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Boiler backplate - Hot water needed for heating

Discussion in 'Plumbing and Central Heating' started by matburton, 9 Apr 2021.

  1. matburton

    matburton

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    Hi,

    I have a pretty old gas boiler system, it's a two channel setup.
    Oddly for the central heating to work the hot water must also be on :confused:

    Recently I've swapped out the mechanical timer with a Drayton Wiser Hub.
    So for the first time I now have a thermostat (y)

    But that leads to a problem, the Wiser Heat app has no way to specify that heating requires hot water as far as I can see, making the room thermostat quite a bit less useful.

    I was wondering if I could solve this problem with a modification to the backplate wiring?

    upload_2021-4-9_12-16-40.png

    Maybe I could do something like link CH ON to HW ON with a diode?
    (Sorry my electronics knowledge is pathetic, I'm a n00b)
     
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  3. oldbutnotdead

    oldbutnotdead

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    Depends how your system is plumbed. You may have an old-school system- gravity hot water and pumped heating- in which case you can't have heating on without hot water. Clue will be any motorised valves on the system...none= gravity/pumped.
    Diodes won't switch ac as your plan, the Wiser doesn't have volt free contacts (as i've just discovered, costing me £5 for some relays).
    Your original room stat-2 wires? Bridge it out. Then at the backplane, CH on should connect to the pump & boiler power. HW should connect to boiler direct (assuming gravity). Pics of the original wiring would help...

    EDIT
    Ah yes. Your original controller will have been mechanically interlocked (so heating wouldn't/couldn't be commanded on unless hot water was on. So i'm wrong above- switched live to boiler comes from HW on, switched live to pump comes from CH on.
    2 options.
    1 Tell Wiser you want hot water 06.00-23.00. Heating you can then have between those times (controlled by app).
    2 (maybe beyond your current skill set) connect HW on to boiler as normal.
    Acquire a 240v coil DPDT relay. Coil live goes to CH on. COM1 and COM2 connect to permanent live. NO1 connect to boiler switched live. NO2 connect to pump live.
     
    Last edited: 9 Apr 2021
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  4. ianmcd

    ianmcd

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    As already said sounds like you have Gravity HW, post a pic of your HW Cylinder
     
  5. matburton

    matburton

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    Thanks for the responses!

    Blast! Any other simple options if I can't solve this via wall plate wiring alone?

    Not a problem as it looks like this system has never had a room thermostat.

    If the heating is turned on without the hot water I can hear 'pump-y' sounds but the boiler doesn't fire-up.

    Here's the wall plate wiring, which I haven't made any changes to.
    IMG_20210409_144833839.jpg

    The original controller was an SM2.
    IMG_20210409_145135573.jpg

    There does seem to be a motorised valve, sorry this pic is a bit dark at the bottom.
    IMG_20210409_144343401.jpg

    I was hoping to avoid that, I think that could be fairly expensive in terms of wasted gas.

    Ah that sounds like wiring that would indeed be beyond my current capability.

    Sorry about the mess. There are no valves etc around the cylinder.

    IMG_20210409_144157524.jpg

    Thanks for your help! I realise the system simply needs replacing, but that's something I'd need to save-up for over a few years and I was hoping maybe there was a simple way to tide us over until then.

    IMG_20210409_144259048.jpg
     
    Last edited: 9 Apr 2021
  6. ianmcd

    ianmcd

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    When you switched the CH on with the SM2 did the HW slider move accross at the same time ?
     
  7. matburton

    matburton

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    I admire your optimism! ;)

    No, the LED would light, there would be pump-y sounds but the boiler wouldn't do anything.

    This was never a problem for the previous owners (or us up to now) due to the lack any thermostat.
    (I found a date, the boiler is 36 years old)

    Either you moved the pins around together for timing or for manual control you'd just have to know that if you want heating you need to slide both.

    This annoyance has only been highlighted now because it's the first time the system has had a thermostat attached I guess.

    I wonder if there's any potential that I could smooth this over with software?
     
  8. ianmcd

    ianmcd

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    Nope sorry you have gravity HW and you cant use the wiser for that, it may be possible to alter pipework and install another zone valve, not really sure what that one is doing to be honest
    The way your system is wired, HW on switches the boiler on
    CH on switches the pump on
    on HW only the water is gravity circulated
    ON CH both are on, the HW switch brings the boiler on
    and the CH brings the pump on

    Not really there is a little adjustment inside the old programmer that you can set so both sliders will move together if you select CH, the lazy installer has just been two lazy to set it properly
     
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  9. matburton

    matburton

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    That's fair. The Wiser wasn't expensive (it's used) and is still useful, I mainly bought it for the timer and remote control. The pins on the SM2 are pretty dodgy making it unreliable as a timer.

    Now I look at it closely I'm not sure it's wired to anything? o_O

    WHAT! :LOL: I wish I'd known that! But then again I guess that wouldn't have avoided the problem of the dodgy pins.

    I'll instigate whether I can 'supplement' the Wiser' behavior. Maybe there's some IoT approach I can take, or maybe the Wiser itself has an API etc.
     
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  11. ianmcd

    ianmcd

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    Nothing to do with the wiser, it is a plumbing set up, you could install a Hive Dual Channel, it can be set to Gravity HW
     
  12. oldbutnotdead

    oldbutnotdead

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    With the old controller the DHW will probably have been active all day so your gas bill won't be any worse....
    Without a relay, you can't have real 2 channel operation. Your choices are;
    DHW on constantly (doesn't mean boiler will be running all the time long as your cylinder is well lagged), heating whenever you want.
    DHW only heating when CH is on (problem with yr new controller is CH ON will only go live when Wiser thermostat is calling for heat). This option will suck in the summer but work quite well in winter.
     
  13. matburton

    matburton

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    Darn! That would have been easier.

    Serves me right for not educating myself better upfront.

    After a bit of experimentation I can detect when the Hub has enabled the heating relay from http://hubhostname/data/domain/ since it contains:
    Code (Javascript):
    {
      ...
      "HeatingChannel": [
        {
          "id": 1,
          "Name": "Channel-1",
          "RoomIds": [
            1
          ],
          "PercentageDemand": 0,
          "DemandOnOffOutput": "Off",
          "HeatingRelayState": "Off",
          "IsSmartValvePreventingDemand": true
        }
      ],
      ...
    }
    And I can enable the hot water relay by asking for a boost by doing a PATCH to http://hubhostname/data/domain/HotWater/2 with:
    Code (Javascript):
    {
      "RequestOverride":{
        "Type":"Manual",
        "DurationMinutes":2,
        "SetPoint":1100
      }
    }
    So I guess maybe if I just check if HeatingRelayState is "On" every handful of seconds and if it is, but the HotWaterRelayState is "Off", then request an override boost for a few minutes. Something along those lines anyway.

    Not nearly as elegant as a properly wired solution, but maybe good enough to make the thermostat useful until I can get the whole system overhauled.
     
  14. ianmcd

    ianmcd

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    Love your coding, but here it is in simple language
    If you want CH , you need the HW on at the same time .....,,,,,,,////////
     
  15. matburton

    matburton

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    He he yeh don't worry, I've known that from before I posted, and that's what the above can (probably) achieve.
     
  16. ianmcd

    ianmcd

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    OR be radical and set the HW on times to be on every time the CH is due to be on, it is called gravity HW settings :rolleyes:
     
  17. matburton

    matburton

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    That certainly works, it's what we did with the SM2. I just want to make better use of the thermostat and avoid using gas when the temperature is already good (y)
     
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