Generally about 12 pounds. Usually automatically set by the pressure in the Extrol tank, or by a pressure reducing valve setting.

It is based on the maximum height above the pressure gauge the system needs to operate at.

If you used 12 PSI in a 3 story house with basement the attic would be rather cold.

Yes, that too.

17-18 PSI would be required for a 3 story, any more and the relief valve (30 PSI) will also require replacement with higher rating (50).

The ASME coded boilers have 30 psi on the plate. How would that work with a 50 psi?

To my knowledge, only a few residential boilers carry a ASME approval for cascading application of multiple boilers is a commercial application. ASME rating on a residential boiler to my knowledge is not required by code in any state. Thats not to include IRI, FM, CGA, UL and all the others that may be required. Otherwise any commercial boiler usually starting around 175MBTUH can be ordered, 30, 50, 75, 125 & 150 PSI.

My company produces 100% ASME coded boilers and most are used in residences.

It's interesting to note that in PA, a home's boiler is subject to registration and inspection by L&I, if it has a sign outside in the yard advertising that a business is being conducted in that home. If it's a business, it has to be coded. I asked our area L&I inspector how he determines what is a business use or a residential use. His answer was: "If there is a sign outside, they are subject to inspection". Even if it's a mom and pop operation !

I have seen the paperwork for "intention to install a boiler"

Act 85 I think?
PA is intense

Outside of PA, I would think the extra $$ for the stamp would make competing difficult unless you only do copper or stainless

Actually, there are areas inside of PA where coded boilers are required in all situations. Also, outside the state.

This has become a problem for one of the local hard coal-fired boilermakers who wants to sell outside of PA and the noose is tightening. They aren't H stamped. Even though these boilers are built to ASME standards, they lack the H stamp. The stamp costs about $2000 for a single unit, which makes them more competitive by not having to pay for it. We decided years ago to stamp every boiler, which substantially reduces the individual cost of a stamp.

It wouldn't matter if you installed a brand new non-coded boiler in a commercial application on Oct. 1st in PA. If L&I came to inspect on Oct. 2, the boiler would have to replaced.