More than likely your boiler is leaking inside the combustion chamber.The leak may not be enough to cause you to see water. Yhe burner vapories it befor it can show up on the floor. You should be able to see the steam from your chimney while the unit is running. Another cause could be a leaking domestic coil plate gasket. Water will run down the hot cast iron and produce steam as well. Carful about inspecting the gasket, it will be dry rotten on such an old boiler and you can make it alot worse. Time for a new boiler anyway, the fuel savings will be significant.