Obvious downside would be the Air con haveing to work harder and chew more power if its on a thermostat, as the room cooled, when you shifted hot air to another room.
Cheapest solution is insulation if you havent done that already.
Thick curtains, carpet and stopping drafts.
Saw a system in the US where a guy built a little glass house over a South faceing(North Faceing for Australia) wall, painting the wall a dark clour first.then ducting the hot air into his house.
The glass house was only 150 mm of the wall just enough to heat up the air between the wall and the glass.
I dont know what they are called but they sound pretty cool.
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