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Nest 3rd Gen & Ideal Logic+ System 30

Discussion in 'Plumbing and Central Heating' started by PhilT246, 5 Dec 2019.

  1. PhilT246

    PhilT246

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    First post for me and chose to ask here as you seem to be a helpful bunch of people with queries like mine.

    Here's the state of play...
    1. CH and HW was controlled with a timer. S Plan with HW tank.
    2. I installed the Nest based on several days of research, tracing wiring and input from someone in the trade.
    3. I have emailed Ideal Technical help, but no response from them.
    4. I did it myself because it's a busy time of year for the trade and I'm not daft when it comes to this kind of stuff.

    Problem : HW (Tank) only being heated when CH (OpenTherm) has the boiler running/heating. Nest HW boost doesn't work.

    Here's my current wiring:

    upload_2019-12-5_10-12-6.png

    CH : Controlled by Nest which opens the CH valve via connector 3 and controls the boiler temp via OpenTherm. Valve orange wire not connected to boiler as OpenTherm replaces it.
    HW : Controlled by Nest which opens the HW valve via connector 6 and I assumed would control the boiler temp via OpenTherm (i.e. Call for higher boiler temp when HW is ON). From a lot of reading it seems like a common issue that people have where the HW is only getting to CH temp. Valve orange wire not connected to boiler as OpenTherm replaces it.

    Possible Solutions:

    1. Don't use OpenTherm and rewire to do traditional call for heat on SL1, as it was with the original timer.

    upload_2019-12-5_10-20-49.png

    2. Don't use OpenTherm and rewire to do traditional call for CH on SL1 and HW on SL2. In the email to Ideal I asked what the difference between SL1 and SL2 is; i.e. Does the boiler behave differently when there is a call for CH or HW. Basically the same as option 1 but leverages any benefit of doing separate CH/HW calls if the boiler behaves differently.

    3. Use OpenTherm for CH and wire up HW valve orange to SL2 for HW. It's not clear to me if this is supported by the boiler and what would happen if both the CH (OpenTherm) and HW (Call for heat) are asking the boiler for heat at the same time. Logically, I would expect that if there is a call for HW, then the boiler heats above the OpenTherm CH temp (I asked Ideal in the same email mentioned above).

    upload_2019-12-5_10-33-43.png

    4. Other ideas...?

    Thanks in advance,

    Phil.
     
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  3. stem

    stem

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    AFAIK OpenTherm doesn't work with S-Plan, I found the same scenario as you when I tried it once. Ever since I have always used the two motorised valves to control the boiler directly and not used OT.

    I guess that unlike the Nest Roomstat, the hot water cylinder thermostat being a simple on/off switch outside of the Nest control can't provide the required signals for OT.

    I would love a more definite answer from Nest. In their literature they only ever show OT connected to boilers that aren't connected to a stored hot water system, but as you say....

    Maybe someone else can enlighten us.
     
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  4. Jajodu

    Jajodu

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    As far as I can recall open therm control will not work with hot water cylinders.
     
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  5. PhilT246

    PhilT246

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    Thanks for the quick replies. Here (https://support.google.com/googlenest/answer/9259578?hl=en-GB) it says the 3rd gen Nest will work with "Separate hot water tank with On/Off control". The Nest also allows you to set CH == OpenTherm and HW == ON/Off. This lends itself to solution 3, if the boiler supports CH OT and HW switched live. I feel like the answer needs to come from the boiler manufacturer, Ideal in this case. Of course the documentation doesn't cover this, nor does it even cover the use of "SL 2 IN" for DHW... https://idealboilers.com/uploads/documents/logic-system-s-installation-and-servicing.pdf
     
  6. fezster

    fezster

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    The nest thermostat does have an anti legionella program, so it has obviously been designed with the hot water tank in mind to a greater degree than just bolting it on.

    However, as above, whether it is smart enough to boost the flow temperature when the HW zone is on, you'd have to just try and see. Should be simple enough to test.
     
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  7. PhilT246

    PhilT246

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    I managed to speak to someone at Ideal today who relayed answers to my questions:

    1. What's the difference between SL1 In and SL2 IN? Answer : There is no difference.
    2. Can OpenTherm be used for CH and SL2 IN for HW? Answer : Not recommended as it might confuse the boiler.
    3. Why do you supply a system boiler with OpenTherm if it can't be used with HW? Answer : I'm afraid all the technicians are busy on other calls....

    Apparently I can't speak to a technician because I'm not a Gas Safe engineer. I'll have to call back to get a clear answer on question 3....
     
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  8. PhilT246

    PhilT246

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    In the meantime I have rewired so that it's not using OpenTherm. CH and HW call for heat are going to SL 1 and SL 2. I'm pretty sure that the call for HW is causing the boiler to heat the water to a higher temp than the the call for CH. I'll do some more testing as I don't feel confident in the responses I got from Ideal.
     
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  9. Jajodu

    Jajodu

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    As I understand it it's not a case of using both but rather use one or the other, the methods of control are mutually exclusive.
    A simple on/off is what you need for hot water. OT uses a completely different method whereby the water is heated to a constant low temp.
     
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  11. stem

    stem

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    My thoughts would be the same as yours. The boiler should use OT for the heating and SL for the hot water. It shouldn't be rocket science for a boiler manufacturer to do this. When only the heating is on the boiler is controlled by OT, when a digital signal is received into SL, the boiler will know that the hot water is required, override the OT and function as normal just whilst the water is being heated. Once the hot water demand signal disappears it could switch back to OT.

    I get the impression though that OT may have had its day, particularly in the UK. New domestic systems now have to be zoned [living areas / sleeping areas] Apart from the difficulty of having two OT connections to terminate, it would be impossible for two OT thermostats requesting different water temperatures to get that from the same boiler anyway!

    How good is OT in practice? I don't know. When I get the chance to speak to someone who has has a thermostat with OT installed on an existing system I will ask what they think. Usually they just mention all the app features and that the thermostat looks nice on their wall. No one has mentioned that they have they noticed any improvements in comfort or running costs, unless I specifically ask. Only one guy who is a sound engineer, with a technical interest, commented that it does modulate the boiler temperature and he thinks there may be a slight energy saving, but wasn't too sure.
     
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  12. fezster

    fezster

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    To be fair to the boiler manufacturer, shouldn't the company developing OT controls for both heating and HW design it so that it is capable of working with both? Evohome manages to do it just fine.

    I also agree that the 230V input could override the OT controls - seems like a simple thing to do.
     
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  13. PhilT246

    PhilT246

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    I think I've found a potential way to achieve OT CH and SL HW (Excuse the crude diagram)

    upload_2019-12-6_9-18-3.png

    By using a couple of relays; when there is a call for HW, the relays close the OpenTherm loop at the boiler (Boiler then thinks there is no OpenTherm control). When HW is off, OpenTherm is connected to the Nest. The challenge here is that the Nest will probably complain that it's lost the OpenTherm connection.

    Whilst researching how to replace the OpenTherm "no contact" with something more intelligent I came across this thread (https://www.diynot.com/diy/threads/intergas-opentherm-and-nest.478222/). Seems like Dan Robinson may have gotten his head around this....

    Then, thinking this through a little further and wondering why I'm designing a workaround, I think... When the NEST has HW ON, shouldn't it speak to the boiler and tell it to run at the correct temp for HW and override the temp for CH.... Maybe I need to knock on Nest's door and see what they have to say, after all the Nest still maintains the SW functionality when OpenTherm is connected, so we must assume that if it wants to control the valves, it must also set the appropriate boiler temp to heat the hot water....
     
  14. fezster

    fezster

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    Yes! As I said above:

    :=)
     
  15. PhilT246

    PhilT246

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    Sorry fezster. I did try that as it was the original configuration. It didn't boost when then HW was on, hence why I went down the rabbit hole
     
  16. ianmcd

    ianmcd

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    Cant remember the post a short time back and @shambolic says that Ideal use the HW off terminal to tell the boiler to go OT, I have tagged him for you , hopefully he can explain it for you
     
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  17. stem

    stem

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    Interesting definition of OpenTherm from Wikipedia.

    OpenTherm (OT) is a standard communications protocol used in central heating systems for the communication between a central heating boiler and a thermostatic controller. As a standard, OpenTherm is independent of any single manufacturer. A controller from manufacturer A can in principle be used to control a boiler from manufacturer B. However, OpenTherm controllers and boilers do not in fact always work properly together. The OpenTherm standard comprises a number of optional features and some devices may include manufacturer-specific features. The presence or absence of such features may impair compatibility with other OpenTherm devices.

    So OT connections are the same and 'standard' yet different :confused:
     
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