I have found that if the water pressure is above 90lbs in the house, and you don’t have an expansion tank that in heating the pressure will rise to where it will cause the relief to open.

MY utility says 40-80lbs is normal 60 ideal but they also say their only requirement is to provide “adequate” pressure and if it is too high then you need to add a regulator.

15 psi? Are you talking about a boiler for hot water heat and not a water heater for your potable water? That is more like the pressure a boiler runs at.

your pressure reief valve will open at 150 lbs , or 210 degrees ……… normal operating pressure depends on your local water authority – all in all i agree 60 lbs is normal -

your average water pressure will be 60 psi

150 max