Yes, you can use Nest. Electrically speaking Nest is just an on/off switch exactly the same as your existing thermostat.
There are different models of Nest. Nest-e only controls the central heating. It can replace the existing thermostat, and the existing programmer / timeswitch would be set so that the central heating is permanently on 24/7 to allow the Nest to take full control. If there is existing time control somewhere for heating the hot water, that won't change. Nest-e doesn't have hot water heating capability.
Nest 3rd gen on the other hand, provides control of both the central heating and heating of a hot water cylinder. So, then the Nest Heat link replaces the existing programmer.
If you require further assistance, please provide details of the other controls (programmer) and the type of heating system you have. Fully pumped? [eg S-Plan? Y-Plan?] Pumped central heating with gravity circulation of hot water? Something else? And of course which Nest product you are considering.
It's a nest E Thermostat. A friend is giving it to me after they've renewed their boiler.
We have the acl lifestyle lp241 programmer on a y plan pumped central heating.
I found an online compatibility checker for the nest E and says its not compatible when I select the wire labels as "1,2,3,4" - wondering if I chose the wrong selection?
The online checker you have used is for the North American version of Nest that controls combined furnaces / air conditioning. So I assume you have the UK Nest-e version which is much simpler:
The red wire in the thermostat terminal 1 goes to Nest-e Heat link terminal 'C' (Common)
The yellow wire in the thermostat terminal 2 goes to Nest-e Heat link terminal 'NO' (Normally Open)
UK Nest-e does not require connections to the other two wires so the blue neutral and green/yellow earth should be insulated / isolated separately and tucked safely out of the way. I normally would use a couple of screw connectors.
The central heating should be set to be permanently 'on' at the LP241 and all control of the heating done using the Nest
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