[engi_nerd]

No. The unit saw that it was not reaching its setpoint, so it kept trying to reach the setpoint. But there is apparently no feedback from this.

Instead of A/C, the unit activated heating. So the temperature kept rising even further beyond the setpoint. So, more heat.

Positive feedback -- exactly what I didn't want.

[maxerickson]

I can see both sides of including detection of improper wiring in the software. It's a nice feature when it works right. On the other hand, it's something new to go wrong that is only helpful in one specific case (that you would rather prevent using some other method).

(I'm presuming enough QC in the manufacturing that improper wiring is the most common trigger of positive feedback)

Have you gone far enough into it to compare how the wires are hooked up on the heat pump side to the nest diagram? Not trying to heckle you about it, it's a puzzle, it must be solved.

[engi_nerd]

"it's a puzzle, it must be solved."

I agree with you. I love solving puzzles, that's why I'm an engineer. But I had to balance solving this puzzle with the ongoing puzzle of maintaining domestic harmony. My then-pregnant-wife insisted I get the system working properly ASAP. The best way to do that was to reinstall the Honeywell -- the system worked properly within 15 minutes. As life circumstances dictated that I was away from home for most of the time -- and we moved a couple of months later, the best solution was to get a refund.

Did I have some wiring wrong? Most likely.

Not to beat my credentials against my chest, or to imply that they mean that I could not have possibly made an error... I have engineering degrees and have done a lot of instrumentation system design work. Figuring out how to wire up strange sensors to an existing system is something I do all the time. If I had this kind of difficulty, what kinds of problems are ordinary people having? Or maybe it just reinforces the aircraft industry joke that "you shouldn't let engineers touch tools, they'll break something".

[maxerickson]

Other people are posting about problems with professional installs on heat pump systems. The two more likely explanations of that are faulty nests and wiring problems. If it's wiring problems, it suggests there may be something that is easy to overlook or misunderstand.