No C-wire in new home and seems jumper done on wires

[Meloserj]

Hello mates

I moved from my old apartment to new one, and the new apartment has a non smart cheap Thermostat.

I can see 5 wires came in to back plane of old one, and 3 wires used with label and 2 not connected. Below are the wires on old Thermostat:

1. Rc "RED"

2. G "Green"

3. Y "YELLOW"

4. B "BLUE" (Not connected)

5. B "BLACK" (Not connected)

I measured the voltages and below are the result:

Rc---Y (26V AC)

Rc---G (108 mv)

Rc---Blue (3.7V and reducing)

Rc---Black (4.2V and reducing)

However, the fan is not working independently and only turns on when cooling is on. So I am suspecting somewhere on PCB a jumper has been made (or maybe no jumper at all)

Unfortunately I can't see the PCB itself and check on terminals, as its in a really bad location and I prefer first to understand the situation and later call for building maintenance staff.

The AC unit is "Hitachi RAS-5U2S" and I managed to get the wiring diagram from web that I am attaching to this post.

My questions are:

1. How to run fan independently on my nest with the existing wiring (should say that I have connected my nest to wires but although it shows fan is running, but the fan is not starting, BTW I know that I should use ECO mode and try the fan running as nest is not accepting only fan option)

2. How can I get a C-wire, at the moment voltage between Rc-G is almost zero so I think Rc is jumper with G. Can I get the C-wire from the transformer directly or should use a nest converter ?

Thanks for your kind help.

[Patrick_Caezza]

@Meloserj 

Can you post a picture of the current thermostat's wiring here?

[Meloserj]

Here you are bro @Patrick_Caezza 

As I said above, only 3 wires used on old thermostat and 2 black and blue spared. I connected the 3 to my nest 3rd gen as per pic.

[Meloserj · Accepted solution]

Hi mates

As nobody understood my issue, I went up to attic and start tracing cables. I understood that stupid building maintenance person mixed the RC and Y cable, indeed Y was 24v and Rc was cooling! So I corrected them and now fan is working independently as it should when I choose from Nest.

I also managed to make a C-wire and now I am getting 200mA for battery charging.

Hope this helps people who might face the same issue. Never trust your voltmeter in HVAC connection tracing, that's why seeing is believing 🙂

Cheers