Jesse Grandbois is one of the techs who reads the tech tips and he wrote a few tips that he wanted to share on some gas furnace control basics. This tip is about the basic terminal designations on typical 24v gas valve. Thanks Jesse!

Have you ever noticed the TH / TR terminal on a gas valve?. When I was in school, I’ll admit I didn’t know what it was. I’ve worked with people that are experienced and still think you need to use it. 
Nobody ever explained to me what it was. Being a newbie at the time it never clicked when I looked at the wiring diagrams. All my teachers would give the same response, “it’s a common.” 
That’s where the confusion set in. It’s a common. We throw this word around like it’s going out of style it seems in the HVAC industry. Everything is a “common” and technically a common is just a “common” point of connection but it really doesn’t clear up it’s intended purpose, or what part of the circuit it is intended for.
Following the diagram below while reading the rest of the article should clear things up.
Lets look at what each of these terminals mean:
TH – The 24v hot leg from the thermostat on a call for heat (R+W closing) to the gas valve (TH terminal) to open the solenoid to allow gas to flow. This is assuming the transformer is good and the high limit is closed.
TR – The 24v common/return side of the transformer.
TH/TR – This is not internally wired to the gas valve. Not using this makes no difference to the circuit. You could run your 24v hot from the transformer directly to your NC high limit in the above example and be just fine. This is nothing more than a convenience terminal. Joining these wires with a marette (Wire nut or terminal multiplier) does the same thing as this terminal.
Hope this helps clear up any confusion.
— Jesse
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