...not really
In theory it could be done. The temperature of the cpu would raise the temperature of the coolant, which would go round to the radiator and be dispersed...yes. But, conventional radiators dont really lose their heat, hence why when you touch a watercooling radiator it's still relatively cool, whereas a proper radiator conducts the heat from the water into the metal and so stays warm. Because the heat from a watercooling radiator is dispersed quickly into the air due to the fans, the heat quickly dissipates into the air and so will bring the temperature up slightly, but the heat from the radiator would also be cooled by the low room temperature...
In an air tight room, it would work, although no where near as fast as a proper radiator. But, with windows, and cracks in doors and everything in a room, clean air gets in, and so you arent actually heating a room, any hot air you give is quickly dispersed through your whole house, and to outside, and I think we can all agree a radiator cant heat the planet
The central flaw to this is that 1) household radiators arent actually radiators they are convectors, and 2) the purpose of a household radiator is to heat up a room, whereas the purpose of a watercooling radiator is to cool something down...
a household radiator must keep the heat within itself, and get hotter and hotter, up to about 80 degrees C to heat up a room, and the only way to get a watercooling radiator up to that temperature would be to 1, run it without fans, and 2, encase the rad with metal so heat is conducted to the metal and dispersed from there. As this would be good for heating a room, as you can guess it wouldnt be for the processor underneath, which would need to be running at around 110/120 degrees to sustain the high temperature of the radiator. Also, household radiators are a lot bigger and so can convect heat into a room quicker, whereas watercooling radiators have less surface area and so will struggle to do this. So, you could get a massive watercooling radiator yes...but then it's gunna be even harder to get that up to 80 degrees to heat the room.
Also, it's not exactly gunna be cheaper to run a computer like that all day in the winter than just paying for gas bills anyway...