Nest on Ice Air 8RSXC18 heat pump PTAC: R reads DC, not 24VAC. Relays + separate transformer?

kadinrabo

Renter here. I'm trying to swap the Riseem thermostat on my Ice Air PTAC for a Nest (4th gen) and ran into a power issue. I saw the older thread here about a Nest on an Ice Air PTAC, but my situation looks a bit different. Pics attached: my before/after wiring diagram, the unit's wiring label, the board, and the thermostat wiring.

Unit info: Ice Air 8RSXC18 heat pump, chassis ELA-13347, logic V3.3_20230817. The control label says WG7010010001400 and the main board is Wellgo WG3220010004100.

What's going on:
The wiring label says R is 24VAC, but when I measure R to C I get 20+ V on DC, around 6 V on AC, and about 0 Hz. Same reading at the board and at the thermostat. The Nest shows "No power" even with C hooked up.

The Riseem works fine because it runs on batteries/power stealing and doesn't need C. It's basically just a switch connecting R to Y, W, etc. If I jump R to Y or R to W at the thermostat wires, the unit runs, fan and all.

From what I can find, Ice Air's own factory Nest option lists a "12V DC control signal," so I'm guessing DC on R is normal for this unit and not a fault.

What I want to do:

• Plug-in 24VAC transformer to power the Nest by itself
• The Nest's Y, W and G outputs each drive a relay (Functional Devices RIBU1C)
• Each relay just connects the board's own R to Y, W or F3, the same thing the Riseem does now
• The transformer side and the PTAC side never touch. Board C stays disconnected

Question:
Would this work, and is there anything I'm missing or should watch out for? I'm also a little unsure whether the relay contacts will switch reliably at whatever tiny current these inputs draw.

Thanks in advance!

109A_5

Well I can't find anything that supports your claims of DC control voltage, everything I read claims AC, did you try a second meter ? I'd like to read the information for my self, any links ?

Anyway if you want to power the NEST with a transformer and buffer the control functions with relays, that should work.

The RIBU1C contacts are rated at 28 VDC. So it should work.

With Direct Current contact bounce (upon contacts closing) and when the contacts open causes arcing. By limiting the DC voltage it keeps the contacts from burning up due to excessive arcing.

HVACNUT

Have you asked the landlord or building manager if they know how?