Replace Honeywell Programmer and Room Stat with Nest 3rd Gen

[Lisa Woolley]

Hi

Looking for some advice....I have just purchased a Nest 3rd Gen and are looking to install myself. I have a Honeywell ST9400C programmer for heating and hot water positioned next to my oil fired Firebird Combi boiler. I also have a Honeywell room stat.

I have photographed the existing wiring:

Image_67168513

• Lisa Woolley
• 4 Feb 2021

Image_67151105

• Lisa Woolley
• 4 Feb 2021

From researching I need to swap the wires in the programmer as follows:

Honeywell N - Nest Heatlink N
Honeywell L - Nest Heatlink L
Honeywell 4 - Nest Heatlink 3
Honeywell 3 - Nest Hot Water Link 6

My question for the swap is - what do I do with the earth wires? And I also have two wires going into L and N in the programmer - do these need to be joined before they can go into the Nest Heatlink?

In relation to the wall stat:

This currently has a 240V supply which is too much for the Nest Thermostat - how do I get around this with the Nest and can I replace this with the Nest room stat as it is?

Any help is much appreciated for a budding novice electrician learning the ropes

 

[picasso]

You sure you got a combi boiler ? it looks more like a standard s plan setup.

 

[CBW]

Put the earth wires in the earth terminal at the heatlink. The existing thermostat wiring can be swapped over but you need to connect them at t1 and t2, but you’d have to find them, otherwise: leave existing stat on full, link the 2 wires 1 and 3, or remove at other end.

Forgot to mention, as Picasso just reminded me, you have a hot water on, but a Combi Boiler works on opening a tap.

 

[Lisa Woolley]

I have an Enviromax Combi C made by Firebird which is an oil fired system. It has no thermostatic controls on it and the programmer is the only way of switching the hot water on or off and likewise with the heating. It's a condensing boiler with no separate hot water tank. Does this make sense and mean I've got the wires corresponding to the correct Heatlink numbers as per my original post?

Any ideas on whether I need to join the two Live and Neutral wires currently going into the programmer before I swap into the Nest Heatlink?

Thanks very much for all your help

 

[ianmcd]

what do I do with the earth wires

Join them together and insulate, the nest doesnt need them

I also have two wires going into L and N in the programmer - do these need to be joined before they can go into the Nest Heatlink?

makes life easier as the nest terminals are very tight, you can squeeze two wires into the same terminal but very tight.

 

[stem]

From researching I need to swap the wires in the programmer as follows:

Honeywell N - Nest Heatlink N
Honeywell L - Nest Heatlink L
Honeywell 4 - Nest Heatlink 3
Honeywell 3 - Nest Hot Water Link 6

You also need to connect Nest L to both of the common terminals 2 and 5

The earth connection is only required if you connect the Nest thermostat to Heat link T1 and T2 for its power, which I guess is probably why the earth terminal is next to them instead of N & L.