What makes a giannoni or any other heat exchanger ASME approved. For example the heat exchangers in a LAARS Mascot or a Baxi. Should the H stamp always be on an ASME heat exchanger or is the H stamp an additional approval on somthing.

I am not refering to a brazed plate heat exchanger. It just kinda funny Gianonni says one thing but the ASME heat exchangers I have seen dont seem to match up to the giannoni ASME exchanger examples.

Is it the welding of the manifold or the welding during the construction of the entire heat exchanger??

The classic heat exchanger is what I have seen on most wall hung boilers but it seems here it does not have an H stamp.