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Bus driver, Believe me, if I had screwed this up, I'd be taking the...heat for it. (Pardon the pun. :)) I own a crappy little house that needs loads of work, and I post about my...renovational shortcomings on different web sites on practically a weekly basis. Here's what the electrician did: 12/2 (the yellow-sheathed stuff) comes up through the wall and into one end of the heater, where it connects to another length of 12/2 (also yellow) that runs underneath (not really "through" the heater, I guess; it goes via the knockouts) to the other end. That's where the thermostat is. Yes, it looks suspect and should be covered somehow. (I have a call in to my electrician, but I was hoping that a simple thermostat wiring was within my grasp. Maybe not.) But beyond the code issues (and I WILL address them), what is likely my problem with the heater not turning off? The only difference between my stat and the one in Marley pdf above (see Step 15) is that the Marley stat has a short red wire that leads (I think) from the element wire to the fins. (Wrong terminology, I'm sure, but it's the short red wire.) In my stat that little red wire doesn't exist. Instead, there are two (not one) black wires that I've nutted. Apart from that, everything else is the same. BTW, you're right: my present knowledge (and probably future knowledge) is inadequate to diagnose and remedy this problem. Trust me, if my explanation doesn't give you or someone else here an aha! moment, I won't start guessing. Thanks for your help.

There are "smart" thermostats that monitor system performance over time in relation to how fast the temp drops during a setback period and how fast it rises during recovery. They adjust the start-up time for recovery and ramp the temp up slowly to avoid triggering the auxiliary. Most systems also have an option to add an outdoor temp sensor and lock-out the auxiliary from running at all unless the outdoor temp is below a programmable level (it still runs during defrost cycles). When I was working two jobs, I routinely did 10°F setbacks for 12+ hours per day. Sometimes the indoor temp dropped by only 6 to 8 degrees so the system never ran during the setback periods. Took some unusually cold weather for the "smart recovery" function to really come into play. In any case, it wasn't unusual for the recovery to fall 1°F to 2°F short of the target temp at the target time, which is what triggered the auxiliary to quickly make up the difference ... which is what prompted me to have the outdoor temp sensor added for the lock-out. Not a problem for me if the room temp was 68°F instead of 70°F at the target time. The system was blowing warm air so it was perfectly comfortable in the house. I'd rather it run longer without the auxiliary than trigger the auxiliary unnecessarily to quickly make up the difference.

"People who do not have a multimeter or have one and don't understand how to use it shouldn't monkey around with electricity." The only way you learn to trace circuits and use test gear is by doing it. It's nice to have expert guidance, but you can also get help from books, which is how I learned to use a VOM as an adolescent. (No DMMs in those days. Also, no internet. We had libraries, though. :) But here we are more than 2 weeks after the original post, and if the OP has done any circuit tracing, I haven't seen reference to it. My guess (and I hope he'll correct me if I'm wrong) is that he doesn't want to get into the project quite that deeply, and is hoping he can just buy a solution. His proposed purchase is a thermostat which has a similar configuration to the old one. That seems like it ought to work, but a digital thermostat still works on a completely different principle from a mechanical one. So the NEW new thermostat may work when connected like the old one was. Or it may not work at all. Or it may work only partly. Or it may go pop or BANG. Or it may just quietly release a little puff of smoke and stink. There's even a slight chance he could get hurt or damage something else, the worst case scenario. I just think that if he doesn't want to do the hard technical work of tracing things out (and there's nothing wrong with having different priorities), it's probably best to turn this over to a pro, and get it over and done with and WORKING.