On 13 Aug 2020 11:38:08 GMT, David wrote:

Can only some systems be run in reverse?

The heat pump will only work one way.

I assume that there is some extra plumbing?

Simplest form is just running chilled coolant from outside through the
fan unit inside.

Simplest is to have the all the heat pump bits outside,ie compressor,
condensor and evaporator and a couple of dampers that determine which
of the condensor or evaporateor the air flow into the room travels
via. Condensor to heat the air, evaporator to cool it. May even have
options to recirculate or use fresh air.

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Cheers
Dave.