With a boiler and radiator system, how clean the water is and how well it circulates are also factors in how much heat you actually distribute to the house compared to how much the boiler is heating. Sediment from minerals in the water, iron from the pipes all contribute to make heat transfer less efficient. Have your boiler serviced by a company that regularly deals with hot water systems along with the previously mentioned weatherizing tips and you can have some measure of peace of mind knowing you did everything reasonable to save money while keeping warm.