@Olga11 When the guests booked your listing, was Hot Tub a listed amenity, with explicit details about its proximity to the condo? If this is the case, and the heat pump issue prevents you from offering the amenity in its advertised state, a token rebate like @Ann783 suggests would be appropriate. Otherwise, it's not really necessary.
Either way you go, I'd recommend waiting until the end of the stay before offering or discussing partial refunds. Otherwise, you risk establishing a pattern in which the guests start coming up with all kinds of (non-)issues to try to get more refunds out of you. It should be clear that anything you offer is a kind gesture and not an entitlement.