Gary,
Your posts are long but full of good detail. Keep it coming
Sounds like this is not defrost and you've got something else going on like your system is too sensitive to increases from the tstat as mentioned. It is possible you're somehow wired wrong. Check out
this post it might shed some light.
Curious, what happens if you go to em/aux heat. Does that run just the furnace, as expected?
Your tstat settings look correct after reviewing
this. Without an outdoor sensor, I think you're right, the HP is making all the decisions. So we need to figure out why the HP is bringing on your heat from the HP's controller perspective. Your first upgrade BTW should be an outdoor sensor so this or any other tstat can make all the decisions for you.
I'm looking at doc
036-33182-002-B-0108 (can't seem to find your doc, but I think this is close). Which wiring configuration do you think you have in that diagram, or can you dictate wire colors from tstat to air handler to HP? Do you have the YorkGuard VI control module? We may need to understand how that baby works to fully understand how its reacting?
I would emphasize again not to just replace the tstat until you know that will help. Once you understand your system and/or understand what's going on, you be in a better position to do loads of upgrades. Whatever you do mark things, take pics, etc. so you can put things back the way they were in case something doesn't work.
After that have all the fun you can stand. Just don't let the romance get in the way of other relationships.
I know what you mean about HVAC, for some reason I find its complexity addicting.
