Advice - Nest and Vaillant Ecotec Plus Wiring

NotBR · 7 Apr 2019

Hi all,

I'm wondering if someone can validate my wiring plan for my Nest + Vaillant Ecotec Plus boiler. It's an early Gen1 nest so only does the heating. The boiler is new and the installers were confused by the older Nest so at the moment the Heating and Hot Water both have to be on all the time and I'm reliant on the cylinder thermostat for sensing when there is no hot water demand.

Here are the parts list;

Boiler - Vaillant Ecotec Plus 415
Programmer - Drayton LP522
Valve - Honeywell V4073A
Cylinder Thermostat - PT100
Central Heating Thermostat - Nest Gen 1 (Heat Only)
Pump - Grundfos Alpha2 L

I've been through and documented all the current cabling and here is what I think the new wiring plan needs to be in the attachment - but I'd like someone to look over it to make sure it's right - I'm unsure about the Switch Live / RT part on the boiler mostly but I think it's right...

Let me know if there is anything that's useful to know.

stem · 8 Apr 2019

Are the dashed brown wires still in place? or do you plan to remove them?

Are you planning to keep the Drayton programmer to just control just the hot water on/off times?

The RT (SL) connection looks right, it should be connected to the motorised valve's orange wire and also the cylinder thermostat call for heat, both of which it appears to be.

NotBR · 8 Apr 2019

Hi @stem! Thanks for the reply.

The dashed brown connections are all wrong at the moment and I don't know what coloured cables I will use in the wall - so I left them as dashed until I have defined that. They are the ones I need to re-work as part of this re-wiring mostly.

Yes - I plan to keep the drayton for just hot water on/off times.

Good news I have the RT (SL) correct.

My main issue with this, from what I can tell, between the downstairs (where the timer, boiler, and mains supply are) to the upstairs where the wiring centre, pump, cylinder stat, nest and valve are I need the following cables;

Perm Live
Neutral
Pump Live
Switch Live (RT)
CH/ON
HW/ON
HW/OFF
And Earth

Making a total of 7 (+1 earth) connections - I have two 3 core (+2 earth) in the wall currently so I can't see how I can make this work!

An option I have is to move the programmer upstairs in which case I'd need the following

Perm Live
Neutral
Pump Live
Switch Live (RT)
and Earth

So that'd be 5 (+4 earth). Would that work?

Cheers (and thanks again for the quick reply!)

stem · 8 Apr 2019

You will need to leave the existing wires you have shown dashed HW/OFF and HW/ON in place (Danfoss 1 & 3). The HW/OFF wire to the motorised valve's grey wire is essential for a Y Plan to work, otherwise the motorised valve won't go to the heating only position, and the hot water would still be heated when it was set to be off, but the central heating was still on.

The dashed CH/ON wire from the Danfoss programmer terminal (4) should be disconnected, (otherwise the Danfoss will still be trying to control the heating as well as the Nest) along with the grey wire that you have shown connecting it to the Nest terminal (2)

The Nest Heat link Terminal (2) can then pick up a new permanent live by linking it to the (L) terminal as shown below. That way the heating will no longer be under the control of the Danfoss, and the Nest can take complete control of the heating.

So based on that, the only additional wires will be the 3 cores and earth* from the wiring centre to the Nest Heat link.

*If earth is required. With Gen 2, earth is only required if you are using T1 & T2 to power the thermostat. I can't remember if the older version is the same or not.

NotBR · 8 Apr 2019

Hi @stem, Thanks for that.

There is no earth to my Nest Link.

With your 'optimisation' of taking out the Timer for the Nest that'll mean I need 6 cables from upstairs to downstairs to complete the connections so this should then work both on paper and in real life.

How risky is it for me to re-cable this? It seems fairly straight forward or should I get a professional in?

Cheers

stem · 8 Apr 2019

That diagram looks about right to me

I'm a bit unsure though of what exists and what doesn't. As you only mentioned having a "new boiler" I assumed that this is a boiler upgrade / exchange and that the Y-Plan system (motorised valve, programmer, cylinder thermostat, wiring centre etc.) is already installed and you just wanted to add the Nest to it. In which case it is fairly straightforward and hence my comment about only needing to add an additional 3 wires for the Nest.

If you are starting from scratch, then that's a different matter, and if you were, then I wonder why you would buy a two channel Drayton and not use the heating channel of it, when there are single channel versions available that would be ideal for just the hot water time control.

Regarding whether you would be able to install it or not is down to you and your skill / confidence. Your working knowledge of the 'Y Plan' seems reasonable from what you have posted, but regarding your practical wiring skills I couldn't possibly tell from a forum.

NotBR · 8 Apr 2019

It's all there, Nest, Drayton Programmer, Valve, Boiler, CylStat, Nest G1 - just poorly wired at the moment as the installers that replaced the boiler were confused about the wiring and setup and think there might also be a dead cable in the wall (I know!). At the moment the HW and CH both have to be on all the time else I lose control of the CH via the Nest - basically it's been bodged and I want to fix it.

My next step is to check continuity of the 6 cores between upstairs and downstairs to see what's working and what's not I think then decide if I feel capable to re-wiring it myself or to get someone in.

Thanks for the help.

stem · 9 Apr 2019

Thanks. That makes sense now.

If you do get someone in to do the work for you, it can be difficult to find the right person. Heating installers will understand how the plumbing part works, but aren't always good electricians or understand the controls. As you appear to have discovered.

NotBR said:
the installers were confused by the older Nest

Which, and as you have worked out. For the central heating wiring it is exactly the same as the current Nest. It just doesn't have the hot water control part.

On the other hand, good electricians don't always understand how the heating system works. So, it's good that at least you have the connections worked out.