Does this seem like the right place to tap a C-Wire from my heater?

Question

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?

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

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.

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!

Ed Beal · Answer

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.

Answered by Ed Beal on May 29, 2021

Greg Hill · Answer

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.

Jasen · Answer

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.

Does this seem like the right place to tap a C-Wire from my heater?

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