Nest gen3 with Danfloss tp9000/TS2 on two zone system boiler

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Few people already posted about installing Nest heatlink and termostat to replace Danfloss tp9000 and TS2 room sensor system. So I thought this going to be a straight job, but I've got some surprises.

Basically when I removed the tp9000 programmer in the kitchen I've got 3 brown neutrals on N , 3 blue lives on L, grey - hot water on 3, brown - CH on 4, and another brown and black wires connected to remote sensor terminals 5 and 6. Then I've got a remote room sensor danfloss TS2 in the hallway, which has also brown and black wires connected to it. Which I thought easy peasy. I connect these existing wires to Nest: common lives on L,2,5, neutrals to N, and heating and hot water on 3 and 6 on the Nest. Then I had a 'clever' idea to use existing brown and black from programmer to power the Nest thermostat from 12V supply T1/T2 on Heatlink and screw the termostat in place of the room sensor in the hallway. So no need for gastly usb cable on the wall. And this is where I've got stuck.

When I turn the power to the boiler off I still have 5VAC between terminals 5 and 6 on TP9000 back plate and those wires do not go, directly at least, to the TS2 remote sensor in the hall. Because when I disconnect both ends, programmer and remote sensor, I still read about 5VAC on both ends. I'm puzzled. Anybody has an idea why and where from?

I do have a wiring centre up in the cupboard, togethet with a hot water cylinder. I tried to locate 5V AC there, but with no avail. The system boiler is Ideal Heat 15. And there are 2 zone valves, second zone is just for CH master bedroom on danfloss tp5000 switch. None if which I thought should matter for the TS2 connection in tp9000. Obviously I'm wrong somewhere. Help.
 

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Because when I disconnect both ends, programmer and remote sensor, I still read about 5VAC on both ends. I'm puzzled. Anybody has an idea why and where from?

Induction maybe from another circuit, what happens if you turn off the mains supply to the entire property?
 
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I can isolate the whole house or boiler, rest of rcds are just sockets and lights, shower etc. When I flick the master no power is anywhere.

I just don't undestand why one would have the voltage on this circuit. Induction is a good idea, nornally sensor wiring should run separate from mains to avoid interference. But the sensor temperatue reads fine from tp9000, so it can't be that wrong. I just don't want to burn heatlink or thermostat by connecting it to live or parasitic circuit.

The only live voltage I can find when the boiler is off is few volts somehwre on the two pumps? on the hot water tank. But nothing in the wiring centre, if I not missed anything.
 
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It's not unusual for a cable to pick up a few volts by induction, usually because there is another cable running parallel to it for a few metres. Sound equipment cables tend to highlight it because it can show up as a hum over the loudspeakers, so to prevent it signal cables have a grounded screen or braid.

The idea of turning off the all of the power was to see if the 5v disappears and assuming that it does, then by turning the circuits back on one by one until it reappears might help you identify the source.
 
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Thanks. Few volts is quite a lot of emf in my undestanding for parasitics. It is about 15 m cable to the sensor though. Another thing which points to inductive pick up is that this is ac voltage and 5VAC is measured between two cores, but between each core and earth there is 10 and 3 VAC. Which makes no sense again.

Instructions for TS2 specifically says to avoid laying the sensor cable next to power. But maybe the filter on sensor input can deal with ac voltage bias.

What would happen if I connect this to Heatlink and Thermostat which run 12 VDC supply, this is another question. Which I'm not keen to try.
 
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Good question...I struggle to see how or why anyone wiring the original remote sensor directly to the TP9000 would also connect it to something else, and after all it it did work previously. Or at least I assume it did.

If you are worried about damaging the Heatlink, and I totally get that, then you could run a new cable. Or you could try posting the question on the electrical forum. There are some smart guys on there. If you post a thread with a title something along the lines of "Where is this 5V coming from?" By avoiding reference to Nest or TP9000 in the title, you will probably have a better chance of a reply as it doesn't give the appearance you are looking for advice about installing a Nest. Replying to which can be tedious and can turn some folks off reading it.
 
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Thanks. I asked on electeical forum.

I do not relly want to run a new cable, this would be a major job. It is about 10m through four walls, kitchen units, toilet etc to go through. A lot of plasteting too. Yuck.
I will spend weekend playing with circuits to see if i can get rid of it.
 
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Just to say it was parasitic pick up at the end. I isolated rcds one by one and voltage gradually droped. I put also a 9v battery on one end to test, and ot was fine.

So after contemplating a bit and since laying new cable is out of question, I've connected power supply from heatlink to thermostat using that cable anyway. All good so far. So thanks everybody for advice.
 

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