Good to see different techniques - and that they work! Hope the silver soldering to the boiler goes well. A tip? Try using a pencil to mark off the area beyond the joint where you don't want the silver solder to take. Just "shade" the area, but make sure no pencil where you do want the solder. Also using a paste of flux and water pre-applied ensures it fills the gap to clean the surfaces and encourage the capilliary action when silver solder is applied. (But I reckon you know all that anyway.).