Nest Learning Thermostat 3rd Gen - C Plan Heating System? Help!

[Johnmdc]

The clock in the picture is a Randall 102 clock. It has 3 settings
Hot water which will fire the boiler and run the pump.
Off.
Water and heating which as well as firing the boiler, running the pump would also open the valve for heating.
As has been said it would be better to upgrade to a S plan by adding a valve to the hot water circuit and installing a cylinder thermostat.
However, the Nest can be used on the system as it is by a slight change to the wiring to it.

 

[Wildcat1961]

it looks like a gravity hot water system with pumped central heating.

The HW and CH are fully pumped to which is obvious when looking at the photo.

 

[djsmith83]

@johndmc the controller is indeed a Randall 102 clock. is it really possible to replace that with the nest? What change to the wiring is needed? I guess the nest heat link would go in place of the Randall 102 clock and the nest stat would talk to that wirelessly.

All I want to be able to do Is replace the Randall with the nest and use it in the same way.

Thanks!

 

[stem]

Yes Wildcat1961, I think you are right. Looking at all of the photos again, it does appear that the pipe from the pump tees off vertically directly to the hot water cylinder coil, and then carries on horizontally to the motorised valve which will probably be supplying the radiators. In which case the motorised valve will be controlling the heating.

The odd thing is why the plumber that has physically inspected the installation advised djsmith83 that he had a gravity system, and also the “nest pro” said it was a gravity fed system. I guess I was taking that at face value.

 

[Wildcat1961]

Yes Wildcat1961, I think you are right. Looking at all of the photos, it does appear that the pipe from the pump tees off vertically directly to the hot water cylinder coil, and then carries on horizontally to the motorised valve which will probably be supplying the radiators.

I do hope so as this is what I do for a living day in and day out.

how without a room thermostat is anyone's guess.

Quite simple really.
Terminal 1 of time clock supplies voltage to boiler/pump/grey and orange MV.
Terminal 2 of time clock supplies voltage to brown wire of MV (MV motor).
Live and common are linked together at time clock.

 

[djsmith83]

@stem @Wildcat1961 @Johnmdc
You guys are great thanks for all the info!

So, where does this all leave me with the nest install? Is it really possible to swap out the Randall controller with the nest heat link and with system as it is already configured - hot water on, off and hot water + central heating on just by wiring the nest heat link correctly?

Thanks in advance!!

 

[stem]

I do hope so as this is what I do for a living day in and day out.

And you would have thought that the same would be true of the plumber and Next expert who have seem the installation in the flesh.. Anyway at least we agree in the end this time.

 

[djsmith83]

And you would have thought that the same would be true of the plumber and Next expert who have seem the installation in the flesh.. Anyway at least we agree in the end this time.

Yes @stem ! This has been driving me crazy as you can imagine!!! Espiecally as I’m new to all this and completely fed up with the Randall controller.

So am I in luck?

 

[stem]

Yes, you can install the Nest, assuming the motorised valve is working. You won't get all of the functions though. When you want hot water, the pump and boiler will both operate and heat the hot water. When you want heating the motorised valve will open and supply the radiators.

You won't be able to have the heating 'on' without the hot water though, because the hot water will have to be 'on' to keep the pump and boiler running.

Below is a wiring diagram I did for someone else. The only difference with your installation is that, where I have shown the boiler connected, in your case, it will be the boiler and the pump, and where I have shown the pump connected, in your case it will be the motorised valve. Wired like this, when the heating is selected 'on' it will also start the boiler and pump, which because of the plumbing will also heat the hot water.

 

[Wildcat1961]

And you would have thought that the same would be true of the plumber and Next expert who have seem the installation in the flesh.. Anyway at least we agree in the end this time.

You do slip up from time to time which proves google is not always your friend.
You should have known when looking at the photo that the system is fully pumped and obviously not to be taken at face value.
You give sound advice with regards to the wiring procedure for a simple nest installation.
Out in the real world of controls wiring it’s a different ball game entirely (Dom/Com).

 

[djsmith83]

Yes you can install the Nest, assuming the motorised valve is working. You won't get all of the functions when you want hot water, the pump and boiler will both operate and heat the hot water. When you want heating the motorised valve will open and supply the radiators.

You won't be able to have the heating on without the hot water though, because the hot water will have to be on to keep the pump and boiler running.

Below is a wiring diagram I did for someone else. The only difference with your installation is that, where I have shown the boiler connected, in your case, it will be the boiler and the pump, and where I have shown the pump connected, in your case it will be the motorised valve.

View attachment 138135

Amazing, thanks @stem and @Wildcat1961 everyone else!

So this is basically what is happening at the moment right, and we’d use the nest in the same way? We would be able to have hot water on/off as on it’s own and heating and hot water on/off together?

I’ll show the diagram to another local plumber i know and see if they can fit!

 

[stem]

We would be able to have hot water on/off as on it’s own and heating and hot water on/off together?

Yes.

You do slip up from time to time which proves google is not always your friend.

Not sure what you mean by that. As the op had been advised by two 'professionals' that had physically examined the system that he had a gravity system, I took that at face value, nothing to to with Google. Apologies to djsmith83, I missed the tee originally as it didn't appear in all of the photo's and it was only when I got home and could see all of the photos more clearly on my laptop and after Wildcat1961 pointed it out I could see he was right....maybe I should have gone to Specsavers!

 

[Wildcat1961]

Yes, you can install the Nest, assuming the motorised valve is working. You won't get all of the functions though. When you want hot water, the pump and boiler will both operate and heat the hot water. When you want heating the motorised valve will open and supply the radiators.

You won't be able to have the heating 'on' without the hot water though, because the hot water will have to be 'on' to keep the pump and boiler running.

Below is a wiring diagram I did for someone else. The only difference with your installation is that, where I have shown the boiler connected, in your case, it will be the boiler and the pump, and where I have shown the pump connected, in your case it will be the motorised valve. Wired like this, when the heating is selected 'on' it will also start the boiler and pump, which because of the plumbing will also heat the hot water.

View attachment 138135

I’m sorry Stem but the above wiring diagram is incorrect (it won’t work).

 

[picasso]

 

[djsmith83]

More conflicting information. Frustrating.