Really hope someone can help me with this. I've looked through the other posts and can't find one similar enough to mine to answer. I've got an old gas furnace + AC and i'm installing a Sensi Wifi theromostat. There's only 4 wires and I cannot figure out how to do the G wire to C conversion because my furnace has no terminals. It looks like someone added a 24v transformer at some point and i think i can just tap into that for my new theromostat but i don't want to connect it incorrectly and damage something. I can easily run a new thermostat wire that will have 5 wires, but i'm unsure where to attached that new C wire. Any help would be really appreciated.
WiFi Thermostat, No C Terminal on Furnace
hvac
Related Solutions
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.
Get rid of the external transformer, and pull a proper C wire.
It's not working because you have one side of the contactor coil connected to one transformer, and the other side connected to another transformer. There's not a complete circuit.
When the thermostat is calling for cool, your circuit looks like this
If you follow the circuit, you can see that it's not a complete circuit.
I've highlighted the R wire in red, and the C wire in blue.
Best Answer
Wirenut A is where you should tap C at
Since you have air conditioning, we can pull a little trick here. We start with the yellow Y wire from the thermostat, noting that it is connected to one wire going off to the outdoor unit (condenser) via wirenut E. Since we know that the thermostat is putting 24VAC from R onto Y to call for compressor, we know that the other wire coming back from the outdoor unit must be a C wire to complete the circuit. Following it, we come to wirenut A, which just so happens to be quite a convenient place to make the thermostat C-wire connection at.