Nest 3rd gen Opentherm wiring voltages

[Scottbolly]

 

Hello

I have just replaced my old combi boiler with a new Opentherm compatible Vokera Vibe Max 26C and want to use the OT+ option, but the installer was unsure of the Nest's Heat Link wiring, as he is a bit old school!

He was not sure if the Heat link on/off switching or OT+ outputs were both voltage free, (which the boiler needs) as it is mains powered.

He was running late, so didn't have time to check these himself, so has fitted a standard room Stat for now, but I would like to get my Nest back up and running.

I can wire it in to the boiler myself, just need to know about voltages coming out of the Heat link.

I would assume the OT+ wires are  voltage free as they carry data, but  would be great to get it confirmed.

• Thanks and Happy Xmas!

[Frustratedddd · Accepted solution]

This page is in your booklet that came with the Nest from new. I have copied the page you need, and that tells you all you need to know. Voltages clearly shown on the page, as you requested. Obviously terminals 1 - 6 would be 240v, but you would know that from your previous connections.

[Scottbolly]

Brilliant, thanks for this.

I did think that I would not use any of the first 6 terminals as my boiler needs voltage free connections, so thanks for confirming this.

My old boiler & Heat Link were connected via mains power supply from boiler to L,N & then a Live switch feed from L,  jump wired into 2, with 1 & 3 going back to boiler thermostat terminals, which I don't think is suitable for my newer boiler. My Nest Thermostat is on another room & is powered by a standard Nest PSU to wall mount.

I take it all Opentherm connections run on the same voltage i.e 42 VDC or lower?

Have seen the specific wiring for  Opentherm boilers in the Nest manual and it seems fairly straightforward. Take mains power direct from boiler or separate fused spur into the Heat Link's L & N, then connect 2 wires from OT1 & OT2 back into the 2 OT+ terminals in my boiler, orientation is not important.

Not sure why the installer was having trouble with it? I think the way the Nest had been wired, for a standard 240v system and the fact my new boiler needs voltage free connections may have thrown him off a bit, but not sure why he didn't try the Opentherm option?

Thanks again!

[Frustratedddd]

Yes that sounds about right, the Nest has Max 42v, but I can't see that being a problem for you with a new boiler. I can't help anymore sadly, as at 72 I'm really old school, can you just image the problems!!!

[Scottbolly]

Thanks!

You may be "really old school", but with age comes wisdom!

[Frustratedddd]

👍Have a good Christmas, and keep warm.