Can you use a full bridge rectifier off of 1 AC line?

OK.... It is possible, however I would not recommend it. I've become very conservative in my old age. If you have 24VAC for your controller, there has to be 120Vac nearby.

I would also:

enable the WDT
Use one of the other relays as an override incase one of the control relays fails ON.

Yes. All voltages should have some connection to ground (building, earth etc). If not they might pickup high voltage and damage some components. Hard to explain.

Do you have the wiring diagram for your HVAC? It will show the internal 24Vac has one lead grounded to the AC frame (which is grounded to the mains ground). On the 24 Volt transformer it matters not which one of the leads is grounded but one has to be.

No you cannot get power from connecting a bridge to 1 wire, that would be a form of perpetual motion. For a bridge to work it needs a voltage potential, with 1 wire you will not get it. The system is a 24V AC system and the 24VAC controls are NOT grounded in many systems. You will find the frame grounded but this is not connected to the secondary side of the transformer. This non grounded configuration has been repeated millions of times through the world. This is changing slowly but if it was not a grounded system keep it isolated.

I don't read pretty pictures. Draw schematic.

Would you really expect this light to work? This is exactly how you are proposing to power the Arduino.
light

So lets try to make an example not using electronics.

You are in the ISS space station. There is no gravity. You are floating around and not anchored or pushing against anything.

Now you reach out your finger and try to slowly push a button. You fail because there is no connection between you and the panel the button is mounted to.

Same with a battery and light. To "push" current out the + you need to have the - connected to the same circuit.

In this drawing the blue wire can be considered the "common" wire.

Not the best example. :slight_smile: First, gravity is everywhere even when you feel weightless. And you can push a button in space, but you will float away as well. You have a lot more mass than the button. Your inertia and the inertia from the ISS will allow you to overcome the force of most common button switches. I have seen pictures of push buttons on the ISS. I am certain you can push them when you are floating. :slight_smile:

Hence I specified push the botton slowy. This way inertia Would play an minimum roll.

Try to CAREFULLY pull the cable out of the wall to see if the installer just folded the 5th wire back (smart) instead of snipping it off (dumb but typical), you might still have to connect the 5th wire to the 24VAC common at the furnace.
CAUTION: If you have to disconnect the wires from the thermostat, be sure to tie a "retrieval" string to it first so it can't "escape" back into the wall.

ok, but this would only provide a 24VAC current, there doesn't seem to be any ground in the system.

Can you show a picture of the back of the thermostat?

Voltage is relative with GND.
"24V" indicates that it is 24V when viewed from the GND of that circuit, and there is no absolute 24V from any circuit.
In other words, 24V can exist because there is GND.

EDIT:
It should also be noted that circuit GND is a definitional.

For example, a 1.5V AA battery measures -1.5V from the negative pole in a circuit where the positive pole is 0V (GND).
On the contrary, when the negative pole is 0V (GND), 1.5V is measured at the positive pole.

0V (GND) is always required for your 24V.

I see.
I was checking the terminals on my multimeter, but I didn't see any difference in the voltage. There is certainly a ground in the circuit, just not in the terminals I was measuring


Here is the back of the thermostat
the left side is a battery compartment. I ma pretty sure I haven't replaced the batteries, (2x AA), for years, so I don't know how the thermostat powers itself with no "common" wire/ground. it surely can't last purely on battery power for 5+ years

So it is battery powered but you want to switch it to using the power line intended to trigger the components. So yes, the way it's wired there is no clean ground as it is not needed.
You can check to see if there is another wire, as other's have suggested, then use it as a ground though you'll probably have to connect it.
Or you can run off batteries like your thermostat has been.
Why the big relays, isn't there a more efficient solid state relay or even a MOSFET solution that won't kill your batteries?

Not the best example.

I suggest you come up with a better one than pick apart my attempt to communicate a difficult concept (or a beginner).

Really, here we are trying to help the OP and you criticize my attempt to explain in a different manner with minutia?? Even at that if you are a scientist, perhaps. If you are an engineer, NO.

I understand the mass principle hence my qualification "slowly push a button".

I would be tempted to put an Arduino at both ends, use two of the wires to provide 24VAC to the thermostat and the other two for two-way communication between the Arduinos. Then the thermostat Arduino can use commands to the other Arduino to turn the heater, cooler, and fan on an off.

Better yet, put the batteries and charger by the furnace and send 5V up the wires to the thermostat.

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OK, then name the common from the transformer "ground" if that makes you happy. Or Fred or Shirley. The HVAC people call it "Control".

He did in #14.

No, that's the mounting bracket and wiring harnesses.