air-conditioningfurnacehvacwiring
Trying to add common wire to furnace for smart thermostat. Looking at wiring diagram, it looks like common wire off transformer is blue, switches wire color to white (via wire cap), then back to blue (via connector block), and connects to terminal TH on the ignition control module (SP-735A). Is this understanding correct? Can I just add a wire off the TH terminal (or off combustion blower relay) to get a common?
Yes you can terminate any number of connections on the C terminal. All of the connections will be in parallel. This is very common.
Since in your case, one side of the transformer is grounded. You can simply use a fork or ring terminal, to connect the C wire to the chassis. Though it appears there's already a wire that's attached to ground, and comes right over near the thermostat wiring. I'd just put my C wire in with the other two wires, in that twist-on wire connector near the bottom of the photo.
Best Answer
I am not so sure that the Common of the transformer connects to terminal TH of the ignition control module. It looks more like the TR terminal and it looks like another terminal is labeled TH.
I would encourage you to locate the wire join point depicted below and add your C wire into the existing 3 wire join to make it a 4 wire connection. If you are lucky this may just be a simple wire nut that you can remove. If it is a crimped wire termination then you have a little more work to do cutting it off and then re-stripping the wires to allow use of a proper sized wire nut.