My heater turns on, but it never shuts off. We have to do it manually. I'm hoping to fix this problem on my own. After I shut off the gas to the valve, can I just take the thermocouple off and buy a replacement somewhere? Or should I take the whole valve off and replace the whole thing? I probably shouldn't be so cheap and should just call a repair guy to replace the dang valve, but I'd like to try it on my own! I enjoy your show.
The thermocouple is designed to convey to the gas valve whether there's a flame at the pilot light. This prevents gas from being released when there's no flame.
It sounds like your problem isn't the thermocouple, but the thermostat itself. The furnace should turn off when the temperature hits the thermostat setting, and if it isn't, it's likely because the thermostat isn't picking up the signal. That said, it's very difficult to diagnose what's causing this from distance, and it's always best to have HVAC repairs completed by a pro who does it everyday. If this is your first attempt at replacing a thermostat, my recommendation is: Don't! Though I admire your enthusiasm, it's not a DIY-friendly project for a first-timer. And as a general HVAC note, it's always better to replace furnace parts then to try fixing them. Good luck!
I would suggest that a professional make this repair for you. Seeing that the heater is older there can be other issues with it that you may not be aware of. And putting all the hard work in fixing this may be not the best advice I would suggest. The professional can determine if its time to replace the unit or simply repair it.
Your local utility company can check this out for you and many times this evaluation can be done at no cost if you carry a service agreement with them.
The thermocouple is not necessarily the problem with this unit. As all this device does is prove to the gas valve that the pilot light is on and its safe to turn the main gas on for heat. There is most likely another sensor that runs to the gas valve that senses the room temp and that control is faulty.