Mine have just gone back out in the Pighouse this last week.(Night and day). They were inside at night for winter. My small bedroom is fairly cool (radiator furthest away from the boars indoor cage) My boars outdoor house has a deep hay box, I pop a SnugglePad in for night time.
Mine have gone out every day even through winter with a heater on in their house. They don't have the day heater on now, but I put it on low at night for now. If you give them deep dry hay and a Snugglepad (Amazon are cheapest) or PetHottie (microwaveable pads) - build up their outdoor time?
Last weekend ours had some time in their run- GP1 was lazing about on his fleece blanket on the grass. But this week it's been too cold/damp for grass time