Help Needed - Nest Thermostat replacing a Danfoss TP9000

8 Apr 2020
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United Kingdom
I have been spending a lot of time at home (due to the coronavirus epidemic), and I decided to take the plunge, learn something new and replaced my Danfoss TP9000 with a nest thermostat 3rd generation.

After reading a lot of forum and posts online I was pretty confident that it would be easy swap and all would be sorted after 1 hour. How wrong I was...

The Danfoss box the cables were too short and that made it difficult just to connect them all correctly, after a couple of hours all the cables connected correctly and all kicked started as planned. But I quickly realised I had a problem, the boiler was constantly heating the hot water, and unfortunately, I was not able to find a solution online, so I'm hoping that someone had the same issue as me.

My configuration is as follows:

Danfoss TP9000 w/ Thermostat

Boiler Ideal Logic heat H12

I have put the danfoss back in place as the missus was complaining, but I really don't want to be defeated and pay £85 quid for a professional installer.

Any suggestions are appreciated


  • Danfoss Tp9000.jpg
    Danfoss Tp9000.jpg
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We need to know how you connected the nest to see where you have gone wrong, can you post a pic or diagram ?
I have dismantled it and replaced it with my the old thermostat, however, I have followed the below diagram and ended up with the same Heat Link as per picture.

I also have a system boiler, with a water tank if it makes a difference.


S plan 1.png
splan 2.jpg
splan 3.jpg
The black wire in the original pic is hot water on but on the nest you have a black connected to heating on, do you have the heating on and hot water on reversed ?
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I have noticed you have a grey wire for hot water call, which appears to be connected to neutral on the Danfoss? Also the wiring going to t1 and t2 look a different colour than original, although could be lighting?
Just messed around with it again. And I found out the constant hot water issue. It was the salmonella prevention system that always heats up the boiler to kill the bateria. Because I was always turning the thermostat off it restarted again the procedure.

Problem solved.

Cheers for your help and input, I'm much appreciated.

Stay safe

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