Google Nest 3rd Gen on a Thorn M120 / 150C.

4 Dec 2023
Reaction score
United Kingdom
I am looking to Install a nest 3rd Gen onto a Thorn M120 / 150C.
The hot water tank is currently not used, just the central heating so only looking to control that (but will set the hot water up if possible)

Currently the boiler has a remote room thermostat (to be removed) and a mechanical timer with the 'heating' 'off' 'water and heating' options.

I have the instructions for both boiler and nest, I've fitted nest before to several combi boilers which was straight forward enough but this seems like it may not be as straight forward.

Any ideas of wiring connections?
Will be running all new 4 core (or whatever needed) for the heat link and the nest thermostat is going on its standalone USB power.
Sponsored Links
Could you be more specific on the type of system? Y plan, S plan, C plan, W plan? What timer is it? Sounds like could be gravity hot water.
I have VERY Limited knowledge to be able to answer that but I believe it will be Y Plan as one of the switch positions is 'water and heat' and the other is one of those only. There is a hot water tank connected on the floor above so I believe the hot water system is gravity pressure if that's what you are asking?

This sounds like it is not going to be as straight forward as a combi as I expected.

I will get some pictures and more details in the next couple of days of the control etc.

I'm already half way decided I'm just paying for an installer to do this!
  • Thanks
Reactions: CBW
Definitely not as straight forward as a combi.
Sponsored Links
From attached I'm going with it being a gravity system as the blue plug is connected (second diagram out of shot shows red plug for 'fully pumped')

Turns out both boilers at the same premesis are due service in January anyway so installer contacted to just do it all at the same time.

Yeah they are minimum 25 years old, I would not be suprised if someone told me they were 40 yrs old. I've looked at having them replaced with modern combi's in the past but was told the incoming gas pipes are not large enough for a combi and its a listed building so issues just replacing the pipe work. I may investigate this again as it seems ridiculous that there is no way round it.


  • IMG_20231204_210035_247.jpg
    221.1 KB · Views: 28
slight change of subject, could anyone give guidance on the efficiency of this age of boiler vs a modern combi from someone such as Baxi?
Given it's age and the system I'd assume 60% efficiency (and it could be worse)...and a properly installed modern combi maybe as high as 95%.
Should've been swapped out 20 years's so old it doesn't appear on the SEDBUK efficiency website (and that's decades old now).
Completely pointless adding silly smart stats to such old junk.
Cheers, looks like it needs a change then based on efficiency savings alone.
The only feature I need from the stat is the control over internet to save 20 mile round trips just to flick a switch.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links