LP522 - Nest Upgrade (almost working)

I decided to get a Nest 3rd Gen (Learning Thermostat) after reading the forums on here. I'm in a rental property with no thermostat and I want to save money on heating, and I figured I could probably take this with me if I move (I also like tech).

I read this forum https://www.diynot.com/diy/threads/upgrading-lp522-to-nest-wiring-diagram.502626/ - and found it really useful and how is easy it was.

The wiring in my LP522 is missing a wire from the CH. But I still went ahead.

I wired it up as per the above post with the missing CH wire.

The HW didn't work on the Nest, which was a surprise. So I just took a wire off, and it now works fine. It was great to have hot water in the morning without getting out of bed and pressing a button. However, my CH wasn't working on the scheduler, or the dial.

This was my final wiring.

LP522 N - Nest N
LP522 L - Nest L
LP522 1 - Nest 4 (green/yellow) ** I disconnected this and reset the HW worked **
LP522 2 - Nest 1
LP522 3 - Nest 6 (white)
LP522 4 - Nest 3 (yellow)
LP522 L - Nest 2 and 5

It's not a total disaster, as pressing the manual button does work - but that doesn't make sense as clearly it can and does operate the CH.

Has anyone else experienced this, and what important are the missing wires?


Thanks


Rebecca

I’m guessing you have a y-plan? How have you managed to go from LP522 2 to nest 1, if there’s no wire in LP522 number 2? If you can have heating but not work from timer, then it’s a timed fault.

Rebecca_UrbanFox said:

LP522 2 - Nest 1

Click to expand...

Except as @The Novice has pointed out there isn't a wire in terminal 2.



If there wasn't a wire there originally, you don't need to add one. So, there shouldn't be a wire in Heat link (1)

From your above description I suspect that you have you left the LP522 in place? If so that's what will be causing the problem you describe. The HW will be getting an 'off' signal from the LP and an 'on' signal from the Nest at the same time. You can't have two programmers in circuit. The LP522 back plate is removed and the wires are instead connected directly to the Heat link as below. The only extra is the link between L, 2 & 5. [The LP522 has this link made already internally]

Blue wire = Neutral and goes to Nest Heat link (N)
Brown wire = Live and gores to Nest Heat link (L)
Green yellow wire with red tape goes to Nest Heat link (4) Hot Water Satisfied
White wire goes to Nest Heat link (6) Hot Water Call for Heat
Yellow wire goes to Nest Heat link (3) Heating call for Heat

Then you have to link terminals (2) and (5) to (L)

Thanks Both, no I have removed the LP522 completely from the circuit - I just reconnected the wires into the Next 3rd gen Heat Link, and of course connected 2 and 5 into the Live as well.

What's really strange is the manual button on the heat link works and makes the CH work (the radiators heat-up - thank goodness, it's cold right now!). I can even hear a relay (or something switching) in the Heat Link when I adjust the temperature control (on and off as I change the required room temp up and down) on the Nest Thermostat, but the boiler doesn't fire up. It's driving me nuts. There is no other thermostat connected to the system.

The good news is the hot water works on the timer OK (and of course when pressing the manual button on the Heat Link).

Any more ideas - is there some other wire I might need to connect up in the Heat Link? Or couple together?


Thanks again everyone, Rebecca

If the manual button on the Heat link brings on the heating and hot water, then the wiring to the Heat link is most likely OK, with one exception I'll mention later.

When you use the Nest Thermostat to switch the heating on & off does the indicator below on the Heat link turn on and off correspondingly? If not it would appear there might be a communication error between the two. Are they too far away from each other, or screened by a large metal object? Although if you can hear a relay click when operating the Nest thermostat that shows it is receiving a signal.



Was everything working OK before you installed the Nest? I ask, because based on the way the original wiring was connected, your system should have a 3-Port motorised valve, and they can develop a fault that prevents the central heating operating unless the hot water is on and actually heating up at the same time. When the Heat link is operated manually, of course it turns both on, so a problem might not be immediately obvious until the hot water heated up, and then the heating would go off with it at the same time.

It also might be worth checking the wire in the Heat link terminal (4) to make sure it is tight and the screw is clamped on the metal conductor and not the insulation. This is the wire responsible for putting the motorised valve in the heating only position. Using the manual override puts both the HW and the CH 'on' so this wire won't be made live under these circumstances, and a fault with it would not be evident immediately.

This is really helpful, thanks. I am planning to do the same switch on mine (not removed anything yet) but had a quick question about the existing thermostat. I assume it is mains wired from the junction box in the hot water tank cupboard. If I wanted to use the same location for the Nest thermostat (a good one) could I short the switching cables at the wall but install a 12V transformer next to the junction box and use the existing cables to supply 12V to the Nest?

Hi Pittso and welcome to the forum.

For future reference you should really have started your own post. Tagging your question on to someone else's is called Hi-jacking and is against the forum rules. Also, new posts are flagged as such and are better seen by those that can help you. The reason you haven't received a reply earlier is because no one has found your new post buried at the end of an existing thread.

Anyway back to your question. Once the original thermostat cable is disconnected completely, you can then safely use it to connect the 12v supply to the Nest thermostat.

Personally I wouldn't use anything other than either the T1 & T2 12v terminals of the Heat link, or a plug in power supply supplied by Nest to provide the power to the thermostat. However quite a few on here have used a separate power supply successfully. There's an example here.

Thank you Stem and apologies.