have open system empyre outdoor boiler going into forced air system. no trouble last two years heating house, this year can't get above 70F on a 25F day. Water temp at 180F. previous years, water line going into heat exchanger in forced air furance was hot, and return line coming out much cooler. Now we check and feel the lines are both very hot. Would think if heat is being removed the return line should be cooler. Did try putting on new circ. pump, made no difference. Fan for furnace is running. could air in lines somehow be a problem? any suggestions appreciated. by the way, our HVAC installer placed the radiator in the cold air return due to problems with space, so air is drawn thru rather than blown over radiator. system worked last two years, don't know why this would cause problem now, but????