The answer depends upon how close the design of the system is to the actual heat requirement of the home. As most installations are grossly oversized for homes, it is not predictable. If your radiators and boiler are twice as large as necessary, you could simply wind up with more drafts in the house in the rooms with the turned-off radiators and no money savings. You could set the high limit on the boiler to 140F and theoretically cut the gas cost in half. Set the thermostat to 50F in the house and use the stove heat in the occupied room and you could theoretically save 50%. Time will tell.