Nest 3 wiring onto Worcester 35CDI ?

[lucky1313]

Hi Guys

I was hoping someone might be able to help me work out which wires go where ?

The boiler current has a Honeywell CM927 thermostat with only 5 wires one being a shared live so really only 4 wires as shown in the pictures, N L L and then ABC can anyone advise where these go on the Nest Unit, the Nest Unit is the 2nd picture with the Nest wiring diagram on it ? Hopefully the pictures make it more obvious

 

[lucky1313]

I should probably add that I think the hot water is permanently set to on, it’s a combi boiler and we just have hot water on demand, no heater tank or anything it comes straight out of the 35 CDI to the tap if that makes sense

 

[lucky1313]

So I’ve been reading and I think I’ve got it, I’ve measured the voltage across A and B on the BDR91 and it’s 8v which means I have a low voltage switch. So from using the BDR91 as an example I would connect the existing connections to the Nest as so:

L->L
N-N

Shared Live -> 2 on Nest
B -> 3

How does that sound ?

 

[lucky1313]

Hi

Has anyone got a second to check I’ve got the wiring correct on this swapover

L->L
N-N

Shared Live -> 2 on Nest
B -> 3

 

[stem]

I'm a bit puzzled why you are asking. You have posted the answer yourself:

Existing stat

Nest

The wires, including the link, simply move across from the Honeywell to the Nest terminals that have exactly the same function: The two (L) terminals at the Honeywell are connected together, so are electrically actually the same terminal.

So..

Honeywell CM927 (N) = Nest (N)
Honeywell CM927 (L) & (L) = Nest (L)
Honeywell CM927 (A) = Nest 'Heating Common' (2)
Honeywell CM927 (B) = Nest 'Heating call for heat' (3)

The green/yellow wire is being used as a live wire. This is bad practice. As a minimum it should have a brown sleeve over it to identify it as a live wire.

The link is also a live wire and should be brown not blue, also bad practice.

I take it you have a combi boiler and don't have a hot water cylinder that requires connecting to the Nest Heat link.

 

[lucky1313]

Awesome thanks Stem

Worked a treat, I’m nesting it up to the max now

 

[HERTS P&D]

Stem is THE man!

Andy