Space heater, 20 bucks. In the past I chose to heat the bedroom a little on cold days and otherwise suck it up and dress while shivering (no point burning a ton of gas to heat an entire bedroom). Now, I use 1.5 kW for ~3 minutes (0.01 kWh) to have hot air be blown on me while getting dressed in the morning.
Because it changes how warm I feel while going downstairs, I also don't need downstairs to be as warm to get warmed up again. I'm already warm and can sit in a normal temperature room to work.
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Air purifier (40 bucks on offer I think). We got it for unrelated reasons, it didn't help for that. Unexpected uses:
1. Neighbours' smoke occasionally comes into our apartment somehow and now I can do something about it. Before, I would just suck it up and try to convince myself that the little residue coming through whatever wall isn't going to impact my health.
2. This winter, people seem to like to heat the house with whatever old painted glued rotting wood they can find (or maybe also dead bodies, it's hard to tell). I can wait a few hours, but if it doesn't clear up and I want some fresh air before sleeping then what I do is open the bedroom window wide for a few minutes to replace the air, doors closed, then close window and turn on purifier on high for 30+ minutes, and then go back in there to sleep.