Hope I'm not breaking any forum rules posting notes, rather than a question, but I wanted to share this basic info in case it's useful to any one else doing the same thing How I wired my Nest thermostat kit in to a Vaillant TurboMax boiler (UK) NOTE: if you're not very technical, or are unsure then get an expert installer to do this for you. These notes are only for reference, and not professional advice. First buy the Nest thermostat kit, and some cable to connect it to the boiler with. Best to get the 5m option in case you more length for the Nest transmitter/receiver box. Wire up the Nest transmitter/receiver box before you open the boiler up (image below), using the new cable you brought. Note to remove the bottom bit of plastic tab (under the cable), so the lid can close later. Now you can turn off boiler using it's front switch, at the wall power plug, and also remove the power plug from the wall. This is important as it'll be linked to the same power supply, and it should not be live while you're working on it! This my model of Vaillant TurboMax boiler I installed it in. Open the boiler, then remove the circuit access panel. In the boiler note that the power cable is threaded through a secure hole. Loosen it with a screwdriver, so it can be pulled through to work on... Now loosen all 3 of the cables that come from the power cable in to the circuit board, and also remove the red 'looped' cable in 3 and 4 holes. Now thread the new/additional hive cable up through the bottom of the boiler, following the power cable route, and install 2 cables per hole where needed (2 live, 2 neutral, 2 earth) from the original power cable+the new one, then the black and grey cables from the new cable, as per the image. Once you're happy its all secure, tighten up the plastic screws that hold the power/hive main cables in place, pop the plastic access panel back on, then close back up the boiler cover. Now wall mount/pop the front cover on to the Nest transmitter/receiver box. You should now be ready to turn the boiler to the middle 'manual/always on' option, then put the power cable back in, and turn the boiler on. After this you're ready to do any required next steps on the Hive setup phone app etc.! NOTE: if you're not very technical, or are unsure then get an expert installer to do this for you. These notes are only for reference, and not professional advice.