There's nasty stuff in home heating systems to stop the radiators rusting. This would be likely to clog up your cpu blocks. I'm not sure what pressure heating systems run at so can't say whether it would be appropriate or not.
One of these days I'm going to get around to running pipes out of my computer, into a standard radiator, which is immersed in a water tank. Heat goes from radiator into the water, but the liquids don't mix. Heat exchanger fashion. So I would propose submerging a radiator in the water tank but keeping the actual liquids separate.