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TH, TR, and TH/TR Gas Valve Terminals
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 a 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 who 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. In the HVAC industry, it seems like we throw this word around like it's going out of style. Everything is a “common,” and technically, a common is just a “common” point of connection, but it really doesn't clear up its 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.
Let's look at what each of these terminals means:
- 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 that 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.
I hope this helps clear up any confusion.
Nice work Jesse. I’ll be using this simple tip for a five minute tech training.
Nice. Where are those five minute tech training tips located?
On milivolt powerpile gas valves replace the TH-TR with TH-TP and the TR marking with TP. The TH terminal will remain the same.
How do you hook up a WiFi thermostat that requires the C wire? Thanks
To answer your question you need to understand a few basics:
– TR (TRansformer) terminal is the common (or neutral, which carriers current back to the source). The C wire for the thermostat could be connected here or it could be connected directly to the transformer’s common terminal (the same terminal the TR terminal on the gas valve is already connected to.) ***If the power to the thermostat cuts out during colder months when you’re heating regularly, see my comments below.
– TH-TR (THermostat-TRansformer) terminal is the 24VAC from the transformer. (Important to note that this terminal does nothing for the gas valve. Its only purpose in life is to connect the R terminal on the thermostat to the 24VAC terminal on the transformer. It serves no real purpose for the gas valve’s operation. You it’s easier, you could wire the thermostat directly to the transformer which is often the case with newer gas valves that don’t have the TH-TR terminal at all.)
– TH (THermostat) terminal is the 24VAC return from the thermostat to the N/O (Normally Open) relay switch on the gas-value. When the thermostat calls for heat (using the W terminal), 24VAC is sent to power the relay which closes it as current flows through the relay coil, out the TR terminal and back to the common terminal on the transformer.
*** So the power to your thermostat is cutting out…
The diagram shown will result in the same problem as what I’m about to explain below… the power to the thermostat will be disconnected if the limit control relay is activated!
Anyway, I had this issue because of a safety feature; The Fan/Limit Control. It’s amazing how little information there is on datasheets assuming you can actually find one.
Essentially, a fan/limit control is two temperature controlled relays. If the temperature is within the range you’ve set with the adjustment levers on the dial, the N/O (normally open) fan relay will close and turn on the fan and/or the N/C (normally closed) limit relay will open and turn off whatever it’s controlling by literally disconnecting it.
I have a Honeywell L4064 Fan/Limit Control which can be configured 3 ways according to Honeywell’s datasheet:
A: Limit in low voltage circuit
B: Limit in line voltage circuit
C: Limit in line voltage circuit without jumper
My furnace was configured with option B (Option C is essentially the same thing when you really look at it, they’re just tying the hot leg of the fan and limit relays together, which is just another way to connect or jumper the two terminals together). The fan control is what it is, it works and isn’t affected if the limit control is activated. The limit control on the other hand was connected to the line voltage (or 115/120VAC) which is basically like putting an on/off switch directly in the path of the hot wire (black wire) before it reaches the transformer. If the limit control is active, no power reaches the transformer and the 24VAC on the other side of the transformer will be off as well.
If you have the same setup, you’ll need to change to Option A. To do this, TURN YOUR FURNACE BREAKER OFF FIRST, then you remove the jumper from the Fan/Limit control. This makes it possible to control low voltage devices with the limit control and still control the fan with line voltage. Next, you find the (typically red) wire that runs from the limit control side and the transformer. On the transformer end, disconnect the two wires and reconnect the transformer wire to the hot (black) wire. On the limit control end, remove the wire from the terminal using a small flathead screwdriver to release it. Next, disconnect the “W” wire from your thermostat to the “TH” terminal on your gas valve and reconnect it in the same terminal on the limit control that you previous removed the (red) wire from. Finally, connect another wire from the other terminal on the limit control side to the “TH” terminal on your gas valve. Done.
You should now have a transformer the will always be powered on no matter what. The common is connected to the C on the thermostat and the TR on the gas valve. The 24VAC is connected to the (optional TH-TR and on to the) R on the thermostat. At this point, the thermostat has 24VAC power and a C (common) that completes the circuit from the transformer so it will stay powered on, as long as the transformer is on. Finally the W on the thermostat is connected to the N/C (normally closed) limit control relay which is finally connected to the TH on the gas valve. When the furnace gets too hot and reaches the temperature the limit stop is set to, the limit relay will open and disconnect the W on the thermostat from the TH on the gas valve which prevents the thermostat from turning the gas valve on, effectively preventing your furnace from heating until the limit control relay detects the temperature has dropped low enough and closes the limit relay again to continue normal operation.
Hope this helps!
Thanks for your detailed response on this subject. I am attempting to install a NEST too and don’t have a C wire coming to the thermostat. My fan control unit is also a L4064B, which already has the jumper removed. Any chance you could provide images or further assistance on the wiring from the limit side to the transformer?
I’m also interested in making this work