Nest Thermostat - Convert Zoned To OpenTherm

[Townsmcp]

My current Nest 3rd Gen thermostat setup consists of 2 zones (upstairs and down stairs) -  2 x HeatLinks (both about 1m away from the boiler and zone valves), 2 x Nest 3rd Gen thermostats, 2 x Myson Power Extra motorised zone valves, Potterton Promax Ultra 40 boiler. Everything works fine however I have just found out that my boiler is OpenTherm capable (single OpenTherm wiring port).

From reading the Nest Installation Guide (here), I can see the gist of how the Heat Links are wired up (page 20) - I have had the front panel of the Heat Links off and can see I have cables in the N, L, 2 (wire goes from 2 to L), 3, T1 and T2. When I look at page 24 of the manual, it looks like I only need N, L, OT1, OT2, T1 and T2 connected.

My thoughts on how to get this working:

So I am aware that I can only use 1 Nest thermostat with OpenTherm (such a shame considering other thermostat manufacturers have got this sorted to allow controlling OpenTherm within a zoned system using thermostats as master/slave devices and only the master then talks to the OT on the boiler, but I am in the Nest ecosystem and don't want to replace thermostats).

My intention is to use zone 1 (down stairs) thermostat as the controlling thermostat (master effectively). I want to leave the zone 2 (upstairs)  thermostat in place (without a heating schedule via the app), and if someone turns the temperature up/down on that I will use automation to turn the downstairs thermostat temperature up/down as needed.

I can see on my zone pumps I have a slider on top stating 'Manual Override Only' - the sliders are currently at the Auto position.

My Questions:

So my questions are:

1. If I want to go through with reconfiguring the setup to use only 1 thermostat connected to OT on the boiler, and leaving both zone valves on manual override, can I just put electrical tape around the wires in the Heat Links that I disconnect (effectively around wires in ports 2 and 3)? This will then allow me to switch back to original zoned setup in the future should OT not work well.
2. Can a zone valve be left permanently on manual control (I would end up moving the slider to close the valve probably every 6 months to stop it from seizing up)? Will it keep trying to call for heat from the boiler even if I dont want it to heat? I am guessing it wont as call for heat port 3 on the Heat Links wont be connected.
3. Do I just need to run 1 new cable, from the down stairs zone Heat Link into the boilers OT terminals? I understand I will need to change the Nest Pro setting on the master thermostat to Open Therm which I am fine to do.

[Townsmcp]

Just to add, depending on what people think will work, if my points above are true, the on the 2nd heat link I assume I would also need to put electrical tape around upstairs heat link cables 2 and 3 so that it also doesnt try to control the valves

[Townsmcp]

I have also seen the following thread but from what I read everywhere else I cant see how it would work or how I could apply the same setup:

https://www.googlenestcommunity.com/t5/Nest-Thermostats/Opentherm-and-multi-zone-heating/m-p/631152

I guess I would still need to electrical tape the downstairs heat link wires 2 and 3 but leave upstairs wires alone? Wouldnt this mean that upstairs zone is working on on/off heat call while downstairs is working on OpenTherm? If so, it leads me back to the whole turn it to 1 zone so I reap the benefits of OpenTherm, especially where I have done some calculations and can take boiler return temp down to 40c and all the radiators working together would be able to dissipate the heat (my boiler can modulate down to a minimum output of 6kW and the ΔT50 of all the radiators is 7.1kW - 3.3kW for down stairs and 3.8kW for upstairs)

[GabrielaG]

Hi @Townsmcp,

Thanks for reaching out to the community. I understand you want to connect both zones on the Manual Override Only. I'm here to help you.

I see the configurations you want to do with your Nest Thermostat and the Manual Override Only. We advise having one Nest Thermostat per zone; the electrical tape may not be secure for you, your Nest Thermostat, and OpenTherm security.

We advise contacting a technician who can help you to configure your system with the Nest Thermostat.

Feel free to keep us posted if you have more questions.

Best regards,

Gabriela

[Townsmcp]

Hi @GabrielaG 

Thanks for replying. I have seen that the gap in the heat link should be large enough for a Wago connector which will be safer than just electrical tape. 
If a heat link only has L, N, T1 and T2 connected, will the thermostat display still power on without error messages and allow the dial to be turned? Or will it detect that cables are not connected, specifically Common and Call For Heat (ports 2 and 3) and show an error?

[GabrielaG]

Hi @Townsmcp,

Thank you for your response. I'm aware of your wiring connection L, N, T1 and T2 connected. We don't recommend using a Wago connector or electrical tape. For the configuration you want to do, it is preferable to contact a technician who can help you configure your system with the Nest Thermostat.

Feel free to keep us posted if you have more questions.

Best regards,

Gabriela

[Townsmcp]

Hi Gabriela, I will contact a heating engineer, however just for my own information, with only the 4 terminals connected, will there be an error message on the upstairs thermostat? This will influence how I proceed - if upstairs thermostat is effectively rendered dead then I would be seeking to remove the entire thermostat rather than leaving it on the wall 

[GabrielaG]

Hi @Townsmcp,

Thank you for your reply. I understand your concern. If the Heat Link is connected with only the 4 terminals L, N, T1, and T2, you might not have error messages in your upstairs Nest Thermostat.

The Nest Thermostat is created to work out of the wall, but you need to have the Heat Link properly installed.

Feel free to keep us posted if you have more questions.

Best regards,

Gabriela

[Townsmcp]

Hi Gabriela. My next question is, when the thermostat is set to OpenTherm as the control type, there is an option at the bottom called 'Supply'. I am aware this is the maximum flow temperature the boiler is allowed to operate at as is configurable on the thermostat - is there also an option to adjust that 'Supply' temperature in the Nest/Google Home app? Or can it only be done on the thermostat display itself? I am aware the hot water temp can be adjusted from the app so it would make sense the central heating Supply temp i also configurable in the app.

Thanks

James 

[GabrielaG]

Hi @Townsmcp,

Thank you for your reply. I understand your concern about the Nest Thermostat being set to OpenTherm as the control type for the option at the bottom called "Supply."

I see the configurations you want to do with your Nest Thermostat. For the configuration you want to do, it is preferable to contact a technician; they will answer your questions about your OpenTherm and how to use the "Supply" button.

Feel free to keep us posted if you have more questions.

Best regards,

Gabriela