Our 30 yr old condominium in southern California has 14 units in 2 buildings. Heating is from a central boiler, AC is from separate AC systems in each condomimiun. Heating and AC share a common heat exchanger in each condominium. We are told by contractors that replacement parts are not available and that systems in individual condominiums will have to be replaced by reverse heat pumps for each condominium. Is it true that these are units are obsolete and cannot be restored? Are reverse heat pumps the only alternative, or are there others?