System Boiler (Y-plan) with Google Nest 3rd Gen and OpenTherm?

[EvilDrPorkChop]

I decided to leave the system in the simple on/off mode for now, which is how my heating engineer has left it. He will come back to issue the gas safety certificate and has offered to make any installation tweaks I still want at the same time.

One thing I'm confused about is that he left my old Honeywell controller to work alongside the Nest Heat Link base station. I would have thought that the Honeywell controller is no longer needed if the Nest is wired in Y-plan on/off mode (page 23 of the nest installation guide.) With the cylinder thermostat directly connected to terminal 4 on the Nest.

I have attached some photos of my installation as it is wired now. Am I missing something and in fact the Honeywell controller is still required? Or can I ask my heating engineer to remove the old Honeywell controller? I'd like to leave the system in the simplest possible way, with as few components that can fail or confuse future installers.

Sorry I haven't read the thread fully. But why can't you use Opentherm without the S/L to the Ideal?

I have a system boiler using Opentherm and it works perfectly, but another brand. When you refer to Y Plan, you ideally to use Opentherm you should either use a Diverter rather than a Mid position (Referred to as W plan I believe) Or S plan using 2 two port valves.

 

[albert85]

@Basher that sounds like a likely explanation.

Judging from the attached photo I can see the cylinder thermostat wire going into the wiring centre, then to this controller, and then I think it goes directly to the boiler + Y-plan valve I (I think the Nest is bypassed because if I define a heating schedule in the Nest it doesn't appear to stop the boiler from heating the hot water cylinder.)

Do you think it would be possible to replace this Honeywell controller with a fused switch, and then go into the Nest's terminal 4, so that I can use the Nest to control the hot water schedule, in addition to the heating schedule? I like the idea of being able to turn the entire system off remotely, including the hot water.

 

[albert85]

Sorry I haven't read the thread fully. But why can't you use Opentherm without the S/L to the Ideal?

I have a system boiler using Opentherm and it works perfectly, but another brand. When you refer to Y Plan, you ideally to use Opentherm you should either use a Diverter rather than a Mid position (Referred to as W plan I believe) Or S plan using 2 two port valves.

Haha just when I had resigned myself to the idea that I won't be able to get it to work 🙂

Are you using a Nest 3rd gen thermostat? Do you mind telling me a bit more about how your thermostat controls the valves?

 

[EvilDrPorkChop]

Haha just when I had resigned myself to the idea that I won't be able to get a system boiler to work with Nest + OpenTherm 🙂

Are you using a Nest 3rd gen thermostat? Do you mind telling me a bit more about how your thermostat controls the valves?

It should work fine. Basically you need HW priority, so that it heats the HW first before any CH demand. This is so that it can demand a high flow temp to reheat the HW quick, then switch over to CH with lower flow temps.

I'm using Honeywell Evohome, but it's the same principal.

Regarding your timer, think your CH engineer has bodged it up. I think you need to start a fresh with the wiring.

In terms of my valve set up. I have 2 port valves, I have multiple zones. Basically when the HW is demanded the power to the other valves is killed so that the higher flow temperatures can't go round. So you could technically do it with the 3 port Mid position valve using some sort of relay, but really you'd do it using the W plan.

 

[EvilDrPorkChop]

Sorry, you'd have to have some way of detecting the HW temp too. I'm not sure how Nest operates with this? Evohome measures the tank temp.

 

[Basher]

Do you think it would be possible to replace this Honeywell controller with a fused switch

This is really a question of how the fixed wiring in your property is configured.
If the supply to your heating system is a spur from a ring, it absolutely needs an FCU on that spur before being connected to the heating components.
If you have a dedicated radial from the CU to the heating, there should at least be a switch in the vicinity, but again better a FCU, fused to protect the cables in the heating system.

So the answer is "yes"!

You might ask your heating engineer what was the reason for leaving the Honeywell in position?

 

[Basher]

Sorry, you'd have to have some way of detecting the HW temp too. I'm not sure how Nest operates with this? Evohome measures the tank temp.

Nest has a binary hot/cold input from the thermostat, not a temp input from a thermocouple.
(I have Evohome 👍)

 

[Basher]

Haha just when I had resigned myself to the idea that I won't be able to get it to work 🙂

As you see I'm no expert on this, but I still don't understand why you can't follow both the Y-plan and the Opentherm schematics, but disconnect the (switched) feed from the cylinder stat and valve to boiler SL, make the boiler SL permanently Live, and then get Opentherm to control the boiler. Or maybe Opentherm can't turn the boiler on and off?

 

[EvilDrPorkChop]

Nest has a binary hot/cold input from the thermostat, not a temp input from a thermocouple.
(I have Evohome 👍)

Ah, see that would work. I didn’t realise you could do this with Nest. Open them should then just control the boiler.

 

[hometech]

I'm inclined to adopt the occam's razor principle.
Your "engineer" does not know why it works but it does
so don't touch it just in case it stops working