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Thread: Honeywell evohome and OpenTherm integration

  1. #21
    Automated Home Legend paulockenden's Avatar
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    Sorry, perhaps I wasn't clear. Evohome doesn't MEASURE flow temperature.

    With the OT bridge it can send a target temperature to the boiler (although I suspect it's sending a percentage output - that's an option too).

    But Evohome itself only measures air temperature in the zones.

  2. #22
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    From what I was told whilst troubleshooting my setup, EvoHome (controller) doesn't deal with/ know anything about flow/return temperatures.

    EvoHome controller deals in demand % aggregated across all HR92 based zones. Any zone requesting 100% gets the highest ch flow temp the boiler is set to (or not in my case!)

    The R8810 OT Bridge is the system component responsible for deriving set flow temps based on demand % input. This is not firmware upgradble (realistically, not technically)

    Suspect that's why the Viessmann problem is not realistically fixable in EvoHome side.
    Last edited by StephenC; 30th July 2017 at 03:09 PM.

  3. #23
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    Thanks for the replies.

    If Evohome & OpenTherm doesn't monitor flow temperature directly, I don't see how it can ever aim to get the boiler into condensing mode.

    StephenC: I'm fairly sure your boiler isn't the problem here. There are plenty of OpenTherm boilers that surrender control to the outboard controller because that's how the system is supposed to work. OpenTherm is all about closely handling flow temperature and making subtle alterations as the need arises.
    Forum member HenGus is fortunate that his Atag boiler lets him limit the maximum flow temperature, but even so and going by a recent post of his, concerns remain about how long his system takes to settle into condensing mode.

    BTW: Has your boiler been 'range rated', that is, had its maximum (kW) power output reduced to say 50% of max?

  4. #24
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    @Blowlamp,

    Yes tested reducing output. This just takes longer to reach silly CH flow temp! Doesn't affect temperatures though.

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    Quote Originally Posted by blowlamp View Post
    If Evohome & OpenTherm doesn't monitor flow temperature directly, I don't see how it can ever aim to get the boiler into condensing mode.

    StephenC: I'm fairly sure your boiler isn't the problem here. There are plenty of OpenTherm boilers that surrender control to the outboard controller because that's how the system is supposed to work. OpenTherm is all about closely handling flow temperature and making subtle alterations as the need arises.
    Forum member HenGus is fortunate that his Atag boiler lets him limit the maximum flow temperature, but even so and going by a recent post of his, concerns remain about how long his system takes to settle into condensing mode.
    I am confused here? Why would it take time to 'settle' into condensing mode?

    If the Tmax is set to 75 degrees (or lower if system design allows this) and the system has been designed for a 20 degree Celsius flow/return differential (pump correctly sized & adjusted and radiators correctly sized and balanced) the boiler will be at condensing temperatures naturally?

    I think you are confusing the thermostat actually setting a reduced Tmax based on its calculated 'load' and the ability of the boiler to use latent flue gas energy by having a return temperature of 55 degrees Celsius and below?

    My point is even with a flow temp of 75 degrees the boiler would still 'condense' if the system was setup correctly.

    Have you guys used an OpenTherm monitor to see what is actually going on? I think it is poor of Viessmann to try and put this back on Honeywell? There is no requirement in the OpenTherm specification for the controls to set a limit for Tmax. If an installer has designed a system for a 70 degree Celsius flow temp, not being able to put the 'brakes' on Tmax at the boiler end is an oversight or maybe even arrogance on Viessmann's part.

    Intergas have the ability to adjust Tmax at the boiler (as does any truly good OpenTherm compatible boiler).
    Last edited by The EVOHOME Shop; 30th July 2017 at 07:46 PM.

  6. #26
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    Quote Originally Posted by The EVOHOME Shop View Post
    I am confused here? Why would it take time to 'settle' into condensing mode?

    If the Tmax is set to 75 degrees (or lower if system design allows this) and the system has been designed for a 20 degree Celsius flow/return differential (pump correctly sized & adjusted and radiators correctly sized and balanced) the boiler will be at condensing temperatures naturally?

    I think you are confusing the thermostat actually setting a reduced Tmax based on its calculated 'load' and the ability of the boiler to use latent flue gas energy by having a return temperature of 55 degrees Celsius and below?

    My point is even with a flow temp of 75 degrees the boiler would still 'condense' if the system was setup correctly.

    Have you guys used an OpenTherm monitor to see what is actually going on? I think it is poor of Viessmann to try and put this back on Honeywell? There is no requirement in the OpenTherm specification for the controls to set a limit for Tmax. If an installer has designed a system for a 70 degree Celsius flow temp, not being able to put the 'brakes' on Tmax at the boiler end is an oversight or maybe even arrogance on Viessmann's part.

    Intergas have the ability to adjust Tmax at the boiler (as does any truly good OpenTherm compatible boiler).

    In previous posts of this topic HenGus mentioned a return temperature of 65C in relation to his system and of being slow to get into condensing mode. I can't comment further about his setup as I've never seen it, but I am aware that a boiler will condense with a return temperature of up to ~ 55C, although there is the point that a system is even more efficient at lower flow & return temperatures and less stressed too. Others are concerned with unnecessarily high flow temperatures and room temperature overshoots. Isn't this what OpenTherm is there to control?

    My neighbour has a condensing boiler and all too often it's impossible to see his garage due to the huge plume of condensate emitted by it - I presume due to being at the higher end of the condensing temperature range. I can proudly state that my weather compensated Intergas/OpenTherm iSense combo has the smallest plume in the area thanks to its consistantly low flow temps.

    My Intergas combi has relinquished control of hot water temperature and weather compensation to the offboard OpenTherm controller (Remeha iSense) which incidentally also controls the maximum CH flow temperature, so why should Viessmann be considered negatively for requiring similar? I'd have thought anyone with their boiler in the loft or garage would appreciate the convenience.

  7. #27
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    Quote Originally Posted by blowlamp View Post
    My neighbour has a condensing boiler and all too often it's impossible to see his garage due to the huge plume of condensate emitted by it - I presume due to being at the higher end of the condensing temperature range. I can proudly state that my weather compensated Intergas/OpenTherm iSense combo has the smallest plume in the area thanks to its consistantly low flow temps.

    My Intergas combi has relinquished control of hot water temperature and weather compensation to the offboard OpenTherm controller (Remeha iSense) which incidentally also controls the maximum CH flow temperature, so why should Viessmann be considered negatively for requiring similar? I'd have thought anyone with their boiler in the loft or garage would appreciate the convenience.
    I am not sure that we are comparing like with like. As I understand it, your system is single zone. My system comprises 12 zones with 19 electronic TRVs (HR92s) and, as others have posted:

    Quote: ..... with OpenTherm, the system still only starts to ramp down demands when all zones are within 1.5 degrees of their setpoint? Unquote

    It would seem that Evohome was designed to get individual zones up to their target temperatures ASAP: hence, the fact that the boiler cranks up to max flow temperature. My observation - over a very short period of time - was that the boiler didn't drop back into a condensing return flow temperature as quickly as I believed it would. What I did observe was the boiler modulating downwards - if noise and fan speed are anything to go by - followed by a slow drop in flow and return temperatures. That said, a 'test' over 3 weeks in April with very few zones demanding heat wasn't particularly scientific.

  8. #28
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    Quote Originally Posted by The EVOHOME Shop View Post
    I am confused here? Why would it take time to 'settle' into condensing mode?

    If the Tmax is set to 75 degrees (or lower if system design allows this) and the system has been designed for a 20 degree Celsius flow/return differential (pump correctly sized & adjusted and radiators correctly sized and balanced) the boiler will be at condensing temperatures naturally?

    I think you are confusing the thermostat actually setting a reduced Tmax based on its calculated 'load' and the ability of the boiler to use latent flue gas energy by having a return temperature of 55 degrees Celsius and below?

    My point is even with a flow temp of 75 degrees the boiler would still 'condense' if the system was setup correctly.

    Have you guys used an OpenTherm monitor to see what is actually going on? I think it is poor of Viessmann to try and put this back on Honeywell? There is no requirement in the OpenTherm specification for the controls to set a limit for Tmax. If an installer has designed a system for a 70 degree Celsius flow temp, not being able to put the 'brakes' on Tmax at the boiler end is an oversight or maybe even arrogance on Viessmann's part.

    Intergas have the ability to adjust Tmax at the boiler (as does any truly good OpenTherm compatible boiler).
    It may well be that my existing heating system isn't properly sized or balanced to work with opentherm; however, I suggest that this is going to be the case for most consumers that buy Evohome, Nest, Tado Atag One etc.

    What type of monitor are you using to monitor your opentherm connection?

  9. #29
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    Quote Originally Posted by HenGus View Post
    I am not sure that we are comparing like with like. As I understand it, your system is single zone. My system comprises 12 zones with 19 electronic TRVs (HR92s) and, as others have posted:

    Quote: ..... with OpenTherm, the system still only starts to ramp down demands when all zones are within 1.5 degrees of their setpoint? Unquote

    It would seem that Evohome was designed to get individual zones up to their target temperatures ASAP: hence, the fact that the boiler cranks up to max flow temperature. My observation - over a very short period of time - was that the boiler didn't drop back into a condensing return flow temperature as quickly as I believed it would. What I did observe was the boiler modulating downwards - if noise and fan speed are anything to go by - followed by a slow drop in flow and return temperatures. That said, a 'test' over 3 weeks in April with very few zones demanding heat wasn't particularly scientific.

    All my radiators are fitted with electronic TRVs but aren't capable of activating the boiler to call for heat, so in that respect my setup falls short of what is offered by Evohome and is indeed different from yours. I can't grumble too much though as once the heating is on, I feel the zones are well controlled.

    Despite these differences, both our boilers see just one zone, as presented to them by their controller/thermostat.

    As my system is currently weather compensated, it follows that flow temperature will be guided by the heating curve in accordance with the outside temperature (plus a few more variables) and therefore zones can drop in and out as they need to whilst the boiler modulates flow temperature through OpenTherm to suit the circumstances.

    The following is taken from the iSense manual and details the strategy of room temperature control without weather compensation and so is more comparable to an Evohome OpenTherm setup.

    RTC: (Modulating control) Room control The control measures the room temperature in the reference room (the room where the thermostat is located). The required flow temperature based on an intelligent control system is now calculated. The boiler works by modulating. It controls the output on the basis of the flow temperature and return temperature of the water. This allows it to operate as efficiently as possible. With as constant a water temperature as possible.

    In this instance, the reference room would probably be the coolest and thus have the largest heat requirement of the property. This, in my opinion, is broadly similar to that of the aggregated demand from an Evohome multi-zone system.

    What I pick up from the above, is that with iSense OpenTherm, the emphasis is on well controlled, steady flow/return temperatures and how that contrasts with Evohome's OpenTherm approach of running flat out up to the 1.5C proportional band and throttling back from there.
    Last edited by blowlamp; 1st August 2017 at 12:00 AM.

  10. #30
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    Quote Originally Posted by blowlamp View Post
    In previous posts of this topic HenGus mentioned a return temperature of 65C in relation to his system and of being slow to get into condensing mode. I can't comment further about his setup as I've never seen it, but I am aware that a boiler will condense with a return temperature of up to ~ 55C, although there is the point that a system is even more efficient at lower flow & return temperatures and less stressed too. Others are concerned with unnecessarily high flow temperatures and room temperature overshoots. Isn't this what OpenTherm is there to control?

    My Intergas combi has relinquished control of hot water temperature and weather compensation to the offboard OpenTherm controller (Remeha iSense) which incidentally also controls the maximum CH flow temperature, so why should Viessmann be considered negatively for requiring similar? I'd have thought anyone with their boiler in the loft or garage would appreciate the convenience.
    As Hengus has said, the tests he did were performed for 3 weeks during April and his heating system isn't necessarily correctly designed or balanced for a condensing boiler. In my opinion there are various ways OpenTherm can be used to achieve heating control strategies and it is understanding this aspect that I am most interested in.

    At the end of the day the OpenTherm Protocol Specification does not require the 'master' to do every OpenTherm command and because of this, the boiler manufacturers should make their boilers capable of modifying the settings that the controls are not required to do. By Viessmann not allowing these settings available, they are in essence forcing people to buy Viessmann's own controls or an OpenTherm controller that is capable of this. In my opinion this is damaging to the OpenTherm brand as people will start seeing 'incompatibilities' with some OpenTherm products and this is no fault of the manufacturer that has produced them.

    Quote Originally Posted by HenGus View Post
    It may well be that my existing heating system isn't properly sized or balanced to work with opentherm; however, I suggest that this is going to be the case for most consumers that buy Evohome, Nest, Tado Atag One etc.

    What type of monitor are you using to monitor your opentherm connection?
    OpenTherm doesn't require the system to be designed and correctly balanced for it. The heating system design specifies how the system should operate and condensing boilers require a 20 degree differential between flow and return. Most design for 75/55 but you can design the system at a lower temp (70/50 for instance) to aid condensing effect (you would need to increase radiator sizes to use lower temps). The fact is that the heating installer is the expert here and if they feel the new boiler is not capable of maintaining a designed temperature differential of 20 degrees because the radiators are undersized, they should advise that they are changed.

    I am using an OpenTherm monitor from here - http://otgw.tclcode.com/otmonitor.html

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