How to wire 2 Nest thermostats in 2 zone central heating system (2x 2port valves) + Opentherm

[meloni]

Hello,

I spent some time reading threads on forums to figure out how my heating system should be wired. I need guidance from those more knowledgable than I.

My current set up consist Intergas Xtreme 36 boiler with two Opentherm inputs. Nest 3rd gen connected to one Opentherm input to control the boiler. Second Nest thermostat E is waiting to be wired in.
I also have 2x 2 port Honeywell V4043H1056 zone valves installed for zone 1 (downstairs) and zone 2 (upstairs).

My intension is to create 2 zone central heating system across the house (downstairs and upstairs as zone 1 & 2 respectively) by using 2 motorised valves and two Nest thermostats (Nest 3rd gen for zone 1 & Nest thermostat E for zone 2) connected to both Opentherm inputs on the boilers PCB.

Would wiring look similar to this diagram created by @stem for similar installation? I'll insert picture of Stem's design into this thread for reference.

How to connect two new Nest 3rd gen heatlinks? (photos)

should a link from the brown permanent live at terminal 3 to the two grey wires at terminals 4 and 5? Eh?? you said you were using wiring terminals 4 and 5 for the 2 x Heat links 'heating call for heat' Don't see any grey wires at terminals 4 & 5, they appear to be the brown wires to the...

www.diynot.com

I would imagine that Nests connected via Opentherm to the boiler would call for heat and trigger the boiler to fire up instead of motorised valves or it should be wired in the same way as per diagram (valves call for heat) and Opentherm thermostats only control heat demand? T1/T2 on Heat Link would not be used, OT1/OT2 would be used instead and connected directly to the boiler Opentherm inputs (terminal 1/2 X13 & 1/2 X12).

I would greatly appreciate some help on how to get it wired properly.

 

[stem]

My diagram doesn't use OT and I'm not sure how OT would be connected on a two zone system. OT modulates the boiler lowering its output when the room temperature approaches what is set on the room thermostat. Quite how one boiler would respond with two separate thermostat inputs that could be sending conflicting requirements I don't know. Although as your boiler has two OT inputs that sounds promising.

Hopefully someone will be along later who can assist with the OT part.

EDIT

I can answer the non OT parts

T1 & T2 on the Heat link is to provide the thermostats with 12V for operating power (they don't have batteries)

The Heatlink's would still need to be connected to the motorised valves as they need to be opened when heating is required. One motorised valve by each Nest otherwise no water would flow to any of the radiators.

 

[meloni]

You're right and I realise your diagram doesn't use OT. I am just hoping it can be slightly modified to allow motorised valve to open when there is heat demand from Nest. Not sure whether Heat link connected to OT input would trigger the boiler to fire up or it has to be the valve sending signal/voltage to the boiler?

 

[meloni]

EDIT

I can answer the non OT parts

T1 & T2 on the Heat link is to provide the thermostats with 12V for operating power (they don't have batteries)

The Heatlink's would still need to be connected to the motorised valves as they need to be opened when heating is required. One motorised valve by each Nest otherwise no water would flow to any of the radiators.

T1 & T2 won't be used as the Nest 3rd gen is powered from separate power supply and connects to Heat Link wirelessly.
OT1 & OT2 on Heat Link will be connected to OT input (terminal 1 & 2 X13) on the boiler PCB.

Edit:
Yes, zone 1 Heat Link should be connected to zone 1 motorised valve to open when thermostat calls for heat, zone 2 Heat Link connected to zone 2 motorised valve to open when thermostat calls for heat. I'm assuming that thermostat would trigger the boiler to fire up and control heat demand via OT.

My diagram doesn't use OT and I'm not sure how OT would be connected on a two zone system. OT modulates the boiler lowering its output when the room temperature approaches what is set on the room thermostat. Quite how one boiler would respond with two separate thermostat inputs that could be sending conflicting requirements I don't know. Although as your boiler has two OT inputs that sounds promising.

Hopefully someone will be along later who can assist with the OT part.

OT can control several zones. It aggregates to the highest demand. There are not usually huge discrepancies in required temperatures across a property either space or flow temperatures.

 

[stem]

Great sounds like you've answered your own questions then

 

[ericmark]

EPH does a system using OT and zones, with thermostats set as master and slaves, don't think any others do this?

 

[meloni]

Great sounds like you've answered your own questions then

Just sharing my thoughts and I'm not convinced it will work with Opentherm.

EPH does a system using OT and zones, with thermostats set as master and slaves, don't think any others do this?

Any particular solution?

Any ideas on how to make Nest work on Opentherm with Honeywell V4043H1056 (N/C) valve controlling central heating zone?

 

[ericmark]

I know EPH works with relay contacts to zone valve and OT to boiler, but as far as I am aware Nest does not have a master/slave capability so would not really work.

I think the Drayton Wiser also does a twin room thermostat with OT, but Nest is it seems designed for open plan or hot air with no zones.

Except for USA version which has remote sensors, but not for UK version.

 

[meloni]

Thanks ericmark. Will look into EPH and Drayton Wiser systems.

According to Google, Nest thermostats are compatible with zoned systems:

Learn about zoned system compatibility with Nest thermostats - Google Nest Help

Nest thermostats work with zoned boiler systems. In zoned systems, each thermostat controls heating independently in order to heat different areas of the house. Zoned systems save energy by allowing

support.google.com

In the meantime I have ordered myself two (one for each valve) 230VAC relays - 2 pole, 2x N/O contacts to allow Heatlink via Opentherm to fire up the boiler after motorised valve opens and to prevent the boiler firing up whilst valve did not open.

 

[Rodders53]

I think the Drayton Wiser also does a twin room thermostat with OT,

No.

Drayton state that only the Wiser 1 (for combi boilers) works with OpenTherm (currently). It also requires that "Any motorised valves must be disconnected and set to open". {Th user and install manuals are available for download}

They supply the OT plugin module with the other Wiser 2 and Wiser 3 hubs (well my 3 one at least). Perhaps it's in development to support OT with multizone in the future?.

 

[stem]

In the meantime I have ordered myself two (one for each valve) 230VAC relays - 2 pole, 2x N/O contacts to allow Heatlink via Opentherm to fire up the boiler after motorised valve opens and to prevent the boiler firing up whilst valve did not open.

I would expect to use the Heat link terminal 3 to open the associated motorised valve and at the same time the OT from each Heat link will tell the boiler to run and at what level, instead of the microswitches inside the motorised valves.

Only my thoughts though, but I would be astounded if there was a need to add additional relays. Personally I would also be concerned about the effects of abruptly terminating the OT signal when the relay opened, in case it generated an error message.

You could always ask Nest technical support, either online, or by phone. The number used to be 0808 169 2307 but I've not used it recently.

 

[RandomGrinch]

I would expect to use the Heat link terminal 3 to open the associated motorised valve and at the same time the OT from each Heat link will tell the boiler to run and at what level, instead of the microswitches inside the motorised valves.

The OT protocol is quite interesting - version 2.2 from 2003 is linked below.

As you say, according to the protocol, the Heat Link would be responsible for powering the valves and the Nest would communicate the call to heat through OT; there are no commands within the OT protocol to deal with motorised valves etc. (although manufacters are free to add their own bells and whistles, onto the standard).

OT is a point to point communications protocol allowing for one 'Room Controller' and one boiler, additionally within version 3, there is an allowance for a 'Gateway', to coordinate OT messages.
With two OT terminals, the OPs boiler possibly has an inbuilt gateway.

. Personally I would also be concerned about the effects of abruptly terminating the OT signal when the relay opened, in case it generated an error message.

The frame error handling in OT is quite robust; as the communication needn't be fast and isn't safety critical - if an error is detected, the whole frame is discarded.
The device simply waits until it's next scheduled communication.

However, for the OP, there should be no need for any relays - the room controller and boiler should be in constant communication. Any sustained breaks in communication, could lead to OT reverting to a deprecated mode - with full functionality only returning after a reset.

It may be worth checking your boiler has a functional automatic bypass first!

@meloni , please let us know how things go