a pain.

  4. #14
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    Quote Originally Posted by HenGus View Post
    I have spoken to Viessmann again and they are clear that only the 100 boiler series will work with Opentherm. They sent me a helpful diagram showing a two terminal connection and the removal of a terminal jumper lead. Intergas tell me that they are not aware of any Opentherm/Evohome compatibility issues with their boilers and say that Evohome have visited to test the HW bridge. That said, other forums are reporting Opentherm/Intergas issues with multi HR92s.

    @Sandyman. Do you have a 'S' or 'Y' configuration? If HW is on a BDR, then presumably HW heat Demand over-rides the Opentherm CH requirement (i.e., max boiler temp) until such time as HW has reached sensor temp - or am I misreading this part of the Opentherm/HW BDR equation?

    Sorry, to be a pain.
    I used to have an S ( S plus actually as up and downstairs were on separate CH zones). I now technically have W (not Y, I had to read up on the difference, I kept getting challenged about it on DIYNOT !) because the wiring to my CH and HW zone valves has been done such that one is always open and the other is always closed (they swap, depending on state of the system) . This make sure that the 79 degree flow from the boiler's "HW mode" recharges the cylinder as fast and efficiently as poss as it doesn't let any of it go to the rads. The HW heat demand does indeed override opentherm CH requirement by setting max boiler temp until HW has reached sensor temp.

    if you are going viessman 100, make sure you get the extra hot water kit. My installer didn't know about it initially (hence all the grief I had).
    its a little box separate to the boiler. It takes 230v in (taken from the output switched live of your HW zone BDR) and gives a low voltage DC out, which you wire to terminal pair 5 of the boiler (red spade terminals in the wiring loom, not in the terminal blocks). This puts the boiler in "turbo hot water mode" - a "tap" symbol appears on the screen, and it sets a target flow temp of 79 degrees. works a treat.

    If you don't fit that kit you are hoping that opentherm will signal to the boiler to say "HW demand , more power please" and that doesn't seem to work at all, or in fact perhaps doesn't even exist in the opentherm specs.

  5. #15
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    Quote Originally Posted by sandyman View Post

    if you are going viessman 100, make sure you get the extra hot water kit.

    If you don't fit that kit you are hoping that opentherm will signal to the boiler to say "HW demand , more power please" and that doesn't seem to work at all, or in fact perhaps doesn't even exist in the opentherm specs.
    Thanks - scratches head! Are you saying that the Evohome HW kit will not do the same thing; i.e., sensor demands heat which via Evohome sends demand to BDR; motorised valve opens and boiler comes on. Is this problem unique to Viessmann? Opentherm is looking less attractive than it was.

  6. #16
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    Quote Originally Posted by HenGus View Post
    Thanks - scratches head! Are you saying that the Evohome HW kit will not do the same thing; i.e., sensor demands heat which via Evohome sends demand to BDR; motorised valve opens and boiler comes on. Is this problem unique to Viessmann? Opentherm is looking less attractive than it was.
    right up to a point. the evo hot water kit is a necessary part of the system regardless of how you do the rest: HW sensor demands , HW zone valve opens, sends demand to boiler, boiler comes on. question then is what is the flow temp of the boiler?

    DHW needs a really hot flow (70 plus) to recharge the cylinder quickly to 60. My experimental evidence shows that (at least for viessman) the boiler tries to keep flow and return 20 degrees apart - but if the return gets too warm, it shuts the boiler down, there is an inbuilt limiter. what then happens in practice is that your cylinder with a target temp of 60 degrees never gets to target because the boiler keeps shutting off. it interprets return@ close to 60 degrees (which is what it will be , as your cylinder approaches 60) as "gosh the system is too hot, I better shut down". So the viessman hot water kit is to be used AS WELL AS the honeywell one - they do different things. The honeywell evo one controls the zone valve and talks to the controller. The viessman one is there purely to tell the boiler "the demand now is for hot water, so ignore your "return too hot" limiter, and just give it full whack until told otherwise".

    I think its almost a generic problem with condensing / low temp target system boilers (certainly for viessman 100) and DHW. You do not get any different behaviour with an evo boiler demand BDR instead of an OT bridge (I know, I have had both on the exact same boiler and system).

    I had hoped that the evohome OT bridge / OT protocol would have some additional smarts that could tell the boiler "actually the demand now is HW, please do the right thing", but it appears not.

    others on here may have different experience with their hot water cylinder reheat behaviour if they have different system boilers (might have a different behaviour with high return temp), I can only speak for the way my viessman 100 behaves.

    viessman cylinder demand terminal box wiring details http://www.viessmanndirect.co.uk/fil...inal%20Box.pdf
    It doesn't cost a lot (compared to new boiler and all the work!) think it was order of 100 quid.

    oh and there's one more option with viessman if you have the option to change your plumbing - 4 pipe. separate flow/return circuits for CH and HW, can run them at different temps thus achieving the same as I have described but on separate circuits. I'm not the expert, just read about it. This was not an option for me due to the physical layout requirements (can you run more flow/returns in the right place?), talk to your installer..
    Last edited by sandyman; 15th October 2016 at 08:08 PM. Reason: added viessman kit link

  7. #17
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    @sandyman Thanks again.

  8. #18
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    Hi,

    others on here may have different experience with their hot water cylinder reheat behaviour if they have different system boilers (might have a different behaviour with high return temp), I can only speak for the way my viessman 100 behaves.
    Just to confirm that I've had exactly the same problem. I've previously posted about my Glow Worm Ultracom system boiler which doesn't have any way of separating the DHW demand from the CH demand when connecting an Evohome system. It took me quite a long time to work out that the boiler was modulating its flow temperature which prevented the DHW hitting its target temperature. The only solution for me was to set the CH target temperature high enough on the boiler to meet the DHW requirement. Prior to my Evohome install I used a Glow Worm external controls module for the boiler which did separate the two types of demand, but this module could not be linked to the Evohome system .

  9. #19
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    My own Vaillant 438 ecotech open vented boiler works ok. it is supposed to be capable of handling two different temperatures but only when used with Vaillant's own controls. So currently my HW knob on the boiler does nothing.
    Using the Evohome and the Opentherm bridge, the OT just kicks the boiler into a 90C target when there is HW demand. My boiler is set to a 70C max and that's as far as it will get to, even when the OT is telling it to go to 90C. This seems to be enough to recharge my unvented cylinder.

  10. #20
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    Forgetting about Evohome for the moment, how then does a condensing boiler work with a 'S' plan system? Presumably, they all have to have some form of HW priority built into them.

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