Furnace not shutting off when thermostat switched to "heat"
Hello, I'm remodeling my living room and I have been somewhat hard on the furnace lately. It's colder in here because I'm working so when it gets cool in the rest of the house my wife just tells me to turn the heat on for a bit and then I turn it back off. Well I got to the point of mounting the thermostat back on the wall and bringing everything back to normal operating when I notice that the furnace will not turn off after switching it to heat. If it's 74 in the room and I set it at 68 or something and then turn the thermostat to heat it kicks on blowing heat and doesn't shut off until I turn it off at the thermostat. It went all the way to 83 before I shut it off manually. I went to the manufactures site and checked the wiring, it's correct. I tried a reset on the thermostat and it didn't help. Today I went out and got a new thermostat thinking that the sensor in this one was bad from all the dust or something. I installed the new thermostat and the same problem still occurs. Turn it to heat... and on it comes until I turn it back off. My next thought is that a contact on the furnace itself is stuck? I went downstairs to check it out but I really don't know which contacts I should be looking at or if that could even be the problem. Any advice is appreciated and thanks in advance. Details of hardware below.
Furnace "Goodman GMP100-3"
Thermostat "Ritetemp GPM8022 C"
replacement thermostat "Honeywell RTH2300B"