Add a c-wire to 25+ year old Rheem furnace

Judging by the Honeywell R7184A Controller manual, you have one of these:

You described it as terminal 4 but the diagram just shows two terminals labelled "T", but that is fine:

I found a manual for an EnviraCom device which shows terminals 2 and 3 are 24vac power:

This means we have the right connections, and according to the R7184 manual:

EnviraCOM™ Current Available: 150 mA

So the liming factor here is simply the current available. I can't find any specs at all for the thermostat you posted, but so long as it needs 150 mA or less (at 24 Vac) then it should work. You would make the following connections:

Burner             Thermostat     Desc
Terminal 4 T       W              Heating call
Terminal 3 T       R or Rh        24Vac
Terminal 2         C              24Vac "Common"

Note: your current wiring may not have W and R connected correctly, because with the two-wire system it doesn't matter. Now that you need a C wire, it is important to have R connected to constant power. If wrong, your thermostat simply won't get power.

If your thermostat draws more than 150 mA, you're going to run into various strange problems that may range from occasional glitches to your burner not working at all, and I'd highly advise against doing this.

If you do need more than 150mA, normally you could upgrade the transformer -- but in this case, it's all an integrated solid-state unit. I'm actually not sure you could wire this up without damaging the burner controller. The safest thing would be to use a separate circuit with a relay, but that is far beyond the original scope so I won't post how do to that unless necessary.

After looking closely at the images you've supplied, this is what it appears is going on in the last photo.

From what I know about HVAC systems, I would guess a broader view of the system would look something like this.

• The transformer supplies power to the thermostat through the RED wire.
• The BLUE wire serves as the COOL call from the thermostat.
• The BLUE wire connects to the WHITE wire, which is connected to the coil of the contactor in the condensing unit.
• The GREEN wire is the other side of the contactor coil.
• The GREEN wire connects to the BROWN wire, which is the other side of the transformer (C).

When the thermostat calls for COOL, the BLUE wire is energized. This energizes the WHITE wire leading to the contactor, which pulls the contactor closed. The GREEN wire connects to the BROWN wire, which leads back to the transformer, thus completing the circuit.

Therefore, if you want to use the YELLOW wire as C. You should include it in the twist-on wire connector with the GREEN and BROWN wire.

You'll just want to make sure the transformer is sized properly to supply the additional current draw of the thermostat.

If it were me, I'd rewire it so that the YELLOW wire was the COOL call, and the BLUE wire was C. As that's a more common color coding.