Without heating the seat post or the frame, how can I free a bike seat post that is stuck at one height ?

Get some spray lube that penetrates, like "Liquid Wrench", or "PB Blaster".
Spray it, wait for a while, then try to move it.

Good luck.

Agree with above.
I've used WD-40, then some oil.

Here are 17 different ways, from the master:
http://www.sheldonbrown.com/stuck-seatposts.html