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I need a C-wire for my thermostat but my heater only has R and W connectors (red circle). On the schematic on the heater, up top (red arrow), looks like:

5 is 24V Gnd (bit hard to read but i've double checked that's what it says) and

6 is 24V

I assume the 6 24V is the same as the R connector.

So would it be safe to assume that 5 would be the equivilent of a C-Wire?

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UPDATE: after some hunting I managed to locate the service manual :) I've included a page with the circuit overview:

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And have revised my though on a candidate spot to tap the C-Wire. Based on the text in the green box and the comments, I am assuming the top right of the middle connector along the bottoms (with the green arrow) would be the best location.

Update #2: I've now located the block diagram for the S8065 INTERMITTENT PILOT MODULES (block at the top of the picture). And from it, one can clearly see that connectors 2, 4 and 5 and joined together. I confirmed this with a continuity test and they 2, 4 and 5 and the chasis of the unit are all connected.

I then checked the potenital between these points and the R connector and it was 29.5VAC.

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Just one last question.. Is it normal that the voltage is 29.5VAC, this seems quite a bit higher than 24VAC. I just wouldn't want to fry a modern thermostat!

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  • Hello, and welcome to Home Improvement. I wouldn't assume that; for all you know there's a large voltage between the supplies. – Daniel Griscom Aug 14 '20 at 11:02
  • It's certainly not clear from the diagram whether the pin #5 you are referring to is an INPUT or an OUTPUT. The "C" wire is the "common" side of the 24V transformer output. Assuming the transformer in the diagram is a 240:24 step down, I suspect the point you are looking for is one of the outer two wires coming from the bottom side of the transformer. Take a few measurements with a voltmeter and you should see it. – jwh20 Aug 14 '20 at 12:11
  • Hi @jwh20, as stated in my edit above, I've found the service manual. would you agree that the location with the green arrow seems like a better spot? I will do as you suggest, some checks with my multimeter – Guy Taylor Aug 14 '20 at 13:13
  • Thanks @DanielGriscom! From the circuit from the service manual, it appears the 'R' connector and the point marked with the green orginate from the same supply. I assume if I measured the potential across those 2 points and it's 24V, that I should be ok? – Guy Taylor Aug 14 '20 at 13:23
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We've seen exactly the same wiring diagram.. with almost the same question asked.. over there: Where to connect C wire if no C terminal. But you've found additional information about the appliance controls!

It looks like the pin 6 you've identified is not always-on 24V. Instead, it is the switched line coming from the thermostat's call for heat.

The text from the service manual does lead one to believe that pin 5 is the common terminal you're looking for. You can confirm it with a meter by measuring voltage from pin 5 to the R terminal, or you might consider the answer I gave on that other question.

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  • Ha! it looks exactly the same! Based on finding the new block diagram I posted above, it looks like the 3 connectors do indeed lead back to the other side of the transformr – Guy Taylor Aug 14 '20 at 23:46
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It looks like the word common Or ground are in the boxes. I would double check with a meter just to be sure, or trace the transformer connection on the board to the transformer low voltage connection they are probably bonded and in that case it would not matter which one you chose. But if not bonded you need to be on the transformer not the ground.

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  • Yes, I double checked and it was 29.5VAC from 'R' to all the points mentioned above – Guy Taylor Aug 14 '20 at 23:49
  • I just wanted to make sure I figured they were bonded and it would not matter but better safe than sorry. So both locations are the same electrically. 29.5 is the unloaded value when the transformer has its rated load the voltage will drop this normal and the components are designed for this. – Ed Beal Aug 15 '20 at 2:56
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That "System Circuitry" diagram is wrong which leaves us guessing.

The problem is that one one of the transformer wires is shown as connected to anything, and it takes 2 terminals to get power.

I suspect the error is that the red line from the top of the 2A fuse is connected to the wrong transformer terminal. I suspect this because a fuse with one end grounded is most unusual and unlikely to be reliable protection.

If that's the case you can use the ground of the case or the terminal indicated by the green arrow as the C terminal. C is usually ground

Measure the AC voltage from the bottom of the fuse to the case ground if you get an answer that's close to 24 case ground is the common terminal.

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  • I agree, it looks like their red wire is connected to ground which would be a short! I checked continuity between the green arrow, pins 2,4, 5 and the chassis and they were all connected – Guy Taylor Aug 14 '20 at 23:48

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