Replacing Honeywell st699 - wiring ?

Discussion in 'Plumbing and Central Heating' started by Kenelm, 23 Jan 2022.

  1. Kenelm

    Kenelm

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    Hello, blown my programmer due to it getting a drip of water on it. so looking to replace with a new one, but not sure on the wiring

    Looking to get the st9400c as this looks like a simple swap? But open to suggestions and also how much harder would it be to install one with a wireless thermostat?

    Photo of the current wires.
    system is a gas boiler with separate hot water tank
     

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  3. CBW

    CBW

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    If you replace it with just a wireless thermostat, then you’d lose hot water heating capabilities. To have both, you need something like the Honeywell TR6-HW, Nest, Hive etc.
     
  4. Kenelm

    Kenelm

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    Ok thanks, wireless is not important so can stick to something else
     
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  5. CBW

    CBW

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    Yep, plenty on the market.
     
  6. DP

    DP

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    Good opportunity to upgrade controls. Go for T6R, give enhanced operation and facilities
     
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  7. Kenelm

    Kenelm

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    What would the wiring changes be?
     
  8. CBW

    CBW

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    Not much different:

    L - L
    N - N
    Link wires (terminal references included in instructions)
    Heating on
    Hot water on
     
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  9. stem

    stem

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    Presently the functions of the terminals at the ST9400C are:

    st.JPG

    The existing wires move from the terminals at the ST9400C to the T6R Receiver that have exactly the same function.

    t6.JPG

    So from the above diagrams:

    ST (N) = T6R (N)
    ST (L) = T6R (L)
    ST (1) = T6R (F) [May or may not be used depends on the existing system wiring]
    ST (2) = T6R (C) [Unlikely to be used]
    ST (3) = T6R (E)
    ST (4) = T6R (B)

    The receiver also needs links between the second (L) (A) and (D) terminals as shown in the diagram.

    The old wired room thermostat can be left in situ and set to its maximum setting so that it doesn't interfere with the new one. Or, if you want to remove it. That can be done by finding the origin of the cable that goes to the room thermostat. Note where the wires are connected at their origin and disconnect the thermostat cable. Then link the terminals where the two ‘live’ switching wires came from together, so that they are electrically connected.
     
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  11. Kenelm

    Kenelm

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    Thanks CBW

    looks like what’s going to be in my budget at the moment is just the ST9400C

    What’s the wiring changes on this one if anyone can help?
     
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  12. stem

    stem

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    Looks like our posts crossed :)
     
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  13. Kenelm

    Kenelm

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    i

    anychance you could help with a change of the st699 that I have currently- see photo for current wire set up to the st9400c?
     
  14. CBW

    CBW

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    Have a look at this:
     

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  15. Kenelm

    Kenelm

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    Looks pretty straight forward then and just loose some of the link wires not needed
     
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  16. stem

    stem

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    Ah sorry, I misread your post, I thought it was a st9400c you wanted to replace with a T6R, and not a st699 :whistle:

    Anyway @CBW has provided the info on the st699 connections; and the bit about the old room thermostat is still applicable.
     
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