Fire extinguishers can make very good boilers, 2 things tho'- pressure test full of water, – You can't use it in public, needs boiler inspection for insurance.

Yes, although that does depend on the intended operating pressure and thus the operating temperature. As it's riveted the soldering is probably for sealing rather than strength in any case.

What is the volume? If it is less than 2 litres (if I remember correctly) or operating at less than 0.5 bar there are no specific legal requirements other than your duty of care.

Russell.

You may well have difficulty silver soldering an item that has already been soft soldered as the old soft solder is likely to have been lead based and that will contaminate the work.

A vertical boiler will have the firebox at its base so that is likely to be the hottest part so the pressure/temp won't come into it but on the other hand you will not need the caulking to retain steam in the firebox section

Thats a nice boiler, he uses rivits and the expanded ends of the tubes for structural strength and soft solder to just seal or caulk all the joints. Generally this method is not used as much in the UK now instead all joints use a stronger "silver solder" with a much higher silver content

The boiler mentioned above