No power, questions about HVAC wires

I recently purchased a used HestiaPi from @terrarium that he listed on eBay.

I printed a base/case for it and tried hooking it up the other day. My current thermostat is an analog unit, so when I pulled everything off, I didn’t see the blue common wire.

Pulling the old wall mount off revealed a group of wires that were not used

I wired up the unit for US/HVAC, turned the breaker back on and got nothing. No LED from the Pi, no LCD turning on, nothing. Since the design of the PCB blocks the microUSB port, and I don’t have any pi power hats, I wasn’t able to test the board by itself. So I got out my multi-meter to test the cables from the HVAC.

If I turn the breaker on and connect the red to the green, the fan turns on and I register 24~25V. That seems correct. However, connecting the Red to Blue gives me only 3.3V. Connecting the Red to Black or Brown gives me ~1.1V.

The blue should be wired to common/C, right? And that should give me a full 24V AC if I run it through a tester, right? Does the 3.3V indicate it’s not connected correctly? I only have a crawl space, so I guess I should get back under the house and try to find where all this stuff is wired up at?

I guess I could also test the Pi itself by wiring the green (fan) to C, right? It would turn the fan on, yes, but it would at least allow me to make sure the Pi gets power and boots. Then I could diagnose the wiring issue?

Before you test your wires and know which is which I would suggest to leave HestiaPi completely disconnected.
Some houses follow different color code. Measure with the multimeter in Voltage position and AC mode the voltage between red and black without shorting any wires first. I cannot tell if two of your wires are actually black.
Your original thermostat is a passive thermostat that means it only needs to short two wires to turn heating on without requiring a common wire present. If you cannot get 24V AC anywhere it would be possible that no common line is actually connected to the other end of those wires.
If you get 24V AC turn the breaker back off and then continue with the wiring C-BLACK, R-RED, W-WHITE, Y-YELLOW and G-GREEN and turn breaker on.
Please report here your findings

I thought I mentioned all that. I get 24V between Red and Green (24V->Fan), but only 3.3V from Red to Blue and ~1V from Red to Black.

So I think it’s not connected at the control box under the house. Will go splunking for it later.

Oh, I didn’t read it that way. Sorry. At least you have enough wires coming out of the wall :slight_smile: Some people have removed the old thermostat and find only 2 wires which means you would to find 24V AC or mains elsewhere.
BTW 24V AC between Red and Green but turning the fan on when you sort them means you cannot draw (enough) power off these 2 wires for Hestia.