hvacthermostat-c-wirewiring
I want see if someone help me on hooking up this new Honeywell thermostat with Wifi I just purchased. I have an old ac unit and I only have 3 connections to my circuit board which does not have an option to connect my wire. Any help would be appreciated. Thanks
You guys down voted the only answer that is correct and up voted one that is completely wrong!!! You CANNOT use just one wire from a transformer and expect to get it to do a thing for you. The simple answer to your question is that if you add a transformer you want to tie the lines from it to Rh and C. Most modern thermostats I know of expect to pull power between Rh and C. You do NOT want to tie Rh and Rc together since your heating and cooling are on separate transformers and you don't know if they are truly floating.
The complex answer is that you should check to see if the control board of your heating system doesn't, in fact, have a common terminal and nobody bothered to run the wires for it. If it does, it might be simpler to just pull 18/3 wire from the heater to the thermostat. Depending on the run, you can use the old two conductor wire to pull the three conductor wire through.
First off, the green wire should connect to the G terminal on the thermostat. It's connected to the G terminal in the furnace, and so should also go to the G terminal on the thermostat.
At the thermostat, the extra blue wire should connect to the C terminal. Depending on the furnace (read the documentation, and analyze the schematic to be sure), you should be able to connect the other end of the blue wire to either of the C terminals in the furnace.
Best Answer
TLDR: connect the c-wire to the chassis or to pin 6
Here is the link to the GMP075-4 Installation Manual
The manual has a diagram exactly similar to the picture you posted, which is good because it's the foundation of my explanation:
Notice the
BLwire (the blue wire) attached to pin 6. This wire has two tell-tale signs of being your C-wire:Blue is a color usually reserved for the common wire. This has been a standard practice for a while now - I'm not sure when thermostat wire colors became standard, but it'll definitely apply to any units within the last 30 years.
The common wire's main function is to behave like a ground wire. Nothing in the system likely needs a grounded thermostat wire except for a newer thermostat. For current to flow through a device, there needs to be a difference in voltage between two wires connected to the device. In the case of your thermostat, it will see 24V on the red wire (red reserved for constant power), and 0V on the blue wire (0V means ground, or 'common'). Thus, there is a difference between 24V and 0V, so power can flow through the thermostat. This wouldn't be possible with any other wire (white, green, yellow) because they are not providing a voltage difference from the 24V red.
So based on the standardized practices of color codes, and based on the functionality of what a c-wire should be, pin 6 on the diagram is where you connect your c-wire. You could also connect it to the body of the blower if the blower is grounded (and it should be). If connecting it to the blower caused your fuse to pop, I suspect there's another issue at play, and you should most definitely check the AC voltage difference between the blower's chassis and pin 3 (the red wire). It should be 24V. Anything else and you may have a different problem.