Tuya smart thermostat Vaillant Turbo Tec

[Milenkoste]

Hello, I've bought the thermostat in the picture and have connected the 3 wires from my old thermostat. The thermostat turned on but it won't turn on the boiler. I have boiler board photo attached. Please help.

 

[RandomGrinch]

I'm afraid your existing wiring isn't sufficient for your new stat.

Was your existing stat battery powered, or mechanical?

Your new stat turns on, because it is receiving a Live feed from the boiler (brown, or blue wire), but the other wire is a Live return - it is acting as a false neutral, providing a current path back through the boiler.
The green/yellow cable is connected to the earth back at the boiler. This isn't a switched live wire.
For the new stat to work correctly, you need to provide it with a Neutral wire, preferably through a new 4 core cable.

 

[Milenkoste]

I have all these 6 wires pulled to the thermostat, can I do something? The old one was battery powered.

 

[RandomGrinch]

I have all these 6 wires pulled to the thermostat, can I do something?

So two cables come out behind the existing stat?
Is there a fused connection unit nearby?
If so, can you post a picture of all the wiring available?

 

[Milenkoste]

No, there isn't a connection unit. On the picture above the 6 wires that are connected to the boiler are pulled through to the thermostat but only L, N and ground where connected to the old unit. 3,4 and 5 are just left behind tied.

 

[ericmark]

Hardly surprising it does not work. A look at a Vaillant manual shows. And also so it would seem 3 and 4 go to volt free contacts, and 5 is a neutral, but it does not seem to show which is feed and which is return with 3 and 4, but easy to test with a meter.

However one wire to com and nothing to normally open is so clearly not going to work, I would say you don't have the skill required for the job.

I would suggest you get a battery powered thermostat like Nest e which only needs two wires. Or employ some one with a little electrical knowledge.

 

[Milenkoste]

One more thing that is bugging me is that only when I've connected the ground wire into 3 COM the thermostat turned on.

 

[Stuckinarut]

Hardly surprising it does not work. A look at a Vaillant manual shows. View attachment 362017 And also View attachment 362018 so it would seem 3 and 4 go to volt free contacts, and 5 is a neutral, but it does not seem to show which is feed and which is return with 3 and 4, but easy to test with a meter.

However one wire to com and nothing to normally open is so clearly not going to work, I would say you don't have the skill required for the job.

I would suggest you get a battery powered thermostat like Nest e which only needs two wires. Or employ some one with a little electrical knowledge.

3 is live out/common and 4 is live return/switched live on that boiler.

@Milenkoste - you need to be careful before you damage yourself and or the boiler. Guessing can be dangerous. I don't see at the boiler end a green and yellow CPC (ground as you call it) being connected to anything other than the earth terminal.

At your new thermostat you need power (L and N) and 3 and 4 from the boiler to Com and open (ironically the same numbers on the stat). When you have finished, make sure you test the boiler for safety as the cover forms part of the combustion circuit.

 

[RandomGrinch]

One more thing that is bugging me is that only when I've connected the ground wire into 3 COM the thermostat turned on.

Your replacement thermostat is designed for underfloor heating actuators.
A live output is provided on terminals 4 and 5 to open and close the actuator.
Terminal 3 is a Neutral terminal (even though labeled COM, its a little confusing) - connecting it to earth would allow the stat to power up, but shouldn't be done; it may also lead to nuisance tripping.
If you must use that stat, you should use terminals 6 and 7.

 

[Stuckinarut]

If you must use that stat, you should use terminals 6 and 7.

Looking at the thermostat on Amazon it states not to connect live wires directly to PC1 and PC2 as a short circuit may occur. 3 and 4 from Vaillants are live and switched live. Seems a return to Amazon might be in order with a good gas engineer and decent thermostat

 

[RandomGrinch]

Looking at the thermostat on Amazon it states not to connect live wires directly to PC1 and PC2 as a short circuit may occur. 3 and 4 from Vaillants are live and switched live.

Interesting point.
It's difficult to get full manuals for Tuya, to confirm anything, but I suspect that the wording is a slight mis-translation.
The connections are listed as 'dry contacts', with a 3A rating, which suggests live switching should be fine.
A reviewer also helpfully left a pic of the PCB, which again shows a mains rated 3A relay with dry contacts (the one in the middle)...

I don't know what the words really should read - but without clarification from a manual, as you say, better to stay on the safe side!

 

[RandomGrinch]

No, there isn't a connection unit. On the picture above the 6 wires that are connected to the boiler are pulled through to the thermostat but only L, N and ground where connected to the old unit. 3,4 and 5 are just left behind tied.

Coming back to the existing wiring, the wires that we see behind the thermostat are these...

Connecting to terminals 3, 4 and earth.

If there is another set of wires tied up behind, are they connected to anything?
The other cable at the boiler, connected to the L,N and E terminals provides the power to the boiler.
If the other wires behind the stat aren't connected to anything, it is unlikely to be this cable, or the boiler wouldn't have any power.

can you post a picture of all the wiring available?

...behind the existing stat?

 

[ericmark]

I find some of the rules, regulations, etc, quoted when connection thermostats, leave me scratching my head as to why.

1) The cover to the electrical connections to/from the boiler need checking as part of the EICR with rented properties, so it would need a clear notice this cover must only be removed by gas safe personal, clearly an electrician is authorised, so it would need to say gas safe or if old corgi. I have worked on 5 boilers in the family home, only non had room sealed covers to the mains or thermostat supply terminals, they were room sealed, but not the part needing to be removed to check mains supply.

2) I realise there are thermostats without volt free contacts, and that fitting some thermostats without volt free contacts could result in low voltage being supplied to the extra low voltage terminals, and care needs to be taken to ensure any fuse, including internal fuses within the boiler, are not by-passed when fitting a programmer or thermostat. So where low voltage (230 volts) is used for control it should in the main be supplied from the boiler, only thermostats and programmers which are battery powered do not need a supply from the boiler. However, we do have some rather strange thermostat arrangements, the standard backplate has three options, one volt free, the other two linked to the low voltage supply, clearly an accident waiting to happen, when the wrong thermostat is fitted. And the Nest Gen 3 thermostat can be supplied from a USB supply, so will fail with a power cut even if the boiler is battery backed, that also seems a silly idea.

3) The only device in the central heating with a reasonably reliable colour code is the motorised valve, I have seen many times green/yellow used for other than earth, be it to the tank thermostat or the room thermostat, even when they were clearly original fitments. Since 1966 even non-pendent lights have required an earth taking to them, even when class II, most thermostats even when class II have a parking terminal for the earth. So why are so many thermostats wired with twin and earth? Three core and earth is really the minimum requirement, yes I know since around mid 90's we have seen battery powered thermostats, but to no use 4 cores is to spoil the ship of a ½D of tar.

4) Why does anyone buy a thermostat before checking at what the existing wiring will allow? I found my supply from programmer to boiler had one core open circuit, which is why I fitted Nest Gen 3, as power, central heating control and domestic hot water control could all go on the same two wires from the thermostat to the hub. I have since used wireless thermostat to hub, but since Nest is also still fitted, not relying on batteries for whole central heating to work.

Some posts seem to be like buying a car, then saying how do I get a licence to drive it, most people get a licence first then buy the car. Only my wife would buy a manual car when she only had an automatic licence, she said it was cheap!

 

[Stuckinarut]

@ericmark - I understand that covers need to be removed, but I'm guessing it would be in the manufacturers instructions as to whether it forms part of the combustion circuit. A sticker of sorts would be better, but doesn't stop anyone removing the sticker. Most boilers now are going this way now. An Ideal logic being a good example, as their burner seals are porous and an ill fitted cover can then allow products of combustion into the room, no longer making the boiler room sealed.

 

[ericmark]

My boiler was in the garage, so did not really matter, but if doing an EICR one is hardly likely to download the instructions, so unless written on the cover, rather pointless putting it in the instructions, the new landlord law is quite clear, any fixed appliance must be inspected and tested, it would need a law change to alter that.