On Mon, 7 May 2018 09:21:36 -0500, dpb wrote:

On 5/7/2018 9:08 AM, Jimmy Wilkinson Knife wrote:
...

Why on earth could an AC unit not run backwards?* You just reverse the
pump.* Every one I've seen in the UK (commercial and home) has been able
to do this.* The controller usually has settings for heating, cooling,
or automatic.* On automatic you can simply set a range (say 18 - 22C)
and if it goes over, it cools, if it goes under, it heats.

Those aren't "just" AC units; they're heat pumps specifically designed
and built to do so....an AC unit is just that; the condenser/compressor
connected in cool mode without all the other auxiliary arrangements to
operate in heat/cool mode.
and the pump is NOT reversed. There is just a rather complex little
device called a "reversing valve" which switches the high and low side
of the compressor between the "evaporator" and the "condenser" -
making them trade places and functions.