Replacing a Danfoss TP9000 with a 3rd Generation Nest


Step by step instructions on how to wire up a 3rd generation Nest in place of a Danfoss TP9000.

In the first house that I bought with my wife, we had a Nest thermostat installed by NPower for “free” (in exchange for having slightly higher energy bills). When we moved into our second house, it only had a standard programmable thermostat and although a couple of energy suppliers were doing a similar offer of a 3rd generation Nest and installation, I decided to try and install this one myself.

Note: I know less than Jon Snow about electronics so if you’re unsure about anything, don’t rely on my advice here and seek help from a professional!

Step 1: Making sure I could revert back to the original thermostat if anything went wrong

The existing thermostat was a Danfoss TP9000 which was connected to a TS2 remote sensor. This was good because it meant that I could use the wires that go to the remote sensor to power the Nest from the Heat link.

First I flipped the main switch on the fuse board to cut the power and I took the control panel off the wall. The wires looked like this:

Danfoss TP9000 wiring

On the back of the control panel, it explained what each wire was:

Danfoss TP900 control panel

Since I had three different brown wires, I labelled them so I could tell which was which in case they got mixed up. I then disconnected them from the existing panel and removed it from the wall.

By looking at the existing wires and comparing them with the wiring diagrams in the Nest installation manual, I figured out that I have an S-Plan heating system. Therefore, I wired up the Heat Link like this:

Danfoss Nest
Power N and L N and L
Hot Water 3 6
Heating 4 3
Sensor 5 and 6 T1 and T2

There are jumper wires from L to 2 and then from 2 to 5 (black in the picture below). The Danfoss thermostat didn’t have these (presumably because those connections are made internally), but the Nest needs them wired up manually.

Heat Link wiring with jumper cables

You can also see in the picture above that I drilled some holes in the back of the Heat Link so that the wires could go into the wall and not have to come out of the bottom.

I also had to earth the Heat Link too. Using some wire, I connected the earth connection to the metal housing in the wall that had several other earth connections.

Earth wires

Step 3: Wire up the Nest

Taking the TS2 remote sensor off the wall in the hallway revealed the following wires:

Wires from the TS2 remote sensor

I have no idea what the grey and the earth wires were for, but they weren’t connected to anything on the TS2. The brown and black ones are the ones that were connected to T1 and T2 on the Heat Link so I screwed the back plate onto the wall and connected them up, tucking the unused wires out of the way:

Wires from the TS2 remote sensor

Step 4: Turn the power back on!

The last thing to do was turn the power back on! The Heat Link had a glowing green light and I followed the instructions on the Nest to finish setting everything up.

Here are a couple of pictures of the Nest and Heat Link installed on the walls.

Nest thermostat on the wall

Heat Link on the wall

It was surprisingly simple to do! The most difficult part was getting the jumper wires in on the Heat Link since the terminals are quite small. There was quite a bit of wiring behind the Heat Link too so it was difficult getting them all tucked in so it could be fixed to the wall.

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