I used to run Plex on a 3b+ and it couldn’t handle transcoding at a reasonable quality - jellyfin would presumably be fine as long as whatever device you are streaming from can handle your native file format (my library was x265, so I had to use the Plex desktop app because Firefox doesn’t support x265 and required transcoding)
A server produces an amount of heat equivalent to it’s wattage.
A 500W server rack will produce 1/3rd the amount of heat as a 1500W space heater. If your rack draws 100W at idle, than that’s how much heat it produces. So if it’s cold outside you could spin up folding at home or some other thing to burn excess CPU cycles
As long as your server is inside your house it is offsetting the amount of heat your HVAC system needs to produce - granted it is also greatly increasing the amount of work your AC needs to do in the summer
There is a cricket farm in Quebec that heats it’s enclosures with Bitcoin mining rigs.