Technically, you could also just pull refrigerant lines from the heat pump to the bath tub - you probably already run those to the indoor AC units you have in different rooms anyway - or connect the bath tub to the hot water lines you run to the radiators. Both imply a grey water heat exchanger built into the tub.

Depends on setup, of course. If there's a central AC blowing hot/cold air through ducts, you're stuck using a grey water loop. But it is the least attractive setup...