thermostat wiring bh 3000 to 831 vaillant (threads merged)

tmf786 · 13 Jan 2020

Hi All,

I've tried to look for this question in the forums and some are close but don't seem to be exactly what I have.

I am trying to wire a bh3000 thermostat to an 831 vaillant boiler.

I have two wires from the wall (brown and blue) + earth.

I have connected the two wires from the RT230v (brown and blue) on the boiler into the thermostat 3(N) and 4(L)

I have then removed the link from 24vRT

The Thermostat refuses to come on.

Do you what I need to do next?

Thanks

tmf786 · 14 Jan 2020

No worries,

got the answer from here: https://www.diynot.com/diy/threads/wifi-thermostat-install.520076/

It looks like I need four wires

LN to LN on boiler

RT230v to 1 and 2 on thermostat

So, since I don't have LN going from boiler to wall, can I use an alternative power source or does it need to be from the boiler LN? e.g. can I run the power from a socket that is near the wall?

tmf786 · 14 Jan 2020

ok, so I am running:

Boiler LN to Thermostat LN - this gives me power.

s there anyway I can link LN on the thermostat to Dry contact 1 and 2 on the thermostat?

If not, can I repurpose the spare earth wire (which is not being used and remove the sleeve ) then use this to supply L or N where neccessary and link Dry Contact 1 and 2?

There must be a way and I'm sure the experts have found a workaround?

tmf786 · 14 Jan 2020

Hi @stem

Is there anyway to do this with a 3 wire connection at the wall?

I have boiler LN to thermostat LN - this gives me power.

I have a spare earth which I could repurpose (removing the sleeve).

How can I get dry contact 1 and 2 to RT on the boiler?

stem · 14 Jan 2020

First of all, I understand your post to say that you have a single cable going to the existing thermostat with L, N and an earth wires in it, and only the L & N are connected. Is that correct?

Thermostats can be wired with 3 wires, but....

....there is no thermostat out there that has just N & L connections. If you have brown and blue wires for example don't assume that the blue is a neutral. If your existing thermostat has just two wires connected to it they will be a '230V live' and a '230v switched live'. However...

...when I say 'live' and 'switched live' it could instead be '24v' and 'switched 24v'. Some combi boilers are installed using 24v for their switching circuits, others 230V. If the wires connected are 24v then there is no way that you can use 3 wires, you will need 4 as a minimum because for obvious reasons the 24v has to be kept separate from the thermostats 230V L & N power supply.

If you can provide details of the existing thermostat (make & model) and what wires are presently connected to it I should be able to advise you further.

I also need to know if you have a traditional heat only boiler, or a combi boiler. If it's a combi boiler, if possible it would help to know if it has been wired for 24v or 230V control.

stem · 14 Jan 2020

Using the numbers as per your supplied photo

Can be connected to the boiler as below

The 3rd wire (the one between RT and thermostat terminal 2) if you are going to use an earth wire for this, it should be insulated (ie not a bare wire as in twin and earth cable) and it should also have a brown sleeve over it to identify that it is being used as a live wire.

EDIT
I have just noticed this. The left terminal you have marked 'N' is is not a neutral. I have no idea why you think this. Both of the lilac coloured terminals are live connections.

tmf786 · 14 Jan 2020

Thanks for replying Stem.

I have the same model thermostat as the original post.

The thermostat has L N and dry contact 1 and 2. It's a 230v.

The boiler is an 831 vaillant combi boiler.

I have a three core cable going from the boiler to the wall. It has blue, brown and a bare copper wire. Using a multimeter I joined the blue and brown from the LN on the boiler to the LN on the thermostat. The thermostat powers up so I know that I can get power to it but but this obviously doesn't let the thermostat control the boiler.

I know the correct method would be to join dry contact 1 and 2 to the RT 230v on the boiler but I only have three cores. I see that the RT230v on the boiler also has an L indicated on one side of the purple plug.

Thank you for your help.

stem · 14 Jan 2020

I have replied to your other thread already as below. [FYI it's against the forum rules to post duplicate threads and also to hijack other threads.]

Using the info given and the numbers as per your supplied photo on the other thread below.

Can be connected to the boiler as below

The 3rd wire you mention in the other post (the one between RT and thermostat terminal 2) if you are going to use an earth wire for this, it should be insulated (ie not a bare wire as in twin and earth cable) and it should also have a brown sleeve over it to identify that it is being used as a live wire.

tmf786 · 14 Jan 2020

My apologies. It was late last night and I came across your post as I just posted on the other thread and thought that it nearly solved my question.

I will bear your comments for future posts.

Thank you so much for your help. It is really appreciated.

tmf786 · 14 Jan 2020

Hi - so as I understand, the wire that I have going from the wall to the boiler is a twin core and earth so from your reply it sounds like it would be silly for me to use this since the earth is not insulated throughout the whole cable and therefore linking RT to terminal 2 is not possible since it will carry a live feed?

If it is not possible to use this earth then is it significant to have the power running from the boiler? Can I run it from an alternative wiring?

So here is my thinking.

I have a fuse spur switch directly behind the wall where the thermostat is placed which is wired to a socket in the kitchen that is no longer used. (it was going to be used for an electric cooker but we have a Gas Cooker now.

Could I change the 13A to a 3A fuse as per the same as the boiler.

Could I then wire in terminal 3 and 4 from the thermostat to the spare fuse spur to get the power to the thermostat.

Then I could do as your diag says and safely run one cable from RT to terminal 2 with a link from terminal 4 to terminal 1 or since I have two insulated cables, would it be best to to run two cables from 1 and 2 to RT230v?

for example:

Thank you in advance.

Regards,

stem · 14 Jan 2020

The whole heating system, the boiler and everything connected to it should be fed from the same fused spur. This is good wiring practice, and Vaillant put the statement below in their boiler installation manuals.

Ideally a 4 core cable should be used that includes 3 cores you can use for the thermostat, plus an earth that can be actually used a an earth. Whilst the present thermostat doesn't need an earth connection, you are future proofing it in case in future you wish to fit a thermostat that does.

stem · 15 Jan 2020

I forgot to mention that even though the thermostat doesn't require an earth connection, if there's a metal back box behind it then that will.

tmf786 · 15 Jan 2020

It's a plastic back box so no need for the earthing.

A new cable is not an option as it will require walls, ceilings and flooring to be taken up.

I might need to get a New thermostat that only requires the two wires or a new wireless one that I can put the receiver next to the boiler.

stem · 15 Jan 2020

The Tado Smart thermostat is battery powered [Tado say that they last about 2 years and the app tells you when to replace them] so it doesn't need a 230V supply. Leaving just the two thermostat switching wires to connect to NO & COM

You won't need the optional extension box, that's for completely a completely wireless thermostat, but as you have two wires you can wire it in directly as above.

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