Where is 5V ac coming from?

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IndyS

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I have a Danfloss tp9000 programmer and TS2 room sensor which is controlling the Ideal Heat 15 system boiler.

I'm trying to replace it with a smart thermostat by Nest, replicating this set up https://d3vlabs.com/2016/12/09/install-nest-tp9000-ts2-danfoss-3rd-gen-uk/

There are 3 lives, 3 neutrals one grey to call Hot Water T3 and brown to call CH on T4, this is all straight forward more or less. Than there is a brown and black two core cable on terminals T5 and T6 for the remote sensor TS2, which I wanted to re-use to power thermostat via 12V DC supply from heatlink, as I thought this is just a straight connection to TS2.

My problem is when I turn the power off on the boiler I still measure 5V AC on between two cores on room sensor wiring. Even when I disconnect the cable on both ends (sensor and programmer) the ac voltage is there. It goes away obviously when master is off on the board.

I struggle to understand this. If the cable goes somewhere else, why and where? If this is induction then 5V is quite a lot, but sensor is working fine somehow despite this.

I've got a wiring central next to hot water cylinder in cupboard. I tried to find these 5V but there is nothing.
There is also a second zone for bedroom CH, which should not matter I thought, as it runs from another tp5000 and there is no external sensors.

Please help, as I don't really want to try connecting thermostat to try it out.
 

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If you measuring the voltage on a regular digital multimeter, then what you see is induced voltage. 5v is not very much.
If you put a small load across the wires it will disappear.
 
Thanks. Yes, normal Fluke multimeter. This is good point about a load. I will probe it with a resistor. It still shows 5VAC when sensor is connected on other end. 10VAC and 3VAC between eaxh core and earth. So it made no sense to me.

So yiu think it won't interfere with dc supply when connected to power supply and thermostat. I was afraid it was running somewhere else for whatever reason.

Interesting, 5V to me sounded a lot. I used to few mV to uV at most of parasitic.
 
I had the same when I moved in here. House has a combi with wireless room stat, but there was an ancient honeywell stat on opposite wall. When checking before removing the old stat, I was picking up something like 15-20v from memory, maybe even more. I believe 3 core + earth is worse at induced pickup, due to the CPC not being exactly central in terms of the cores.

In my case it was picking up from the upstairs socket circuit which it ran alongside for most of it's route (boiler used to be in back bedroom upstairs).

Hope this helps,

Jon
 
Basically it looks like it picks up a bit from every loop in the house, when I isolate rcds one by one voltage drops away.

Ok. Now the main question will the thermostat and heatlink like such dirty supply line. Any ideas? I connected a 9v battery on one end I read it fine on another, but it still have about 10-8VAC parasitics on top. Is it safe for 12VDC powe supply?
 
All is solved now. Thanks everybody. I've connected the heatlink and nest thermostat via that existing sensor cable after all, as laying another cable is out of question. I thought if the passive thermistor sensor circuit could work fine like that, so dc power supply should be also ok. It works so far, let see how long:).
 
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