Yamaha U3

[Gill the Piano]

Radiators still give off a dry heat; only an open fire gives off 'healthy' heat. Air-conditioning is exceedingly dry as well. If you make sure it's a long way from the radiator, or have some source of moisture in/near it (over here we favour the high-tech solution of a bowl of water on the floor inside the piano!) it might be happier. Your best bet would be to approach a local tuner who's familiar with the climate and knows how various instruments react to it. The Piano Technician's Guild have a page similar to this; perhaps you could try asking them?