Help with Installing Nest Smart thermostat

Discussion in 'Plumbing and Central Heating' started by Dazhazard, 5 Feb 2021.

  1. Dazhazard

    Dazhazard

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    Hi Guys

    Trying to install my first thermostat. I know enough about electrics to not kill myself...thats about it.

    Gone to install the Nest heat link and realised I have 2 'hubs' connected to my combi boiler. Both Drayton. One the controls and one is a Digistat SCR.
    I'm thinking I replace the controls but what do I do with the Digistat (right on the picture)?

    Much love and appreciation in advance
    Can take further pics with covers off to see wiring of needed.

    Cheers,
    Daz
    20210205_210509.jpg
     
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  3. MeldrewsMate

    MeldrewsMate

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    The LP711 is a timer, nothing to do with any 'hub'.
    The SCR is a radio frequency (RF) receiver from a remote room thermostat.

    My best guess is that mains is presently switched by the LP711 to the Digistat SCR when the timing schedule calls for heat. The SCR takes that signal and further switches it to the boiler when the room thermostat calls for heat.

    Thus you may remove both of these devices once you have identified the mains power L, N, and E, and the switched live (SL) going to the boiler. The Nest Heatlink will replace both, and it will switch live (SL) to the boiler when both the Nest schedule calls for heat and the Nest senses the room temperature is lower than that desired.

    If the Nest link is anything live the Version 3 that I last fitted a couple of years ago you'll find the terminals are small and fiddly. Other than that they're great controls.

    MM
     
  4. muggles

    muggles

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    You need to find your wiring centre, which all this is connected to. You'll also need a stand for your Nest if you've not bought one already
     
  5. ericmark

    ericmark

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    Step one have a look as the boiler installation instructions and see if it has option for OpenTherm, if it has better wiring to use it, if not as @MeldrewsMate says not much room inside Nest Gen 3, I put a wiring centre below it so one terminal one wire. Likely it will replace the LP711 and the SCR with wire into LP711 so can go completely.

    But you have three options really, unlike the LP711 you don't need to set Nest at the Heat Link, so it can be mounted where not easy to read and alter as there is nothing to read and alter, so could go right by the boiler or in airing cupboard, so option one is start from scratch and mount near what it controls.

    Second option is mount where LP711 is and used a USB cradle for the thermostat bit.

    Third option is hard wire the thermostat bit, I did the latter, still no sure I did right thing, but boiler was on another floor and the pair of wires was already installed.

    From what I remember of the LP711 it can be configured as 16 option or 10 option with a switch and/or slider, so can be used with C Plan or Y Plan or S Plan, Nest Gen 3 can be used with C Plan that is what I have, but there is no circuit diagram on how to wire as C Plan in the little book that comes with it, so before starting you need to work out what you have.

    C Plan often no motorised valves or tank thermostats on the DHW.
    Y Plan a three port motorised valve.
    S Plan 2 two port motorised valves.
    Combi boiler no control of DHW by Nest the boiler does it.

    All central heating is a compromise, but some thermostats are better suited for some systems, There are three really good ones, the Honeywell EvoHome, Drayton Wiser, and Tado the latter I know very little as to how it works, but it seems the Drayton has the best programmable TRV head which works out how the rooms heats and remembers it, so is the fastest to heat room without over shooting. But non are designed to work with zone valves.

    EPH however can work with OpenTherm and zone valves with a master slave configuration, but it does no it seems link to TRV heads.

    Nest is an odd one, it has got OpenTherm but will not with with either zone valves or TRV heads, it is a stand alone only, a very good stand alone, but does not work with TRV heads.

    Hive works with TRV heads but does not have OpenTherm, it does however have a configuration to work with old C Plan.

    Worcester Bosch is like the Apple Mac of the boiler world, a good boiler but with not work with common third party thermostats, having no OpenTherm but does have it's own e-bus system, but Hive system works well with Worcester Bosch allowing the return water temperature to control the boiler.

    But the house also matters, my old house open plan Nest was great, mounted by the arch between rooms worked A1. This house really do need TRV heads which link to wall thermostat, there is no room on centre level without a door to the outside, and only the hall and dinning room have no alternative heating, I made a mistake fitting Nest Gen 3, I was told it worked with the Energenie MiHome TRV heads, and in a way it does, but not in a way which can be used.

    Non of the systems are cheap, as to if worth have each room with independent control is worth the cost I don't know, I have 9 programmable TRV heads, and they do stop rooms heating up when not required, but they can't heat a room if boiler not running, so in my case the hall is room with Nest in and it and the TRV plus lock shield setting in the hall controls whole house, when I moved in central heating was a mess, so never used central heating without Nest, however my hall is too slow to cool, so key is to ensure it does not over shoot.

    Nest with all its cleaver algorithms can't work out if hall doors are open or closed, so it can never learn how to control as it does not know how many of the 5 doors into hall are open or closed. And with an open fire and a pair of double doors to outside in living room no good putting thermostat there. Dinning room also sliding doors to outside, and kitchen door to outside and a cooker. So this house really needed linked TRV heads.

    Also live style, I am retired and have not left the house since March, other than medical appointments. If my wife and I were going out to work, then the Nest geofencing would be really good, however since Nest also has a built in PIR it needs to be in an area where it will detect movement, so hall is ideal as all rooms lead into the hall. This may not however work out with all homes, in my sons old house where the thermostat was mounted it did not register there were people in the house.
     
  6. CBW

    CBW

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    I doubt he has any plans, as he’s already confirmed (I know folk sometimes get it wrong) that he has a Combi Boiler. LP711 is a single channel, so heating only and only one scr so unlikely to be x2 2 port valves. I also doubt there’ll be a wiring center for this set-up.
     
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  7. MJN

    MJN

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    Terrible aren't they - small recessed terminals, barely legible markings, hardly any room for wiring. The whole Heatlink seems of unnecessarily poor design eg CH/HW demand indicators but they're only active when you use the manual override - they're not used to indicate automatic demand which I would find actually useful.

    The jury's still out for me on that one, although I have only had it for a couple of months. I'm finding the temperature regulation/stability to be pretty poor (compared to the previous CM927 at least) and accessing the (supposedly) raw sensor date via the API reveals it has some serious lagging/damping issues so it's no wonder it can overshoot/undershoot. I also don't like the non-configurable 1-minute minimum 'on' time as by the time the zone valve has opened and boiler performed its pre-purging the boiler barely has chance for any meaningful burn time before it might be asked to shut off again so not great for wear and heat wastage.

    It looks great though, and the geofencing seems to be working well, but that's not much to shout about for such an expensive device.

    I don't have an Opentherm boiler to confirm this but it was my understanding that even when enabled for Opentherm the Heatlink still activates the zone valve outputs. Or have I misunderstood what you were saying on that particular point? I hope it works like this as that was one of the reasons I went for the stat in advance of eventually fitting an Opentherm boiler.
     
  8. ianmcd

    ianmcd

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    Forget the long winded rubbish, your job couldnt be easier, the nest simply replaces the Drayton Scr , and the LP711 is removed show pics of the wiring in each, you may or may not have to make a link when you remove the LP711
     
    Last edited: 6 Feb 2021
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  9. ericmark

    ericmark

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    You can hard wire a zone valve to open/close with Nest, You can't wireless control a zone valve. A electronic head on a TRV makes it a zone valve, all the other makes seem to allow either the wall thermostat to control the TRV head or the head to control the wall thermostat, it said on the instruction the Nest wall thermostat can control Energenie TRV heads, but it only works when using the App to change temperature with, it temperature change is be schedule change or direct on the Nest ring, then the Energenie TRV heads fail to follow.

    Ringing the help line I found Nest support has been withdrawn.

    However in my house the TRV head in winter always shows about 3°C lower than the wall thermostat, shows same in summer so just the way the air does not mix, so having the TRV follow the wall thermostat would mean the TRV is always fully open, so no point in having it.
     
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  11. MJN

    MJN

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    Ah, see what you mean now. I was only thinking of hardwired valves (and when used with OT specifically).

    I can't help but feel that, feature/development wise at least, Nest is dead in the water. Doesn't seem to have been much - if any - innovation for quite some time now. I see there's a new '4th gen' stat in the US but it's more of a pared back version of the 3rd gen with learning and some aspects of physical quality removed. Shame that Google bought out Nest really and stifled its innovative approach (I guess that's exactly why they did do it).
     
    Last edited: 6 Feb 2021
  12. mgmaestro

    mgmaestro

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    Just a thought on nest wiring - as it gets its L N E from boiler supply why does it need live supply and neutral for boiler control, surely switched live only needs to be connected to terminal 3 call for heat
     
  13. ianmcd

    ianmcd

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    No nest wiring terminals can be used for different control voltages, sometimes referred to as voltage free, which isnt actually true, it is just simply a switch, which switches the required voltage, some dual channel units have internal links so will work with what you suggest but Nest doesnt
     
  14. mgmaestro

    mgmaestro

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    Thanks, that's helpful. So presumably terminal 1 is ignored and 2 and 3 are wired to boiler live and live return for the nest to close a circuit and switch the boiler on.

    Mick
     
  15. ianmcd

    ianmcd

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    yes terminals 2 & 3 on the nest swiching the boiler, not all boilers are the same
     
  16. mgmaestro

    mgmaestro

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    Many thanks that's explained it for me.

    Mick
     
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