I've had to put a betta close to an air vent, not right next to it and not right on it though. Then I got him a heater the next day. :)
and who are these "people" saying NOT to get a heater for a 2.5? >:( safely, a 2.5 can be heated with a mini heater, aka a 7-15 watt heater.
better yet put him in the closet. YES it sounds weird. If you have a shelf in your closet, heat RISES and often is trapped in closets. my closet is sooo warm, compared to my room with is soo warm compared to the rest of the house which is soooo cold
Basically the warmest spot in the house, out him. add a blanket or twoel around his tank :)