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Help replacing a Stone Age stat

Discussion in 'Plumbing and Central Heating' started by CFR, 25 Oct 2020.

  1. CFR

    CFR

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    Hi guys,

    I'd like to replace my prehistoric Landis & Gyr thermostat with a more accurate digital one, and I'd like some advise please.

    This is my current thermostat

    IMG_4279.jpg IMG_4277.jpg IMG_4278.jpg

    1. Is it a straightforward process (Like getting a 2 wired Honeywell and connect the red and the yellow wire only) as shown below?



    2. If not, could someone help me with this endeavour?

    Thank you in advance
    Chris
     
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  3. ericmark

    ericmark

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    The Flomasta 22199SX Wired Digital Programmable Room Thermostat (screwfix 6259G) is two wire so easy to wire up, however the video does not really explain what the wall thermostat does, and for many systems not having the neutral wire on the old thermostat makes it better I will try to explain.

    Most modern gas boilers modulate that means to output varies, and most use the return water temperature to control output, there are analogue thermostats most common is the OpenTherm system, but unless going to analogue which is better, then you want the TRV to do most of the controlling.

    However the TRV has one problem, it will not turn the boiler completely off, on warm days the boiler will cycle on/off, anti cycle software may reduce it, but if the TRV turned the boiler off, it would not be able to turn it on again.

    So the wall thermostat is only used on warm days, we should fit it in a room normally kept cool (so it will turn boiler off early enough) with no outside door (clearly if there is an outside door when opened it will cool room) on ground floor (as heat raises) and with no alternative form of heating (think that is obvious) however in the real world the room rarely exists, so we need a compromise, so we would not if the room exists fit a TRV in that room, but if using for example the hall, then often you need a TRV as well to reheat hall once front door opened, and as long as the wall thermostat is set higher than the TRV this actually works, as in the winter the wall thermostat does nothing, it only does anything as spring arrives.

    So once the wall thermostat switches off, you don't want it to switch back on in a hurry, as it only switches off on a warm day.

    However this does not take into account varying temperatures through the day, so in the morning I am active, so 18°C is ample, but by 6 pm I want to sit down and watch TV so want 21°C and by 11 pm getting ready for bed, so 16°C is ample so it can reheat next morning quickly, that is for living room, kitchen finished using by 5:30 pm so that room can go to 16°C much earlier, bedroom the reverse 20°C first thing to get up, 16°C during the day, and 18°C over night. Only giving as an idea of course, but the main problem is the standard TRV is not programmable, so we need to change the TRV head for a battery powered programmable unit, except maybe the bathroom and possibly hall, which is why the hall is a good place for wall thermostat.

    There is a small problem, the TRV can only heat the room if the boiler is running, so with a programmable wall thermostat with a non modulating boiler you can set a small temperature change at same time as the TRV's change temperature to ensure boiler runs, but will modulating boiler not really needed.

    So the programmable TRV comes in two basic types, those which connect to a hub, or wall thermostat, and those which don't, and there is a huge price jump, so a eQ-3 TRV head likely less than £10 if you want bluetooth version I paid £15 note will only work with one phone, so if set to yours, your wife will need to manually alter the temperatures, which is easy. Then the hub connected cost around £38 each I got mine as pairs, and some can cost £60 each. If I knew when I bought my expensive ones what I know now I would not have bought them. So this house 9 programmable TRV heads, and 5 standard heads, one standard in the shower room/toilet, the other 4 in the flat which is only used for storage.

    At £10 I would say worth a go, fit them down stairs, if you like them, then also up stairs, but if not good enough move them up stairs and get hub connected down stairs, leave the wall thermostat as it is, until tried the TRV heads, if you want hub connected TRV heads you want them to match the wall thermostat, for example the Hive wall thermostat has a call for heat feature so if a hive TRV wants heat it turns on the wall thermostat, even if the room the wall thermostat is in is warm enough. But when you look at price of Hive TRV you can see why you should try cheap ones first.

    The exception is geofencing, my Energenie TRV heads using IFTTT (if this then that) can geofence, however they have a rather good anti hysteresis software built in, so a room set at 20°C will heat to 18°C rapid, but rather slow getting that extra 2°C so I would set at 22°C for one hour then to 20°C to get around the anti hysteresis software, Drayton do make TRV's with predictive software which will heat the room faster as a result, but expensive, so having a wall thermostat like Nest which also has predictive software can cut the cost when using geofencing as all done by one device not a collection of TRV's.

    However not so sure about geofencing, I find the house does not warm up or cool down fast, so seems to work better using simple time control.

    I will admit the TRV heads worked far better in last house with a modulating gas boiler than in this house with an oil boiler, there is more of a delay in this house, as TRV opens when boiler not running, I do use Nest Gen 3 for main house, but not because of the geofencing or occupancy detection or the algorithms to detect when to start heating, it was bought as it could control heating and domestic hot water and be powered with two wires, and I only had two wires main house to boiler.
     
  4. Johnmdc

    Johnmdc

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    OMG @ eric

    To answer your question
    1. Yes
     
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  5. CFR

    CFR

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    Thank you for replying! Especially @eric :D
     
  6. ianmcd

    ianmcd

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    Yes that is all you do
     
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  7. CFR

    CFR

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    Well, it doesn't work. I connected the wires according to the diagram (red to live, yellow to neutral) but the boiler didn't fire up, even though the thermostat "clicked".
    I installed the old thermostat and the boiler fired up immediately. Any ideas?
     
  8. ianmcd

    ianmcd

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    yes you havent a clue what you are doing, you need a battery operated Thermostat, with volt free connections, no Neutral no Live
     
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  9. Mottie

    Mottie

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    Excluding the earth, you surely have a live in and a switched live out? No neutral needed if you are using the Honeywell thermostat in the picture. I have the exact same one (DT90E) that replaced my old twisty thermostat. Got a picture of your terminal connections? I think you need to do away with the neutral and earth and then the other two wires can go any way round on terminals a and b.

    If you are switching the live onto the neutral, surely you’ve blown a fuse or popped a breaker or fried the room stat?
     
    Last edited: 28 Oct 2020
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  11. OMFG.How the hell did we get from coal fire to this absurdity.
     
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  12. Johnmdc

    Johnmdc

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    What thermostat did you get?
    The honeywell one you linked to doesn't have a neutral?
    The yellow isn't a neutral it's a switched live.
    The blue might be neutral but it might not be connected at the other end because your existing thermostat didn't need a neutral.
     
  13. CFR

    CFR

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    Right! I bought a Honeywell DT90A. I connected the red wire to A and the yellow to B. That is exactly what the video says; and a couple members here confirmed. I didn't connect the blue or the earth wires.

    There's a photo of my current thermostat attached to the OP.
     
  14. Johnmdc

    Johnmdc

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    Alright calm down!
    You said you connected the yellow to neutral make your mind up.
    The connections are as you say to A and B it doesn't matter which way round.
    Are you sure you have fitted the front on properly, it slots in at the top then hinge down then clips in.
     
  15. CFR

    CFR

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    I am calm :)
    I connected the red wire to A and the yellow wire to B. I said neutral because the diagram on the back of the new stat has an N for B, and I assumed N is for Neutral.

    IMG_4285.JPG
     
  16. CFR

    CFR

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    The weird thing is that if I had connected the wrong wire, the new stat would have been fried, or the breaker would have popped. Right?
    I tested the stat, worked perfectly, (apart from not firing up the boiler) no smoke or plastic smell and the breaker didn't pop. Unless the yellow wire goes to C, instead of B?
     
  17. Johnmdc

    Johnmdc

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    Okay
    Well that is how it should be.
    Like I said make sure the front is on properly. On the back of the thermostat there are 3 pins that push into the holes on the back plate under the A, B and C connections of they are not pushed in properly they will not make contact.
     
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