Opentherm Installation - Ideal Logic C30 Combi Boiler

Hi all, I'm brand new here!

I'm moving into my new house at the minute, and in a burst of foresight I bought the Wireless Extension Starter Kit on Prime Day last year.

The new house has an Ideal Logic C30 Combi gas boiler, and already has the Opentherm Harness installed.

There is already a 'smart' controller installed in the house, but I use Homekit for everything and this one wasn't compatible, and I wanted the radiator control, so it's getting removed and the Tado installed.

So - OpenTherm? Is it worth the effort? I could easily rip out the existing controller and wire the Tado Wireless Extender the same way, but if OT is more efficient, and it sounds like it is, I'd love to wire it up with that.

I've attached pictures of the interior of the boiler - the OpenTherm harness is pretty visible at the top right. It's currently shorted - do I remove the short and connect those two terminals to the OT1 and 2 connecters in the Wireless Extender? Do I also need a power feed, or does that send power as well? If I need power, do I move the current wiring in place to the Extender, or should I use a different scheme?

And here's a closeup of the OpenTherm section - they have handily printed some cabling suggestions on the inside of the plastic housing - I take it the idea would be to feed the Tado supplied cable through the white cable access above, and then along the same route as the existing cabling to the Extender? (I'm a network engineer, not an electrician, so I may be making assumptions that aren't correct here).

And here's the view of the other side - the wiring to the controller is obviously the middle block - does that need to remain, or should I detach it from both ends and terminate?

I know there's a whole bunch of on-device configuration that needs to be run before OpenTherm is configured too. I think I found a guide to that elsewhere on the site here.

Thanks in advance for any help!


  • Popped over to the house a couple of hours ago, took some photos of the internal of the existing control box:

    Bit confused by this - I would have thought the L and N should be Brown and Blue respectively, and I don't know where the Brown wires are going at all…

    Anyone any insight?

  • So I popped over to the house this evening to have another look at the existing controller wiring. I'm a bit confused, to be honest. I'd have thought the L and N would have been Brown and Blue respectively, and I can't understand where the Brown wires are going. Anyone any insight?

  • GrilledCheese2
    GrilledCheese2 ✭✭✭
    edited January 2021

    Are you sure the 2nd blue wire goes into the Live? If you remove the two tiny screws to reveal the wiring terminals I think you’ll see both blues go in the Neutral and there’s a brown coming from behind going into the Live.

    That EPH receiver supports OT. Personally if I was in your position I’d re-wire your existing kit to work as OT and verify you’ve got the wiring in the boiler correct. Then replace the EPH controls with Tado.

    OpenTherm is definitely worth the effort as you will have better control of the room temperature without the over/under heat associated with relay control.

  • I'll have a look later this evening when I'm back over, but what you suggest sounds sensible.

    I would just wire the OpenTherm terminals in the controller to the shorted ones in the Boiler, correct?

  • I’ve never wired one of these boilers, but I’d start by removing the current relay control connections. Then remove the link from the OT connector and run two core cable between the boiler and EPH receiver. The OT connections are polarity insensitive. Whatever you do, keep the 230V wires well away from the OT terminals.

  • Did you manage to do this? I'm thinking of doing the same
  • @healy93 I managed to get Opentherm working with my Ideal Logic C30. Some more info on my set up here: