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Nest to replace very old Timer/Programmer & Thermostat

Discussion in 'Plumbing and Central Heating' started by The_Scotster, 8 Mar 2019.

  1. The_Scotster

    The_Scotster

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    Hi all,

    I'm going to replace my old timer/programmer with a nest thermostat. I understand I will have limitations due to how old my heating system is but I thought it was still better to have a more flexible control system even if the rest was as old as tea.

    From what I can ascertain I have what appears to be a C-Type installation but I think it may even pre-date that as I can't seem to find a control valve or a thermostat on the cylinder for it.

    The timer itself is under the stairs and the wiring appears pretty standard (picture attached) but the connector block inside raised my eyebrow. The thermostat is in the hall (always good to relay the hall temperature to heat your living rooms eh) but I can't seem to locate where this terminates. It's a 2-wire thermostat so seems like a very simple unit, I was thinking it's likely connected to a relay?

    Is it OK to just connect the wires at the thermostat end and plaster over it? I can only imagine that the wire terminates under the floor somewhere, although it may be behind my fire as it's a back boiler.

    Could someone have a look at my current wiring and just check that it's simple enough to swap over using the Nest Link box?

    I had a search around and this looks ideal for me but I'm not 100% sure on what goes where.

    https://www.diynot.com/diy/media/nest-gravity-hw-pumped-ch.101673/

    20190308_163915.jpg
     
  2. The_Scotster

    The_Scotster

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    Please excuse the archaic drawing, limited options so i had to stick with paint haha.

    Does this seem correct? I'm making a few assumptions but I think they are pretty reasonable. I'm positive nothing is getting from the controller to the thermostat directly so I think there must be a relay in there to control the pump. I'm guessing the connector block in behind the controller is connecting up the relay switch positive with the negative being bridged off through the thermostat.

    If my assumptions are correct would I be right in saying it's simple enough for me to completely ditch the block/relay/stat and just connect the pump directly to the neutral. That way the pump would run constant, without the thermostat controlling it, and thus the Nest would then take charge?

    Again, assuming all of this is correct am I right in saying that the diagram I posted earlier would be spot on and I can simply wire it as follows:

    Current N (Boiler Neutral) to N
    Current N (Assumed Relay) Removed
    Current N (Mains Neutral) to N
    Current Block (Pump Neutral) to N
    Current 1 (Mains Live) to L
    Current 3 (Boiler Live) to HW5 (Common)
    Current 4 (Pump Live) to Ch3 (C)
    Bridge Live & CH Com
    Bridge CH Com & HW6
    Bridge CH3 and HW0
    Earths as expected

    My understanding is that when HW is on, 5 & 6 will connect switching on the boiler.
    When CH is on, regardless of the HW position (thanks to the link between 3 & 4) the boiler will siwtch on.

    Nest Wiring.jpg
     
  3. The_Scotster

    The_Scotster

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    All done. Many thanks for all the help
     
  4. The_Scotster

    The_Scotster

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    Not to worry, it was maybe over your head :D
     
  5. The_Scotster

    The_Scotster

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    Terminology does not relate to understanding. I might not know the lingo but I tend to understand most things involving logic. I'm an engineer by trade, studied electronics at college, was a calibration engineer for our aero engine test site (limited to 110v due to no papers) and enjoy dabbling with a bit of everything. Not that any of that matters, but I've seen sparks and gas filters make an absolute see you next Tuesday of jobs so it goes for nothing to know the language. It doesnt take a lot to become a professional... to be a good professional you need to have the aptitude and be good to begin with.

    I'm a methodical chap and as much as I may not use your favoured lingo I managed to figure out my own system and draw out a proposed strategy in paint and execute flawlessly with nothing more than a torch and a head scratch. It was hardly rocket science. Try wiring a 1996 Toyota Supra loom from scratch... that takes more than paint :confused:
     
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