no yellow wire from air exchange (and no 24v)

[prg281]

Hi everybody,

There is no yellow wire coming out of the exchange unit and I couldn't find 24V on any of these wires.

The current (dump) thermostat has a yellow wire connected to the yellow of the outdoor AC unit.

The air exchange unit black wire is connected to the second outdoor AC unit wire.

I think I need to put a C-wire adapter, but I don't know what to do about the adpater's yellow wire that's just left unused.

Here is the air exchange wiring diagram showing no yellow wire.

Any thoughts?

[Patrick_Caezza]

@prg281 

That looks to be a heat-only unit that wouldn't have a yellow Y wire which is for cooling.

How was the original thermostat wired? Can you post a picture of it here?

If you put a multimeter between the Red and Black thermostat wires you should read 24vac. If it doesn't, then check the 3.5 amp fuse.

[prg281]

Thanks @Patrick_Caezza.

Here are the wiring pictures.
Thermostat yellow is connected to AC (not AX); AX black is connected to the second AC wire.
So AX to thermostat is only G, W, R.

I'll check the fuse but both heat and cool modes are functioning; could the fuse be bad without impacting the existing config?

[prg281]

Hi  @Patrick_Caezza, here is some additional info:

- the fuse appears fine (0.1ohm)

- there is a label on the air exchange about the transformer (see attached); could the transformer be bad even though the system is functioning correctly with a dumb thermostat? Is there a way to test the transformer without power?