Anther way to set system pressure is via the rear view mirror
well sort of
IF -- and there is the "caviar" Fish egg Disclaimer -- If you can find a port to test system pressure at the highest point - and In Ancient systems it was where you applied the plug to fix an atmospheric pressure gravity flow system to a "modern" pressurized system -- If you have this point --
3 psi at the highest point is satisfactory
Why did we do it like that ???/ Because old systems are full of old pipe -- runs and fittings are often thin from rot as "new" air was always available at these open tanks and the near pipes grew weak - so 3 there made the system function correctly and didn't severely impinge on old delicate parts.
now your system is more modern =-= but its simply a point of testimonial as to the value you found by reading the BG link
Its all good 9 12 15 - as long as its enough - not enough is bad but it can be a lot too much before its gets bad again
You say potato
I say potato
I could give you the answer
But its better for U to understand why the answer is correct and how to arrive at it - so that's why I annoy you with links and stories and a little BS
boiling point in a residential heating system really should not come into play -- UNLESS the system used to be two pipe steam and the radiation (converted over to hot water) is now so severely undersized that the system will not heat the dwelling without running excessive temperatures. OH the stories of the Nacked City Heating Company
Lets go back to the study of cup sizes at Hooters with Whistlenut.