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Discussion Starter · #1 ·
Hello,
I'm new to this forum and have made this account to ask some questions. I really hope someone can help me.

To cut a long story short, I hired some plumbers to install a new Vaillant Ecotec Pro 24 boiler and they told me that they can do all of the relevant electrical wiring themselves, but it turned out that they couldn't. While the boiler and all plumbing works perfectly, the Honeywell DT90E1012 thermostat does not communicate with the boiler at all. We have paid them for an electrician but have not seen one, so here i am. I have only basic understanding of electrical wiring which is not enough to work this out myself. Also i'm not looking for legal advice or who i should be reporting them to (yet).

From what I can tell the wiring into the boiler itself seems correct and the problem must be in the junction box outside of the boiler and/or the thermostat wiring. I know this because the thermostat powers on and accurately displays the correct temperature but the boiler never responds when it needs to be turning on or off. The boiler is always on regardless of whether the ambient temperature is above or below the desired temp set on the thermostat.

I'll start at the thermostat end and move towards the boiler in a series of photos, showing the wiring.

https://ibb.co/xmtPCbj

Here you can see the inside of the thermostat with the live wire into position A and neutral into position B. The plumbers had used a short length of wire to connect A to C, but i have removed this because while that's what the diagram suggests to a plumber, it's definitely not correct. Take note of the yellow wire.

https://ibb.co/0K49pV0

This is the junction box in a cupboard, under the boiler, in the kitchen. We can see two cables entering from the right and one white cable on the left which leads to the boiler. Of the two cables to the right, i'm 95% sure the top-most, obscured, cable is connecting to the thermostat (due to the yellow wire). The cable in front of that one is mains power connected to a fused spur that powers the boiler system.

https://ibb.co/44kmRJ4

This photo shows how the white cable connects to the boiler's circuit board. Both ends of the white cable have the grey and black wires taped off and are not being used.

A lot of parts are obscured in the photos and one angle couldn't show everything so i made a quick diagram in ms.paint of where each wire is going. I apologise in advance for how amateur this image is, but I hope it helps make sense of what's currently going on here. I have copied the colour of the wires because i'm not fluent in technical wire diagrams. Sorry.

https://ibb.co/9vLT1Fk

There are a few things to look at here:
-The live (red) wire connecting the thermostat to the junction box is terminating with no other connection.
-The yellow wire is not connected to anything but must have been used in the old thermostat. I have a suspicion that this should be in use now.
-There is a short red wire in the boiler's circuit board connecting live to "RT". I don't know if this was fitted in the factory or by the plumbers.
-Also, the thermostat is not a wireless model.

This is as far as i can get with my limited knowledge and understanding of electrical wiring. Though i'm pretty sure something is wrong when only a single wire connects the thermostat to the boiler.

Thank you for taking the time to read through all of this. I hope someone who is familiar with these systems can advise me on any issues they can see and advise me on how to fix them.
 

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What is the make and model of the boiler controller on the new boiler? The new controller may not use an outboard thermostat but may have its own thermocouple control. You should have the installation sheet that shows how the boiler should be wired.
 

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Discussion Starter · #3 ·
It looks like this boiler (Vaillant Ecotec pro24) does not come with it's own controller as standard. In the documentation, the "built-in controller" is referenced as an accessory and there is only a flat face plate where that would be if we had one. It has a small digital screen for setting the DHW temp and target flow temp, that's all.

I cannot find anything about wiring an external thermostat to the boiler in any of the documents. There's only mention of using one, not fitting one, as if it's assumed it would just be there work perfectly every time?
 

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Discussion Starter · #4 ·
I've done some more digging and might be on to something, but i wont go ahead with any changes unless someone can confirm.

http://vaillantgroup.intelliresponse.com/uploads/Vaillant/ecoTEC pro 2/roomstat.jpg
This part of one of the boiler's documents shows where a controller should be wired to (a controller is another name for a thermostat/roomstat?). It says that a controller can be connected as a 24v or 230v configuration. The wiring looks like the plumbers attempted the 230v but only finished half of it, perhaps.

the spec sheet for the Honeywell thermostat found on the right side of this page,
https://heatingcontrols.honeywellho...cts/Room-Thermostats/Wired-Thermostats/DT90E/
in "wiring connections" shows the 'stat to be wired directly into the mains power. Surely this would blow it to bits? Even if it didn't, how does that convey a message to the boiler?

For the 24v connection option to the boiler, could i simply use the existing red and blue wires (from the ms.paint diagram in the op) that are currently connected to the 'stat, and connect the other end to the X106 position on the board, replacing the existing loop? First link above show's the wiring diagram.
IF that is wise to do, which way around would the wires go?
 

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OK, I misinterpreted what you said earlier and though you were seeing a display from the boiler controller. I now realize it was from your room thermostat.

It's hard to follow the wiring scheme being used by looking at the pictures so perhaps it's best for me to describe how it should be wired.

First let me say that the correct circuit wiring will start at a voltage source (may be 24, 120 or 240 volts) that contains 2 wires that supply the power... it will end up at a load that has 2 terminals that those power wires are connected to. Anything in between will in series with one of those wires and will act as a switch to control the flow of power. There may be several of those series devices on the way to the load. OK. that's just basic stuff that you likely already understood but I had to act like you didn't, just to be sure.

If your power source is the incoming line from you electric panel it has 3 wires, one of them is used for a safety ground and not to carry power. The other 2 wires are the ones we want to consider in discussing this boiler circuit. If your power source is a 24 volt transformer, you will have only 2 wires.

Follow each of those wires from their first point on landing until they end up at the control valve that turns the fuel supply on to the boiler heater. That circuit must be complete for both wires all the way to the load device. This is a simple circuit and doesn't require any advanced theory about electricity or even boiler control at this point. We just want to know what is there so we can see what is wrong.

Typically the the power wires will pass through a thermostat and a high limit switch before reaching the load. That way, if the thermostat fails it will have a safety backup by virtue of the high limit switch. Those devices may only switch one of the power wires or they may switch both of them. That need not concern you but just understand that a switching device may have 2 terminals or 4 terminals, nothing to fret over.

So, give it your best look and see if the wires seem to make it all the way from the power source to the end load. The thermostat will only need the 2 wires they have connected and it looks to be on the correct terminals as far as I can tell.

I suspect it is miswired at that intermediate terminal strip. One of the line wires that hook to that strip is required to connect to a 3 conductor cable that runs to the thermostat. It appears that the red wire does that. Then the blue wire of that 3 cond. cable comes back from the stat and connects to one of the load wires that goes to the burner valve. That completes that side of the power line connection. The other side of the line should connect to the other load wire. That should be easy to follow. There will be 2 conductor line feeding the circulating pump also connected in parallel with the lines going to the fuel valve so verify that as you trace on through.

Do not connect the unattached yellow wire at the thermostat to anything. Tape it off safe. I think it's a neutral that is not required for your purpose and could damage the thermostat if improperly connected.

Finally, be sure the voltage source matches the rating of the final load!

That's the best I can do with what you have given me. If you hit a snag post back and I'll see what I can do. Just try to keep track of how the wires change color as they pass across that terminal strip and what kind of cable they go into. That will make for better communications and tracing the path easier.
 

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My WAG would be that the red wire on that terminal strip that has nothing connected to it may be the common line that runs to the end load valve. If that be true, it needs to connect to the common power leg, the non switched leg. That would be almost too easy, what?
 

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the spec sheet for the Honeywell thermostat found on the right side of this page, in "wiring connections" shows the 'stat to be wired directly into the mains power. Surely this would blow it to bits? Even if it didn't, how does that convey a message to the boiler?

For the 24v connection option to the boiler, could i simply use the existing red and blue wires (from the ms.paint diagram in the op) that are currently connected to the 'stat, and connect the other end to the X106 position on the board, replacing the existing loop? First link above show's the wiring diagram.
IF that is wise to do, which way around would the wires go?
The thermostat is rated for line voltage. It's merely a single pole double throw (SPDT) switch. It can be used with 24, 120 or 240 volts so not to worry. The A and B are the correct contacts for your use. Leave the C vacant. Do not connect the spare wire to any terminal.

Verify that your system voltage feed matches the voltage rating of your boiler and connect the thermostat in one side of the line (hot line if 120) before wiring the power through to the fuel valve. I think you have all line voltage equipment but you must verify that before powering it up. If you have a 24 volt boiler, you'll need a transformer to operate it on your line voltage system. You need the line voltage system since it controls the circulator pump, which is likely line voltage. I'm guessing... verify all this.
 

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Discussion Starter · #8 ·
Thank you for all the information Surferdude2, and for taking the time to get back to me.
I forgot to mention that im located in the UK, we have 240v supply as standard, the boiler is a combi-boiler which does not have an external gas, control, or burner valve that requires wiring to. It's a self contained unit that handles all gas and flow operations and is supposed to be the simplest system available. In an ideal situation one would simply connect power to it, and then connect the room stat, and that's it.

I feel that the answer is definitely within your posts, and while i understand most of it, some terms are different to ones we use here so i can't be sure. This is my fault for not stating my location. Specifically "end load valve" and "common power leg" are not terms i'm familiar with likely because this system doesn't have these, maybe?

I really appreciate the amount of information given so i will try to understand it as best i can. It sounds like the thermostat is supposed to act as a switch in the circuit between the mains power and boiler. If that statement is correct, and it was wired as such, would this be completely powering off the boiler when the desired room temperature is reached? As it stops the supply of power to the boiler. As far as i am aware a boiler would normally stay powered on and use the information provided by the thermostat to alter the temperature of the flow. Perhaps this is the desired affect but it's not what i have witnessed with any heating system here.

I'm failing to understand how a thermostat could send a message about temperature to the boiler when it would be only connected to mains power. Surely it needs another connection where it can actually transfer information?

So, give it your best look and see if the wires seem to make it all the way from the power source to the end load. The thermostat will only need the 2 wires they have connected and it looks to be on the correct terminals as far as I can tell.
The 240v power supply does indeed make it to the end load (which is the boiler, i assume) which powers up and is working fine, but the thermostat is not wired in series, or effectively connected to anything. The connections inside the thermostat i agree are wired correctly and the issue is definitely in the intermediary strip, or "junction box" as we would call it.

My WAG would be that the red wire on that terminal strip that has nothing connected to it may be the common line that runs to the end load valve. If that be true, it needs to connect to the common power leg, the non switched leg. That would be almost too easy, what?
I'm starting to think the problem is with this red wire. It leads to nothing but it must go somewhere. Could you please describe common power leg and non switched leg? I must know what that is but by a different name.

I'll describe how i think you're describing this and you tell me if it's correct.
The circuit should be the mains power connecting to the boiler with the thermostat in series acting as a switch.
Mains Live -> Boiler Live terminal
Boiler Neutral terminal -> Thermostat position B
'stat position B -> Mains Neutral
Circuit complete?
This describes the general connections not the physical connections as they would have to go through the junction box.
That would make a super simple circuit that allows the thermostat to switch the boiler on and off, right?

Aside from the above style of wiring it up the installation documents describe an alternative way of connecting the 'stat as a low-voltage 24v controller as it lasts 4 years on the supplied batteries. In my mind this makes more sense as it provides external information separated from the power supply loop.
That is the first article in this image highlighted in blue: https://ibb.co/hYZ8K8c
What is your opinion on this configuration?
 

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There are a few things to look at here:
-The live (red) wire connecting the thermostat to the junction box is terminating with no other connection.
-The yellow wire is not connected to anything but must have been used in the old thermostat. I have a suspicion that this should be in use now.
-There is a short red wire in the boiler's circuit board connecting live to "RT". I don't know if this was fitted in the factory or by the plumbers.
-Also, the thermostat is not a wireless model.


Refer to the lines from your post that I am quoting above with attention to the blue text as to how it fits in to what likely need to be done.

It looks like all you need to do in order for the thermostat to gain control of the boiler is to remove the small jumper from the RT terminal and connect the unterminated red wire to that RT terminal.

Furthermore, as I mentioned in past posts, do not connect that yellow wire at the thermostat to anything. It previously was a ground wire but is no longer used. I think it is disconnected on both ends now. If it were still connected as a ground and then connected at the thermostat, it would be a direct short and would possibly damage the thermostat. That explains why the installers disconnected both ends. Leave it alone as is.
 

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Discussion Starter · #10 ·
Thank for your suggestion, I have just tried it.
I used the spare black wire that was in the cable between the junction box and the boiler to bridge the distance between that terminating live red wire and the RT terminal. I connected the black wire to opposite end of the connector the red wire is on, then to RT. This is probably unorthodox but it creates the desired connection nonetheless.

Sadly it did not solve the problem but there was a difference. With that connection in place the boiler never fires up. I made sure the desired temperature was set way higher than the current temp on the thermostat, 'stat clicks on, boiler doesn't respond. This must mean that there was some dodgy communication between them, or simply removing the jumper stopped all function.

I then wondered whether it's a continuity issue so i connected one end of the 'stat/junction box cable and tested the other end with a multimeter. It showed continuity so that's not the problem. Also it confirmed that we are indeed using the correct cable that i wasn't completely sure of at the start.

I've since put the red jumper back in so the boiler at least fires up.
 

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That test tells us something... the brown phase is what is need to run through the thermostat and come back to the RT terminal (after removing the jumper). If the jumper makes the boiler fire up, then the thermostat will also do it... but it must be supplying the correct line phase.

Look at the picture I have attached and see if those two wires I circled are the ones going to the thermostat. If not, lets find the red and blue stat wires and see that they are fed the proper line phase.

edit: And another question, does the boiler have its own circulator pump?
 

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Discussion Starter · #12 ·
Yesterday i confirmed that those 2 wires are the same that run to the thermostat but i will test again today for peace of mind.

Yes, the circulator pump comes already fitted inside the boiler, as seen here highlighted in red:
https://ibb.co/LvGpB9L

So if the we are using the same red and blue wires to connect the thermostat to the boiler RT terminal, and the heating does not turn, could this mean that something needs to be synchronised? perhaps the boiler needs to be told specifically to use the thermostat or vice/versa. I don't know what setup sequences the plumbers did initially.
 

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Well then, if you are dead certain that those wires I circled are indeed that thermostat wires, then it appears that the thermostat was wired to the wrong line phase. In addition to that, they didn't connect the final leg of the thermostat to the boiler RT terminal but instead put a jumper to make it run continuously.

The fix is as I indicate on the attached picture. Turn off the power. Repeat the procedure that you did before, remove the red jumper from the boiler and run the wire from the RT terminal to the #1 terminal where the red wire is on that other strip.

Then move the blue wire as I have indicated so that it is on the proper line phase. That should get the system under thermostatic control. Turn the power back on.
 

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Discussion Starter · #14 ·
It worked! Everything works as it should! Your direction was perfect.

Thank you so much for helping us fix this SurferDude2. We would have gotten nowhere without your help. You've saved us potentially hundreds in electrician's fees and we greatly appreciate all the time and effort you've spent on this.

Could we possibly send you a gift to say our thanks?
 

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Your success is reward enough. I'm sorry it took so long since it actually was a very simple fix but working from the US to GB makes it risky and I didn't want to get you hurt.

It was fun nevertheless. Since I'm an old retired worker from lots of trades, I enjoy doing this kind of work for pastime as opposed to when I did the actual work... this is much easier and better! I do miss the exchange of blarney with my old associates but c'est la vie.

All the best, SD2
 
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