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.