21 Jun 2020
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United Kingdom
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...


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.



  • Drumoyne Heating - Wiring Junction Box.pdf
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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
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...
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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
Not with the H1 extension, the grey and orange wires are not used
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.


Any help offered greatly received.
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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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.
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.

"switching it off/on stops is modulating" Why? Assuming the on period is not ridiculously short...
"there will be times when only one or two (rads) are calling for heat. The boiler will try to deliver it's full power output to those one or two loads. The loads won't dissipate it all and the return temperature to the boiler will rise. Then ..If the boiler can modulate it will detect the high return temperature and turn the flame down, the return temperature reduces and the boiler stays in condensing mode. If the boiler cannot turn the flame down far enough it will start to cycle as well"
I have not written the algorithms for a domestic water heater, although not a boiler, unless something goes wrong, I will call it a boiler. However to extract the latent heat from flue gases the return water needs to be cool.

My boiler does not modulate, it is simple on/off, and the minimum run time is around 20 minutes, i.e. time it takes to heat the pipe work and for the return water to reach the temperature needed so the feed water limit is exceeded.

So for a modulating boiler to stabilise and reduce temperature we are likely looking at 2 hours, this seemed to play out with late mothers old house, the heating came on at 7 am, the TRV heads set to 22°C at 7 am and down to 20°C at 8 am so by 9 am the circulating water had cooled to a maintaining temperature, so 9:30 to 10 am on a clear day the sun hit the bay window and the living room temperature started to rise, but radiators at this point at around 40°C so when the TRV closed very quickly the radiator cooled and the room likely hit a maximum of 24°C.

However before the TRV heads were changed to electronic and the wall thermostat set so it only switched off when it was a warm day, the boiler would cycle on/off with the wall thermostat, so often the living room radiator at 70°C as it cycled on/off, if at 70°C at 9:30 am even when the boiler switched off, there was still heat from the radiator, and found the room at 32°C, due in the main to the hysteresis generated with a wall thermostat switching on/off. It was not 32°C every time, it depended when the boiler last ran.

However this 84067_P.jpg thermostat did not help, it would have been a very good thermostat with my oil boiler, but not suited to a modulating gas boiler, as the target temperature was approached it changed the mark/space ratio to stop a high hysteresis, however switching the boiler off/on like that, completely messed up the boilers modulating control.

It is possible to set the TRV, lock shield valve, and wall thermostat (on/off type) to work together, specially in a hall, so when front door opened the TRV detects the sudden cold and auto turns off for set time (feature of the eQ-3 TRV head I assume others also do the same) once the time set has lapsed the TRV opens, and the lock shield valve controls the speed of the radiator re-heat so the TRV has enough time to turn the water flow off before the radiator gets too hot, and the hall is heated to around 18°C, the wall thermostat is set to 18.5°C so does not turn off on a cool day, however on a warm day the 18.5°C is reached and the wall thermostat turns off. This theory I tried and can say yes it worked, so it does work in practice, and it means on cool days the boiler and TRV heads control each rooms temperature, and since water is circulating most of the time, simple TRV heads can control room temperature to within 0.5°C of set point as long as the lock shield is set correctly, again did it, so know it works with a modulating gas boiler.

Where it all fell over was when using an non modulating oil boiler, this is when the old ideas still ring true, like not fitting a TRV in the room with wall thermostat, what we need to remember is a modulating gas boiler that can gain the latent heat from flue gases has to be set up to allow the built in algorithms to work, I think easy method is an OpenTherm system and multi-sensing points, however it just makes it easy, as I proved even with a silly Worcester Bosch with only an on/off external control it can be made to work, but takes more effort.

To my mind there are two huge problems, one is TRV heads marked *123456 which is about as much use as a chocolate fire guard, and installers walking away leaving all lock shield valves wide open. Using a differential thermometer the installer can set lock shields to give a 15°C drop across the radiator or other value, but without that differential thermometer it is very hard for the user to set.

With a TRV head that reports target and current TRV_report.jpg then the user can close the lock shield a tad if the current exceeds target, and open a tad if target rarely reached, but they at least need a TRV head which works with °C. The eQ-3 only shows target, so not as easy, but with a thermometer at least one has a chance.

As an electrician I have lost count of the times called to a faulty wall thermostat only to find the lock shield valves wide open. Thermostat working A1 but could not deal with the hysteresis caused by the lock shield being left wide open.

Clearly if the central heating installer had done their job, I would not have been called, so likely I am seeing just the bad installers work, the evil men do lives after them the good is often interned with their bones. So we don't see the good installers work.
To my mind there are two huge problems, one is TRV heads marked *123456 which is about as much use as a chocolate fire guard

If it is at 3 and the room isn't hot enough turn it up, if it is too hot turn it down. Most people don't care if they don't know the precise temperature.

I have a boiler, programmable room stat and normal TRV's. The system works perfectly fine.

You seem to want to vastly over complicate things.
If it is at 3 and the room isn't hot enough turn it up, if it is too hot turn it down. Most people don't care if they don't know the precise temperature.
That works fine, if the lock shield has been set, when I left mothers house, new owners did not want the electronic heads, so the mechanical were put back on, and as you say once lock shield set they worked A1.
1. Rather a lot of text to plough through, so I haven't read it in detail. However, I have a Vaillant boiler with weather compensation. This also does not use the grey and orange wires but the control system decides when the valve are to open, and fires the boiler at the same time, so the grey / orange "boiler switch" wires are redundant.
2. Those grey, white and orange wires should have had the ends insulated.

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