Nest thermostat - combi boiler and UFH set up question


We recently had some work done, a new open plan room, a new boiler and underfloor heating (UFH) put in. We had some confusion, a lot of which comes from not having a basic understanding of how the Boiler and UFH actualy work, but essentially I want to understand the best way in order to set up our Nest Smart thermostats.

I'll explain the setup first, the original House is all via a combi boiler and traditional radiators, we just had a 50sqm extension to add a new room that has wet underfloor heating and is well insulated and is tiled. (i.e this room is nice and cosy). The room has 3 zones, but for all intent and purposes, these zones might as well act as 1 for us, the only reason we have 3 is because its a large room and would heat up quicker. The new UFH goes back to a manifold which is feeding off the Combi Boiler and has its own pump which as I understand mixes in hot water to the cool water returned from the radiators. (for reference the UFH manifold valves only have...

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I bought a second hand Nest Thermostat v1 (the European model with heatlink). I installed it and the Thermostat seems to work. The central heating boiler seems to turn on but not off.

I disconnected the old thermostat wires from the "bus" +/- like shown below and connected it to 2/3 on the heatlink as shown on the second image.

After that I removed the old thermostat and connected the wires (t1/t2) on the heatlink to the Nest thermostat.

At first I used a cable between the heatlink and boiler that might have been not thick enough (phone cable) then the boiler showed a Vaillant F49 fault. But after I disabled everything again, used a thicker cable, and did a reset of the boiler, heatlink and Nest the error was gone.

Now it seems that when I enable the three devices. I can enable the boiler by turning the heat up on the Nest thermostat but it won't stop heating anymore when I turn the temperature down on the thermostat.

Did I...

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Hey there guys,

i'm back with some news about my set up.

Test went well, there was some air leakage with some connectors but that was quickly fixed. the system was filled and the thermolec powered on... so did the NEST thermostat!!!! (we now know that it only needs two wires)

i first increased the temperature of the thermostat so i can turn the heating on and so it did along with the pump..... sadly, after about a minute, the thermolec and the pump shut down.

I increased the NEST thermostat temperature to 30c but with the same results. the thermolec and pump start and i see on the circuit board of the boiler one led, then two, then third one (led indicate the element that starts to heat) - The heat in the pipe is weak then when it reaches the 3rd element, it shuts down starting by the last element it enabled, then second and first before the thermolec turns off.

My plumber tried to bypass the nest thermostat by short circuiting W1 and C and...

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We've a French propane gas heating system that came with the house. It works perfectly well - it's just the controller (the thing that sits on the wall in the next room) that's starting to show signs of decrepitude. It's an obsolete Tybox 120. Replacing it's proving to be a bit difficult as all the controllers I'm seeing are mains voltage. Ours is a battery-powered affair that simply connects 2 wires through the wall to the boiler's control board and shuts it on and off at predetermined times and temperatures. Now I'm happy futzing with computer mainboards and the guts of disk drives, but boiler controllers are something I now know just enough about to be dangerous - and feeding 240v down our 2 little wires will, I suspect, be fairly disastrous for our boiler's electronics. Is there a generic name for these low-voltage controllers? Where can I buy one?Why are they so hard to come by?Can I get one with a remote control?Whose round is it next???...

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