It turns out to be a lot more complicated than that.
I remember when heat pumps were first rolled out in our area. Like you said, they were just AC units with a reversing valve.
And they didn't work for crap, and at least where I was living they created such a bad impression nobody would consider a heat pump for at least a decade. To design one that functions in either AC or heat is easy. But one that does both is a totally different animal.
When was this?
And when was this?
...
Back in late '50s/early-mid '60s. '70s were starting to get pretty decent, but were still a lot of sorry manufacturers hadn't been weeded out of the marketplace yet.
Top of the line units by then would be reasonable (still not what would be considered "good" now), but serviceable.
The TN house built in '78 had middle-the-road unit at the time; it was serviceable initially but we replaced it with the geothermal unit probably by about year 10...by then it was pretty-much shot.
I forget the specific brand now; name, but not highest quality unit could have gotten. Then again, they would have been at least 1.5X if not 2X the initial cost.
Most units installed were probably middle to lower end; in tract housing or new developments, probably the cheapest the developer could find. That likely contributed to the above experience as well.
But basically it is still just a reversible AC unit. Nothing changed in the basics, Bettwer valves, and better optimnization but they still use the same compressor in modern heat pumps that they use in modern AC units - and in air to air systems many ARE identical units, except for the valving and controls.
A lot of "fine tuning" on ground source units, for sure.
About 1975 or '76 - before Hatherton and his associates got in on the qction (in 1978?)
Definitely
50s through 70s were difficult for heat pumps. Although, we old timers do remember those years but some of our memories have faded a bit.
Then the oil/energy crisis hit and a lot of things changed. There was more attention to efficiency in general, and insulation and envelope sealing against infiltration. Both of those had an effect on heat pump acceptance I think, and by the 80s they were doing well. In my faulty memory.
There was another factor. Heat pumps in residential were forced air systems, of course. (some of us grew up with only radiant heat and no AC at all) A furnace put out a blast of perceptably hot air, while a heat pump by comparison was much cooler. Friends of mine in Wisconsin said they would never have a heat pump because they didn't want cold air blowing. That seemed strange to me, either one gets the room to 73 or so, but they liked the feel of that hotter blast.
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