Insert Arduino into Thermostat circuit, but no common

Question

I thought I was all ready to replace a thermostat with an Arduino. I used a multi-meter and bridged some connections to discover the following:
1. bridging 230V (live) with Cool turns on the Aircon outside unit
2. bridging 230V with F2 (brown) turns on internal fan on LOW

Now I would like to insert an Arduino into this circuit, powered from the 230V (using AC to DC module) and not from a battery. The Arduino will control a relay module. Then I realized I do not have a common ground for the Arduino (actually NodeMCU).

The only way to have power run through this circuit is to bridge 230V with either Cool, F1, F2 or F3. I would like to have the Arduino powered on even if the Airco / Fan are not on, since the Arduino should control the relay.

I have two questions:
1. can I use one of the Fan lines as a "common" since it is lower voltage than the live wire?

Like this:

I assume that in this case there may be some "trickle" of power running from Live to Fan through the AC-DC module. As long as it does not consume a lot of power and does not break the fan I would not care too much. Is this right or am I making a mistake here?

2. Are there other options to make this work, without running further wires or using a battery?
   

Answer 1

It might be instructive to read this FAQ entry from Nest, makers of a smart thermostat, about the C wire:

https://nest.com/support/article/When-Nest-needs-a-common-C-wire#wire-may-thermostat

As far as I can tell, what Nest does when "power stealing" -- that is, operating without a C wire -- is charge an internal battery from power that it draws between the 230v and heat/cool wires. This is somewhat similar to what you're proposing, but note: (1) Depending on your fan, it might run while you "steal" power through it, or stop and start, or heat up (because it doesn't normally pass current while not blowing), or something else unexpected. (2) When you do want to call for the fan to actually blow, you will have to short together the 230v and fan wires, at which point you can't draw more power through them: now you see why the Nest has a battery.

The Nest engineers probably spent a lot of time fine-tuning this and playing with different models of heating and AC units to try to get it right; and they still have an FAQ entry noting that, when using power stealing: "in some cases, you may experience odd heating or cooling behaviors ... [y]our system is making strange noises: chattering, stuttering, clicking or thumping - this can be caused by your system turning on and off rapidly ... the system fan is always running or won’t turn on [or] turns on and off repeatedly in a short period."

So I would suggest that this is a tricky endeavor best left to the pros. Have you considered that most thermostats run on double-A batteries?

(Also note: Working with 230v is dangerous. It can be lethal. I feel obliged to note that it's not really a great idea unless you're an expert and meticulously following proper practices. Do you know the measurement category of your multimeter? If it's not Cat III, it's not technically even rated to be safe to measure mains power at the socket.)