Nest 3rd gen to replace existing wireless thermo on Baxi 105e

Diagram below as promised. The left diagram is what you have now and the right shows how I would remove the Towerstat and connect the Heat link to the existing wires. Take care not to mix the two black wires up when moving them.



It appears from your photo that the Towerstat is fixed to a surface mounted pattress box. If so, this can become a junction box to contain the new wiring terminals. You can buy a blanking plate to cover it.

I have adjusted what I said earlier slightly, in that instead of running 4 wires to the Heat link you will see that I've only used 3 and added a link between the Heat link terminals L and 2. Electrically the circuit is exactly the same as before, but doing it this way has two advantages:

1) It will allow you to use standard 4 core cable (3 core + earth) to connect to the Heat link
2) You won't have to try and cram 4 wires into one of the new terminals

Earth wires aren't shown for clarity, but I can see an earth terminal in your photo. That can stay as it is. The earth wire in the new cable can be used to connect it to the Heat link earth terminal.

The wiring to the Nest Thermostat isn't shown, in case you plan to use a separate plug in power supply for it. If you do want to hardwire it, two new wires would connect T1 & T2 at the thermostat to T1 & T2 at the Heat link respectively.

Final note: I don't know what type of wireless thermostat is presently being used in conjunction with your RF receiver. If it is just a basic thermostat controlling only the temperature, and there is a separate on/off timeswitch somewhere else controlling the heating, that would need to be set to be permanently 'on' to keep the Heat link permanently powered and to stop it interfering with the operation of the Nest.

Thanks again Stem, i’ll Be looking at this when I get home. Can I use simple connector blocks inside the patterns, seeing as they will be covered with a blanking plate?

Typically I’m on site trying to sort out an electrical problem .
Think we aaaallll know I’m not well versed in this. Just because the power to my comms cab has tripped and no-one knows where the fuse board is.

The pattress when it has a lid on will be fine as a junction box, so simple connector blocks are fine. Try and make the connections better than they are now, there's too much copper on show. On the other hand don't go to the other extreme, the screw has to connect with the conductor, not the insulation.

If the pattress doesn't have cable clamps, any exposed cables that could be accidentally pulled should be secured, so as not to put stress on the terminals. That could mean either clipping them in place or running them inside fixed trunking.

So, thanks for all your help, I muddled my way through to getting it wired and set up.
However, as lot of of people tend to have, the heating seems to be permanently on, even when I command nest to turn it off.
So for tonight I’ll have to switch off at the fused spur and address it tomorrow, hopefully after some further advice from you guys on here.

There's an easy test that will point you in the right direction. What happens when you switch the heating on and off manually as per the below?



If it does turn on and off manually the wiring is correct, meaning that the software / set up is incorrect.

If it doesn't turn on and off manually, the wiring is wrong.

Think it’s the latter
But I’m sure i’ve Followed your diagram correctly. I even FaceTimed my stepson who conformed the way you’d do it (in my current situation) as the most logical.

When I first installed the heat link and turned the power on, the heating came on straightaway. I’d not even set nest up. Is that normal?

Morning all,
Is it possible that I’ve got my black wires the wrong way round?
If I swap them am I likely to damage something?

Yep, looks like that’s done the trick

Well I did warn you!

stem said:

Take care not to mix the two black wires up when moving them.

Click to expand...

Anyway, glad you got it working in the end.

Just back to say many thanks to Stem for your help. I absolutely would not have managed it without your help. Fact!
Have a nice weekend.

Hi, i'm back...........with an issue.
Boiler seems to be doing what it did prior to installing nest. Just coming on when it wants to, mainly in the wee hours of the morning usually around 1-2am. When this happened previously we assumed it was a knackered TowerST RF setup, so replaced it with Nest.
I've set a schedule whereby every day from 6-9am and 4-9pm its set 21°c, all other times its set to 16°c. For the main part it works, but when it comes on between 1-2am i know its not because its dipped below 16°C cos the thermostat actually reads something like 19°c.
Now, i think i the remember reading that there is a mechanical valve in the boiler that could cause this issue.
Make any sense to anyone? And as always advice is very much appreciated.

If there's a fault with the boiler, I doubt that the Gas Safe guys will give advice over the forum, see here for an explanation. As your boiler is a combi, the components will be part of the boiler and not external (apart from the controls which you have just replaced)

Occasionally we come across a system that has a remote frost thermostat fitted. The frost thermostat overrides the usual controls and fires up the heating when there is a risk of the pipes freezing, (usually in the small hours). If you have a garage or outbuilding that has pipes or the boiler in, and that might be at risk sometimes there may be a frost stat in there. If you do have one, you could check that it's working properly, and that it hasn't inadvertently been turned up. I don't think we have had too much really cold weather just yet that would trigger it regularly just yet.

Rogue triggering of an RF thermostat by one in a neighbouring property is also not unknown, but as you have just changed the thermostat and the problem persists, you can rule that one out.

Thanks.
Certainly got no intentions of DIYing gas. My other stepson is guess what? A plumber.
Although he’s actually a mechanical engineer now and always works away.
Our combi is in the loft and the issue started when we first turned the heating back on late October. Pretty sure the temps hasn’t been cold enough for some kind of anti-frost thermos to kick in.
I thought it might be me not configuring the nest schedule right, but it’s fairly idiot-proof I think, so don’t think i’ve Got that wrong.
Stumped really, hate breaking my sleep pattern to get up and turn it off, as I’m right next to a 1500mm double rad in the bedroom.

kjacko said:

Thanks.
Certainly got no intentions of DIYing gas. My other stepson is guess what? A plumber.
Although he’s actually a mechanical engineer now and always works away.
Our combi is in the loft and the issue started when we first turned the heating back on late October. Pretty sure the temps hasn’t been cold enough for some kind of anti-frost thermos to kick in.
I thought it might be me not configuring the nest schedule right, but it’s fairly idiot-proof I think, so don’t think i’ve Got that wrong.
Stumped really, hate breaking my sleep pattern to get up and turn it off, as I’m right next to a 1500mm double rad in the bedroom.

Click to expand...

The nest has an “eco” mode and a safety heat level. By default I think the safety mode (basically frost stat) is 4°C, the eco mode is about 9°C - if the stat senses 4° or less it’ll turn the boiler on, if it’s in eco it’ll heat up to whatever you have eco set at (so by default 9°). Worth checking those settings (easiest in nest app, click the stat icon the the settings cog thingy) before messi g with the wiring or getting the boiler checked