C wire not charging - wrong voltage when connected to Nest base

[rsworden]

New Nest Learning Thermostat 3rd gen. Existing Amana AMV81155CXA heating (2-stage) and cooling (apparently 1-stage) system in a house I just moved to. I installed the Nest following instructions on the app and it worked for a few days but drained the battery.

Wiring from the HVAC to the Nest: Y to Y1, G to G, R to Rc, W1 to W1. C was not wired. There is no terminal labeled simple "C" in the furnace. I used a spare wire in the cable and connected HVAC terminal labeled "B/C" to Nest "C".

At the Nest base I measure about 27 VAC between the exposed wire plugged in to Rc, and the C wire before connecting the C wire to the base. After inserting C wire into the C hole on the Nest base, I measure the exposed C wire vs. exposed Rc wire and see about 1.5 VAC.

I tried both spare wires in the cable and got the same result, so the problem is not the wire.

I read about the Nest base's 2-step connectors and made sure all are fully connected.

The Nest Equipment display does recognize that C is connected but gives error E79 No Voltage.

[CoolingWizard]

@rsworden every HVAC system has a common side of a transformer and you need to basically send me a picture of the inside of your furnace control board so I can help you find the connection point for the C wire.

AC Cooling Wizard

[rsworden]

It's hard to get a clear shot due to all the wires, but here are several angles plus the schematic. Thanks in advance!

[rsworden]

On the schematic there's a transformer with 24VAC out on Red and Blue, no indication of which is which. I tapped the Blue and compared it to frame ground, and got 0.001V. I tapped the Red and compared it to frame ground, and got 28.x VAC. So I assume the Red can be used to power the C wire.

[CoolingWizard]

@rsworden , in the HVAC control system sometimes the common is on the B terminal, sometimes it’s on the C terminal. The fact that it lists B/C reflects that it’s both. But that is indeed the terminal that you’re looking for. 

AC Cooling Wizard

[rsworden]

Well, as I mentioned originally, I tried B/C first and it did not work. Other info about the Amana system seems to indicate B/C is involved in other equipment, like a dehumidifier, so it may not be powered constantly.

I found the transformer and based on other notes I tried the Blue 24V lead. It showed 0V to the frame, which sounds line the usual definition of "Common". But in this HVAC world they seem to be using "Common" to mean "Constant". Wiring the Blue to the thermostat C wire did not work, the Nest says there is no power on it. The Red lead shows 28VAC to the frame, but no other notes are ever saying to use the Red lead. Rather than mess something up, I have a tech coming on site Friday, from the company who services this unit and knows Amana.

[rsworden]

Technician confirmed that the B/C terminal was acting a Common for both the heat and air, so it was correct to use it for the thermostat too. What he found that I had not was that the thermostat cable had been extended twice and that one of the connections was not good. (I had found one extension near the unit but not the one near the room entry.) When that was solidified the Nest was able to get power on its C terminal, so now it's charging.

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