How do I connect a WiFi thermostat to my furnace's primary control?

Question

I would like to install a Honeywell WiFi controlled thermostat to connect to a Honeywell R7184U primary control. My question concerns the "C" wire. The R7184 has the terminals labeled 1, 2, 3 and 4 with the T1 & T2 being terminals 1 and 2. Which terminal do I connect the "C" wire to? It looks like I have 24V coming at terminal #3.

Edit: Sorry, my error the current thermostat wires connect, white wire to terminal 4, and red wire to terminal 3. The new thermostat is a low amp draw which has to be powered all the time, even if the furnace is on or off, so it can communicate with a router.

Answer 1

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

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

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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.

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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.

Answer 2

So, I have an original(?) Honeywell controller. It has two T terminals, and two F terminals:

The T's go to the thermostat... one provides constant 25v (or higher, mine is 30v), the other is the return to complete the circuit. The constant 25v ties directly to one terminal on the low voltage side of the transformer inside the controler, the return ties into the internal components (limit switches, relays, etc.) that control the furnace components (motors, ignition, etc.)

The F's go to the flame sensor. One terminal is likewise tied into the internal components. However, the other is tied directly to the OTHER terminal of the low voltage side of the transformer.

You should be able to find the electronic schematic for newer controllers in the technical documents on the Honeywell website. Here is the manual for the R8184G Protectorelay controller:
https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-0617.pdf

As you can see on page 1 (refer to following image), the schematic shows direct lines to the low voltage side of the transformer to the terminals as I described above. From the schematic, the outer two terminals are the two terminals of interest. I probed the outermost two terminals on my older vintage controller with a multimeter, and sure enough, I got 30v.

What does this mean? It means you can use the outer F terminal as a C wire for thermostats that need a C wire. I have a Honeywell RTH6580WF WiFi thermostat that I'm trying to get working with my ancient furnace.

Luckily, I had an unused third wire (green) in my bundle, so I attached it to outer F at the furnace to use as a C wire. The red wire is attached to the outer T, and the white to the inner T. The red & white were originally reversed, so I swapped them at the furnace. So, red is in essence tied to the transformer, and carries constant power.

At the thermostat, the white simply attaches to W (heat), the red attaches to R (power), and the green attaches to C (common). Since the thermostat also needs power to Rc, I needed to use the jumper from R to Rc. For some reason, my thermostat did not include the factory made jumper (it looks like a small staple), so I just used a short piece of wire (the little U-shaped green wire).

Presto-chango, thermostat powers on.