How to connect 2 Heater wires with only 1 terminal block slot?


After letting my hestiaPi in a drawer for a few mounths,
I now need it to control my heater. Winter is coming !

But I am puzzled about how to connect my HestiPi to my heater.

I have a HestiaPI Touch V4.0. The EU version with a 4 slots terminal block.
The 4 slots are:

  • H
  • W
  • AC L
  • AC N

I suppose I should connect the main 220v AC current to “AC L” and “AC N”.

Know I have a Heater that is commanded by 2 wires.
I suppose these 2 wires should be connected to the relay on the HestiaPI.
But there is only 1 "H " slot !

I couldn’t find the Heather wring information on the HestiaPi documentation.

Must be something basic I’m not understanding.

Any idea ?


This model supports one contact systems. It returns L (mains Line) to either H or W. Can you check if your system can be adjusted on the furnace side to support just one line?

Thanks for your reply.

I have also found today a related topicon this forum:
The problem wasn’t solved in that case. Altought it was with a triphase installation, wich is not my case.

Installation and maintenance manual of my Furnace (Wiessmann Vitodens 100-W) shows 2 wires for control.
I doesn’t provide more information.
As I understand it, the thermostat should shortcut the 2 wires when requesting heating to the furnace.
And should unconnect them when not requesting heating.
So the furnace probably provides its mains Line on one of the two wires and check if it gets the same voltage level on the other wire.

My concern about connecting L (mains Line) to the furnace is that the furnace itself may not be connected to the same mains Line than the HestiaPi.

Absolutely. Dont simply connect L to the the 2 wires. The safest would be an additional relay outside HestiaPi that will be triggered from the heating pin of the HestiaPi and will short these 2 wires. Otherwise get a professional or someone who is 100% sure of what he is doing and suggest another solution depending on your wiring.

After many toughts I am wiling to go a different way.
I do not want to mix the AC from the HestiaPi power supply and the furnace.

So my plan is:

  • Remove the transformer
  • Use my own USB power supply
  • Solder my USB power supply to the original transformer output location
  • At this stage the L and N tracks are unused
  • Connect my furnace 2 command wires to H and L

Do you think this would work ?

And a last question:
From a previous post:
Something puzzles me:

I’ve cut the AC/5V track near the logo to get a 5V switched signal on the H terminal.

What is this AC/5V track ?
Why are the 220V AC and 5V DC tracks so close ?
Are they actually touching ?
If yes, what is the purpose ?
If not, why would someone cut this ?

Edit: No need to solder the USB power supply. It can be connected to the accessible USB connector (the one that is NOT labelled Power).

I believe so :slight_smile:

If you cut the track and solder the other 2 pads across ‘5V’ together you would return 5V to the output of the relay instead of 220. Don’t change anything for the setup you have described so far.

Thanks for this fast answer.

I’ll keep you informed as soon as I’ve done the hack !

It works ! The furnace is correctly driven by the HestiaPi now.

Removing the transformer was a bit tedious.

For fully mounted kits, a nice option could be to Not provide the transformer.
That could actually reduce the price a bit.
To ease things further, the Raspberry pi zero usb slot for power should be available.
This is not the case at present as a relay is blocking the access.
But that would require a new PCB layout.

It is available :slight_smile: USB connector can also act as a power supply connector as power pins are shorted.

For the HestiaPi Touch V4.0, the power usb slot is blocked by one of the 2 relays.

What I was trying to say is that you can power the Pi from the connector labelled ‘USB’ too.

That’s good news !

It’s much simpler to connect to the micro usb connector,
than to cut an usb cable and solder the red and black wires to the PCB.

Wish I had this info before going the hard way.

That will be usefull for the other folks in similar situation. :slight_smile:

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