Nest Installation

reddwarfcrew

Hi, I want to replace my existing Honeywell ST9400C programmer and Honeywell T40 Thermostat with a nest. I thought it was going to be straightforward, but the wiring diagrams for each aren't obvious and my programmer seems to have a lot of wires. Any ideas, or should I just get someone in?

As a new user, I can't post links, but if you go to instagram and look for allen6000, you will find 1 post by me with 4 pics of the current wiring.

Thanks

silvercar

https://www.instagram.com/p/BZtHiNYAD6J/?taken-by=allen6000

Alex1983

I’ve not fitted a nest but assume it comes with a receiver box, swap the original timer back plate with the nest version then you will need to remove the original room stat cable in the second picture out of the wiring centre in the airing cupboard and replace it with a link.

minicooper272

Yes, get someone in! Anything to do with a boiler should be dealt with by someone who is registered as Gas Safe.

We have a nest, and it only took a couple of hours for someone to install, costing us about £60. It's worth the price to keep you safe.

Gloomendoom

minicooper272 wrote: »

Yes, get someone in! Anything to do with a boiler should be dealt with by someone who is registered as Gas Safe.

Why? There is no gas involved. An electrician would be more appropriate.

Alex1983 wrote: »

I’ve not fitted a nest but assume it comes with a receiver box, swap the original timer back plate with the nest version then you will need to remove the original room stat cable in the second picture out of the wiring centre in the airing cupboard and replace it with a link.

Reading the fitting instructions, it isn't as simple as that. Unlike the rival Hive, the Nest receiver doesn't use an industry standard back plate.

This is where the Hive system scores. Fitting a Hive is a ten minute DIY job. All you need is a screwdriver.

reddwarfcrew

Okay, so I can see that the L/N has 3 wires as:

1 - there is a power switch to provide/cease power to the bolier and programmer
2 - the second set goes to the bolier to provide power there
3 - the 3rd set goes to the old thermostat

So the 3rd set move over to T1/T2 on the heatlink

Terminals 1/2 on the programmer have no connection. T3 is Hot Water on ad T4 is Heating on, so these go to 6/3 on the heatlink.

So that just leaves me with the 'Common' connection on the heatlink (5/2 on the heatlink)??????

reddwarfcrew

Looking at another site (can't link due to new poster in MSE) I just need to create a connection between the live and the 2 commons on the heatlnk....... is it that easy?

Alex1983

Not sure that’s right, I’ll look at the nest instructions for you in the minute.

The cables in your original programmer are as follows from left to right

Live
Neutral
HW OFF
EMPTY
HW ON
CH ON

reddwarfcrew

Just to confirm, the connections on the programmer are:

LIVE: 1 cable to an external fused switch where 240v comes from, 1 cable to bolier, 1 cable to old thermostat
NEUTRAL: as above
HW OFF: No cable
CH OFF: No cable
HW ON: cable to bolier?
CH ON: cable to bolier?

What I can't see is how the old thermostat actually interacts with the programmer apart from pulling power via it?

Alex1983

Looking at the instructions, I would fit the heat link where your timer is and then you will put L&N IN THE L&N connections, HW OFF in to terminal 4, HW ON in to terminal 6 and CH ON in to terminal 3. You would then need to remove the original room stat cable at the wiring centre in the airing cupboard and replace with a link.

Alex1983

There is probably no cables going from the programmer to the boiler, they will all good to the wiring centre in the airing cupboard, except for possibly L&N.