Nest E Heat Link and Thermostat replacing Honeywell st9100c

16 Jul 2019
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United Kingdom
Hi everyone, first timer poster so thanks in advance for any help.

I am looking to buy a Nest E and have a question on how to wire in the heatlink and then the thermostat in my home because the wiring letters on the Nest do not match the Honeywell st9100c controller and thermostat.

I have a Vaillant ecotec pro 24 combi boiler.

Pictures of current wiring below - is anyone able to let me know how my excising wiring will translate across to the nest?

I read the below:
With the Nest Thermostat E you set your existing controller to be permanently on, but replace the wired thermostat with the battery-powered Heat Link E. The Heat Link E is then controlled via the plug-in thermostat, which turns the heating on and off via temperature (as before) and its own internal schedule.

Does that mean i actually keep the Honeywell st9100c controller and replace the honeywell thermostat with the nest heatlink?

Thanks all, photos below:

thermostat-e-base.jpg Nest-Thermostat-E-3.jpgIMG_1608.jpg IMG_1612.jpg IMG_1613.jpg IMG_1614.jpg IMG_1617.jpg IMG_1607.jpg

thanks again!
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First thing I did was look for installation instructions for your boiler and if that link is correct is says on page 39 that you have two options for the thermostat, 24 volt DC or Bus connection (digital room thermostat/controller) and Nest E also has two options either simple on/off or Opentherm I do not know if the Bus connection (digital room thermostat/controller) referred to in the manual is OpenTherm? Page 16 also has some instructions.

I know where is can be used OpenTherm is better than simple on/off with things like the temperature of domestic hot water displayed. However my boiler does not have it, so don't know how much better it is?

I have considered adding Nest E to my system, so would love to know how it goes, I have already fitted Nest Gen 3 and I will assume they work similar, both have volt free contacts so easy to wire. In your case just two wires, stem has already given the method, to simply replace, it's just the question as to if that is best method or if there is a option to use Bus connection.
stem has already given the method, to simply replace.
Sorry I omitted something from my original post and didn't have time to correct it, so asked the mods to delete it to avoid confusion. Here's what it should have said.

You need to remove the original Honeywell thermostat and the programmer, as the Nest e takes over both functions.

ST9100 Programmer
Remove the ST9100 and move some of the wires from it to the Nest e Heat Link as below:

The brown live wire in 'L' then goes to the Nest e Heat Link 'C'
The blue wire in '4' (on) then goes to the Nest e Heat Link 'NO' (This blue wire is actually being used as a'live' wire so should have a brown sleeve on it to show that)
The link between 'L' and '1' is discarded
The Nest is battery powered so the blue wire in the ST9100 'N' is not required, do not connect it to the Nest, it should be insulated and tucked safely out of the way, so as not to touch anything.

T6360 Honeywell Thermostat
Here you need to trace the thermostat cable back to it's origin. Unfortunately I can't tell you where that will be as its location will have been chosen by the original installer based on their personal preferences and the layout of your home. Often it is connected behind the programmer, but not always. When you find the origin, note where the brown and blue wires are connected then you can disconnect and remove the cable to the old thermostat.

Finally, you should insert a wire to link together the terminals where the brown and blue thermostat wires have just been removed from.

Decomission stat Model (1).jpg

You might be able to use OpenTherm as ericmark suggests, but be careful. To access the wiring terminals of some boilers, you have to open up a room sealed compartment (I don't know if this applies to yours or not) if yours is like this then it should only be done by a RGI who can carry out the necessary safety checks to make sure it is sealed up properly afterwards.

If you do use OT then a single cable can connect the two OT terminals from the Heat Link connect directly to the OT terminals at the boiler, and all of the existing wiring goes.
Last edited:

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