Replacing ESI ESRTP6C with Nest Thermostat (3rd Gen) - Dual Zone

[Chigurh]

Hi,

I am planning on replacing the two thermostats in my new build house that control the dual zone central heating. The thermostats are ESI ESRTP6C are connected to a wiring system that is connected to the combi boiler and two valves.

The thermostats have the following wires connected: +5V, 0V & RH.

The wiring system has the following: N, E, L & SL and +5V, 0V, RH (Zone 1) & +5V, 0V, RH (Zone 2)



I want to replace the upstairs thermostat and keep it connected via the existing wiring system. Looking at the Nest Installation information I am thinking of the following for Wiring System to Heat Link:
L > L
N > N

but I am unsure about the SL and RH on the wiring system.

Heat Link to Nest:
T1 & T2 to Thermostat

Will the wire (shown below) for the 5V thermostat be sufficient for the 12V required for the nest?



The downstairs thermostat will be plugged in so that one will not need T1 & T2 for power supply.

Any help will be much appreciated.

Thanks,

Rich

 

[RandomGrinch]

I want to replace the upstairs thermostat and keep it connected via the existing wiring system. Looking at the Nest Installation information I am thinking of the following for Wiring System to Heat Link:
L > L
N > N

but I am unsure about the SL and RH on the wiring system.

It has been suggested elsewhere, that when your existing thermostat calls for heat, it is supplying 5V to the RH terminal on your 'wiring system'.

A simple test would be to disconnect one of your 'stats; place a small link wire between '5V' and 'RH', and see if the heating fires up.

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If the test works, then the Nest heatlinks can replace the existing thermostats. The wiring system, including the 'SL', will be left as it is.

The heatlink connections would be:
L to L
N to N
Earth to Earth (for the heatlink you will use with t1 and t2 connections)
2 to 5V
3 to RH
Do not mix up the (extra) low voltage and mains wiring. No additional links will be needed on the heatlink.

Will the wire (shown below) for the 5V thermostat be sufficient for the 12V required for the nest?

It may be alright. It could depend on the length of the cable run. But it won't hurt to try!
There are eight separate wires in the Cat5e cable, you can split these up so four are in t1 and four in t2, to help lower the cables resistance (keep the same twisted pairs together!)

I hope that makes sense

 

[Chigurh]

Hi @RandomGrinch ,

Thanks for your reply.

Apologies for the further questions as I try to understand the detail.

To clarify, a test would be:
Remove the wiring from a thermostat at the wiring system and place a wire between the '5V and RH' on the board, as per the sketch below?
]
The outcome of this test would be, I hear the valve kick in and the corresponding heating zone heats up?

If this works the proposal would be:
L to L
N to N
Earth to Earth (for the heatlink you will use with t1 and t2 connections) is there an earth on the heat link? I am thinking of using the plug in version downstairs so T1 and T2 would be empty for that zone?
2 to 5V
3 to RH

SL is the wire on the opposite side of the board and this goes to the boiler? and no changes are required for the 4 wires block?

I didn't notice the 4th set of wires at the wiring sytem (one is cut shorter), that's a good idea to use 4 for each for the thermostat upstairs. This wire will be used from the current wiring system and moved to the heatlink T1 and T2. Is there a risk of putting 12V through the 4 ethernet wires? The distance is about 2m but that is plan view and doesn't count for whichever route it takes to get to the thermosat, potentially 6 maybe.

What type of cable/wire do you recommend for connecting the heatlink the wiring system? 3 core cable, and use two for each heat link to avoid mixing 12V and the mains?

Thank again,

Rich

 

[RandomGrinch]

To clarify, a test would be:
Remove the wiring from a thermostat at the wiring system and place a wire between the '5V and RH' on the board, as per the sketch below?

Yes.

The outcome of this test would be, I hear the valve kick in and the corresponding heating zone heats up?

Yes.

is there an earth on the heat link?

Yes, next to T1 and T2.
It is only relevant when you are using these connections to power the thermostat.

I am thinking of using the plug in version downstairs so T1 and T2 would be empty for that zone?

Yes.

SL is the wire on the opposite side of the board and this goes to the boiler?

Yes.

and no changes are required for the 4 wires block?

Yes, no changes necessary, except you can also use those terminals to provide the L, N and E for the Heatlink.

Is there a risk of putting 12V through the 4 ethernet wires?

No.

The distance is about 2m but that is plan view and doesn't count for whichever route it takes to get to the thermosat, potentially 6 maybe.

That should be fine, the worry would be if the cable was running 20m or so!

3 core cable, and use two for each heat link to avoid mixing 12V and the mains?

Yes a 3 core 0.75mm2 flex for the mains and a 2 core to carry the 5V between the 'wiring system' and the Heatlink:

2 to 5V
3 to RH

You seem to have a grasp on this, but a lot depends on the test. If connecting RH and 5V doesn't heat up the zone, a bit of extra work and thinking needs to be done!

 

[Chigurh]

Thanks for the further information.

I did a test on the wiring system by connecting RH to the +V and the boiler didn't wire up.

I swapped the wire to 0V as below and the heating came on and the boiler was running.



This confirms that your proposed method above is the way to wire a Nest 3rd gen?

Thanks for your help!

Rich

 

[RandomGrinch]

I swapped the wire to 0V as below and the heating came on and the boiler was running.

Interesting, and a little unusual.

It would be worth testing that the other connection works in the same way and with both together (...and the system turns off when you disconnect the wires).

It could be that it is designed that way, or the wiring system is expecting a certain protocol from the stats.
If it's the latter, then the reliability of the system can not be guaranteed with a swap to Nest.

Although if it works, when checking on the combinations above, and nothing gets hot! - then it's definitely worth a try with the Nest.

 

[Chigurh]

Hi,

I performed the test as detailed:
1. Wired both systems 0v to RH - heating fired up
2. Removed upstairs connection - heating stopped upstairs, continue to run downstairs and boiler still on.
3. Removed downstairs connection too - heating stopped and boiler stopped heating up the water.

I guess this means that using the method you previosuly mentioned will work? If so, I will purchase one nest and connect it to the downstairs system.

 

[RandomGrinch]

I guess this means that using the method you previosuly mentioned will work?

If you are confident that your testing has proved it does what you want it to do, then yes, swap the connection with a Nest.
All the heatlink does is act as a switch, bridging terminals 2 and 3 - acting just as your wire is doing.

For my interest, and in order to try and confirm the way your wiring system works - it may be useful to have a photo of the bottom of the wiring system PCB.
This really isn't necessary, but I'm curious, so feel free not to!

 

[Chigurh]

Thanks for your help it's been really helpful and apprecitated.

I will report back when I have (hopefully) installed one nest and I'll provide you with a photo of the PCB.

 

[Chigurh]

Hi @RandomGrinch ,

Mission complete.

Two nests installed, upstairs using the existing wiring to the old thermostat and downstairs is on a stand.

Thanks for all your help with this, I am really pleased with the outcome.

Below is a photo of the wiring system PCB as promised.


tantrum emoticons

Rich

 

[RandomGrinch]

Hi @RandomGrinch ,

Mission complete.

Two nests installed, upstairs using the existing wiring to the old thermostat and downstairs is on a stand.

Thanks for all your help with this, I am really pleased with the outcome.

Below is a photo of the wiring system PCB as promised.


tantrum emoticons

Rich

Brilliant, well done!

...and thanks for the photo - only a quick look now, but the components on the bottom are mostly power supply related.
There isn't any protocol linked to the old thermostats, controlling the switching;
If it switches on when RH goes to 0V, that seems to be intended by design.

....I will have a proper look later to confirm, but it looks good for now!

EDIT:
It's impossible to completely reverse engineer, as there are a few traces hidden under the thermostat connectors.
However, 0V turning on the relay can be (more or less ) confirmed:

The ground plane (0V - green), is completely separated from both contacts of the relay coil (red).
If RH switched the relay on 5V, there would be no reason to not have the ground plane connected to one of the coil contacts.

EDIT 2:
I was silly, noticed the flyback diode was facing the opposite way to what I expected - forgot to reverse the thermostat connections on the image

5V is directly connected to the coil - the relay definitely switches when RH is 0V.

If you ever get any unreliable switching - you can try connecting:
+5V to 1 on Nest
RH to 2
And 0V to 3

No need to change anything if there aren't any problems though!

 

[RandomGrinch]

Just for fun, very simple really - it wouldn't be too complicated to remove the 2 valve PSU from the system entirely...

Some guesswork was involved - double check before modifying your own system!

 

[Chigurh]

Wow, you have certainly fully evaluated my system.

What would be the benefit from attempting to remove 2 valve PSU from the system entirely?

FYI - The sytem has been working 1.5 days and I've had no issues with it.

I'm not sure what "issues" I would have or should be looking out for the boiler kicks in and turns off according to the temp on the two stats.

 

[RandomGrinch]

What would be the benefit from attempting to remove 2 valve PSU from the system entirely?

For you, no real benefit.
But for anyone who comes across this thread in the future, they know they have the option of removing it completely; stripping the connectors off the zone valves, and connecting them directly to a Nest.

FYI - The sytem has been working 1.5 days and I've had no issues with it.

I'm not sure what "issues" I would have or should be looking out for the boiler kicks in and turns off according to the temp on the two stats.

If it continues to work, then brilliant, no need for any modifications.
The issue may be, that if the system turns on randomly when it wasn't commanded, you may need to look at the slight rewire mentioned earlier.
At the moment the RH connection could be floating. With the rewire, it would either be at 5V, or 0V and nowhere in between - basically, the relay will be held on, or held off.

 

[comet2174946184]

Hi @Chigurh - how has your system been performing with this new thermostat? Any random fires of the boiler?

@RandomGrinch thank you very much for your extensive advice on this thread. I also mustered up the courage to swap out my ESI thermostats (same models and wiring centre as OP) for Tado thermostats. I’ve got:

Wiring Centre -> Thermostat
RH -> COM
0V -> NO
5v -> NC


Whilst the thermostats do successfully fire the system up, I do get random “calls for heating” on the boiler at times and I wonder if that’s because of additional circuitry in the ESI thermostat units (resistors or diodes?).

The system doesn’t fully power up but the fan does start and the heating symbol does show on the boiler screen.

I’ve currently put the old units on to see whether this was happening previously (newbuild, still getting used to everything)