"however, in fact, very few had / have an indicator light to show that supplemental heat was running."
As far as I know, every Honeywell thermostat that is designed for use with a heat pump has those attributes. My rather basic 1979 model from Honeywell had that feature, and my rather advanced programmable Honeywell on the new system has that feature.
As far as I can determine, all major manufacturers of heat pump thermostats use both indicator lights, and have for years. I'd be interested in hearing if you know of any made today that don't.
Every heat pump system made today (again, as far as I know), also has the control circuits necessary to run both lights, and have had for years.
Yes, it's possible that a series of failures could push the heating system over onto strictly electrical heat, and a less than observant homeowner might go weeks, or months, without noticing. But I have to think that if they're that much of a brain dead dullard, they'll also have problems figuring out when their gas or oil systems aren't running optimally and are wasting large amounts of money, as well. You can only design in so much "stupid" factor for people. After that, they are on their own.
"What I said in terms of frost and defrost is that if the ODT is high enough the HP may still frost but will melt the frost off during the "off" cycle not needing a defrost."
Gotcha. Yes. That's often the case unless for some reason the cooling load is unusually high.
Personally, I think in the North, anything much above the Pennsylvania Maryland border, no one should be installing heat pumps in the first place unless, as you note, they're ground source.
Interesting aside...
I was talking about this thread to my Father (a civil engineer in Pennsylvania) and he mentioned that my Grandfather installed some of the first ground source heat pumps to be used in Pennsylvania, in the early 1950s. Grandpa was a mechanical engineer, in charge of the boilers and mechanical maintenance at the American Viscose Plant in Lewistown, PA. They used the Juniata River as the ground source.
As a further aside, Mom and Dad live in the house that Grandma and Grandpa bought in 1943. They bought it from the estate. The boiler is a Burnham Jubilee that Dad and Grandpa installed in 1956 to replace the coal-fired gravity hot water system that was in the house originally.
Dad and I were talking about putting a new boiler in (50 years is a good life for a boiler), but now that his health is getting worse, it will probably be contracted out.
