This system is from the 60's era – the furnace has a two T connectors for the R and W (thermostat) and two F connectors – one labeled "Thermostat Load 24VAC 0.2AMP, the other for the Burner Motor Ignition. I want to use a Honeywell Sensi wifi thermostat and have no problem running a new wire to the thermostat for "C", but where would I connect it at the furnace considering the limited connectors (T T F F)? Thanks!
Old oil forced air heat only system (R/W wire only) – Is adding a C possilbe to use Wi-Fi
thermostat-c-wire
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.
First off, the green wire should connect to the G terminal on the thermostat. It's connected to the G terminal in the furnace, and so should also go to the G terminal on the thermostat.
At the thermostat, the extra blue wire should connect to the C terminal. Depending on the furnace (read the documentation, and analyze the schematic to be sure), you should be able to connect the other end of the blue wire to either of the C terminals in the furnace.
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Best Answer
This is an interesting case since your oil burner control integrates the 24VAC control transformer. Normally, you'd look for either a C terminal on the control, or the control transformer itself. However, your control lacks a C terminal. What it does have, though, according to the internal schematics from Honeywell's manual, is one of the F terminals for the flame safety photodetector (CdS cell) that goes back to the opposite side of the transformer from the upper T terminal that goes to the red wire.
You'll need a multimeter on continuity (or its lowest ohms range if you don't have a continuity buzzer on your meter) mode for this procedure.