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Viessmann Central Heating Wiring

Discussion in 'Plumbing and Central Heating' started by SweatyEskimo, 22 Jun 2020.

  1. SweatyEskimo

    SweatyEskimo

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    Hi Folks,

    Looking for a bit of help here. We moved into our current house in 2018 and have always had a bit of an issue with the heating. The boiler is a Viessmann 200-W WB2B system boiler, and the central heating is split into the zones, each controlled by a thermostat. The boiler is plumbed as a two pipe system.

    Along with huge gas bills, at times it seemed the heating would randomly come on, but never to full temperature...

    The other day I popped off the cover from the CH junction box and was a bit surprised to see the following...

    DSC_0070.JPG

    The three black cables coming in from the bottom are from the motorised zone valves for the central heating. (the DHW valve doesn't pass through this box). As you can see, the orange and grey wires are left unconnected... Also , the black wires coming from the thermostats are left unconnected too.

    It's maybe not the best diagram, but here is what we inherited...

    Central Heating Wiring.jpg

    Now, to me it seems the thermostats don't actually talk to the boiler - from the junction box there is power to the Viessmann H1 External Extension, but no other connections.

    So when the thermostats are turned up and click, the valves open and when the thermostats are turned down and click, the valves close, but they don't seem to tell the boiler this.

    So it turns out the odd times of luke warm heating are caused by the central heating valves remaining open when the heating is off, but the DHW is on and in turn pumping round the central heating system.

    Now I've figured this out I can stop it by turning the thermostats right down to zero when the heating is off, but it's not really practical.

    So my question is... Normally, do the switches on the valves normally tell the boiler to fire when they are open and in turn the boiler tell the valves to close when the central heating is off?

    The DHW pump and central heating pump (maybe the DHW recirculating pump too - I'll need to check) are all wired through the Viessmann H1 External Extension which in turn is connected to the boiler using the KM Bus. But there seems to be no connection to/from the thermostats/valves.

    Surely they should be connected some how?

    In the Viessmann H1 External Extension, there is a connection numbered 143 which in the manual is described as external blocking/external demand. There is nothing attached to this. Is this maybe where the valves/thermostats should be connected?

    Obviously all this depends on readers knowing a bit about Viessmann boilers and components, but any advice would be very greatly received.

    Thanks.
     

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  3. sircerebus666

    sircerebus666

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    I'm not familiar with your boiler but normally the thermostat acts as a switch and opens valves and fires the boiler when the thermostat is calling for heat

    I'm curious to know how the boiler knows when to fire up when the thermostat calls for heat as I think it's the orange/grey wire on the zone valves which turn on the boiler , as when the motor drives the valve open it flicks a switch which tells the boiler to fire up

    Hope someone knows the answer to that one
     
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  4. SweatyEskimo

    SweatyEskimo

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    Hi sircerebus666 and thanks for the reply. Heating times are set on the boiler itself. For example, set a heating period from 7pm to 8pm, the boiler comes on for that hour regardless of what the room temperature is or if the valves are closed.

    If in the course of that hour I were to turn the the thermostats right down so they click and the motorised valves close, the central heating pump would continue to run and the hot water would simply trundle round the initial first loop of the central heating pipework - as in the loop before the three zones flow and return pipework tees off of and onto. I'll put up a diagram to better explain this...

    Also, the boiler is weather compensated, which might shed some light on things.

    Anyway, on this initial central heating loop there is a gate valve which I believe would preferably/normally be an automatic bypass valve. The probable reason it isn't is because the pipework on this section is... a whopping 54mm diameter. I don't understand the thinking behind this as the flow from the boiler to the DHW tank and central heating loop starts at 28mm, goes to 2.5 inch for the loop that the central heating zones tee off of, then goes back to 28mm for the return to the boiler.

    On the boiler you can set a room temperature, but I struggle to see how the boiler knows if that temperature has been achieved if the motorised valve switch wires aren't connected to the boiler in any way and instead is left dangling in the junction box.

    I'd actually just be happy enough to have the heating set to run for the times programmed into the boiler, but the issue remains that the only way to close the motorised valves following a central heating timed period is to turn the thermostats right down. And unless this is done, when the DHW cycle is running, the water then gets past the central heating valves and warms the radiators - as such it takes considerably longer (and more money) to reach the set DHW temperature.

    All seems a bit odd that there is no wiring from the valves to the boiler...
     
    Last edited: 25 Jun 2020
  5. SweatyEskimo

    SweatyEskimo

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    I'll put some images up tomorrow.
     
    Last edited: 23 Jun 2020
  6. ianmcd

    ianmcd

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    Not with the H1 extension, the grey and orange wires are not used
     
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  8. SweatyEskimo

    SweatyEskimo

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    Hi Ian and thanks for joining in. So what is supposed to tell the valves to close? If the heating has been on and the house is warm, when the heating goes off it needs to get pretty cold before the thermostats themselves click and tell the valves to close. At times this can take long enough that he next heating cycle kicks in and leads to a situation where the valves never close... I would have thought once the boiler was finished it would somehow tell the valves to close so there is no cross flow of heated water when DHW kicks in. So without there being wiring from the boiler to the valves themselves I can't see how this is supposed to be done. The H1 takes its power from the junction box wiring, but nothing else.

    If it helps, nothing is connected to the 143 terminal in the H1 Extension - in the manual this is referred to as as External Blocking/External Demand.

    Untitled-1.jpg

    Any help offered greatly received.
     
  9. SweatyEskimo

    SweatyEskimo

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    Okay. I've put together a diagram of how the system is laid out - as the DHW does what I want it to I've left that part of the system simplified.

    Plumbing Layout.jpg

    Hopefully it makes sense, but bear in mind it is more pictorial than real life. As i said earlier in the thread there is a 54mm loop that the three different central heating loops tee off of and onto, each with their own Honeywell motorised valves once its dropped to 28mm. They also have gate valves on the flow and return tees. These motorised valves however are not wired to either the H1 Extension Unit or the boiler - the orange and grey wires have just been left loose. I've attached the H1 Extension manual, which to me would suggest they should be wired to the 143 terminal.

    The fact the orange and grey aren't wired to anything just seems odd as there is no communication to or from the boiler to the valves, but then there are some other oddities too...

    Looking in the H1 Extension, the central heating pump, a Grundfos Magna 32-100 180, isn't wired to terminal 20 as the H1 manual would suggest, but is instead wired to the boiler. Unfortunately I don't know which terminal in the boiler as I'm not allowed to look, but that's where its cable goes, I assume to the internal terminal 20.

    Also, the DHW motorised valve is wired to terminal 21, the same one as the DHW cylinder pump. Not sure this is normally done and it does seem to work, but the manual doesn't mention doing it that way...

    Terminal 28 has the DHW recirculation pump wired to it.

    I have contacted Viessmann (who's willingness to help I can't fault) and they said,

    "...if zone valves are on the system that require a 240v power to open them, then this could be taken from the plug 20 (pump output for heating). It may be that a bypass may be required if the valves close. As mentioned previously the internal pump may still run to monitor system flow temperature or for overrun after a demand has finished - this may be the reason why the original installer left the valves open."

    Now that kind of makes sense, and there is a bypass valve of sorts, albeit a gate valve - fully closed (20 turns) then turned back 4/5 turns - so the pump shouldn't really deadhead when/if it overruns.

    The other odd thing as I said there is nothing actually connected to plug/terminal 20. The central heating pump is directly wired to the boiler for some reason. I have a feeling the fitter enjoyed soldering much more than wiring.

    Anyway, I still can't work out how a weather compensated Viessmann 200-W WB2B boiler is suppose to tell central heating motorised valves to open and close and vice versa - mine simply aren't connected to one another. My logic would suggest they are meant to go to plug/terminal 143 in the H1 extension.

    I know as a weather compensated boiler it should in theory say "hey, it's a hot day so I'm not putting the heating on", which it does, sort of, but the problem is the valves - nothing tells them to close on a hot day or when the boiler isn't firing. And the thermostats are pretty pants to be honest - unless they are turned way down, the valves stay open - and so we end up heating the central heating with DHW...

    If there happens to be someone with the correct knowledge and experience reading this, I would be indebted if you could explain it to me. As mentioned earlier in the thread, with the central heating valves open, we end up heating the radiators with the DHW...

    Thanks again.
     

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    Last edited: 26 Jun 2020
  10. ericmark

    ericmark

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    Not seen it wired like that before, but it does make sense, the problem with a condensing boiler is switching it off/on stops is modulating as designed, it seems there is a thermostat made by EPH designed to work as a master/slave so you can have multi thermostats on one boiler connected using OpenTherm, I am sure some one will tell me I am wrong, but I have not found any other make that will work that way, and as far as I can work out, the EPH thermostat does not link to any TRV head. It is not the only one, Nest will not link to a TRV head either.

    This is not really a problem for £10 each you can get programmable TRV heads and it is just a case of setting the same schedule. And with on/off switching rather than connection to ebus, we rely on the TRV to do all the work.

    I have never seen Hive in action, but the whole idea of the TRV sending a 'demand for heat' to the wall thermostat seems a good idea if it works, I say if it works, as I fitted Nest which was claimed to work with Energenie TRV heads, but it didn't work, it was to be fair Energenie who said their TRV worked with Nest not other way around, and it seems Nest has withdrawn support.

    However this means simply reading the spec is not good enough, some products do not do what they say on the packet, so reading how a TRV has smart features working out when to turn off so as to heat room fast but not over shoot is all well and good, but until fitted you don't know if it does what it says.

    I have personally fitted eQ-3 bluetooth TRV heads at £15 each and they have worked very well, only thing which I have a niggle with is they can only be paired with one phone, I have 3 on my phone, wife has 2 on hers, however they are easy enough to control without the phone, one button swaps from Eco to Comfort settings. Other small niggle is only shows target temperature does not show current. But what do you expect for £15, non blue tooth found at £10. All in all although the Energenie are wifi and will work with geofencing, will work with multi phones, and do show current temperature, the lack of local controls means in the main the eQ-3 does a better job. But I don't have any other makes fitted, and people lie, I always remember asking my dad what his Morris Marina car was like, he said very good, so I bought one, and found it wavered in the wind, told my dad, and he said so does mine. Point being people don't like admitting there stuff is rubbish, so will tell you how good it is, even when it does not do as it says on the can.

    So the heating engineer really only knows how his own system works, and complaints, so if no one complains their heating is costing too much, he will not know there is a fault. So we see loads of modulating boilers with on/off controls, but we have no idea if that is costing just a little extra in running costs or a lot extra, we can debate on what is likely, but we don't know.

    So if your boiler is OpenTherm I would consider EPH thermostats, but as to how well they work in the real world, not a clue.
     
  11. SweatyEskimo

    SweatyEskimo

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    Hello Ericmark and thanks for replying. Unfortunately what I am looking for is how the motorised valves should be wired to the boiler/H1 Extension to make them open and close in sync with the programmed central heating periods/central heating pump. Unless they can be wired to behave like this, I will still end up in a situation where the DHW flows past the valves and warms the radiators.

    Programmable TRV heads is something I have looked at for further down the line, but the immediate wish is to get the valves behaving properly so that i can cut the heating costs. This is partly down to the size of the seperate heating zones themselves - they are quite large and so the heat loss from having even the pipework heated does add up over the year.

    My short term solution is a bit similar to what you were suggesting - programmable thermostats that simply close the valves just after the heating cycle has switched off to allow for pump overrun, but again its not quite as practical as having the boiler do it itself, especially as I would need to re programme them all if I changed the heating times.

    Thanks for helping out - something for the future.

    Cheers.
     
  12. ericmark

    ericmark

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    From zone valves all oranges together, all greys together and one set goes to L out and other to 1 replacing the link as far as I can tell if I have correct boiler wiring diagram.
     
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