why does the hot water boiler used for heat, open up and release water then close back up?
The relief valve is working as it should,likely the expansion tank has gone bad. The expansion tank has a air filled ballon inside that expands and contracts with the different temperatures of the water. Without it the boiler will fill up to the proper pressure then when the water gets hot the water pressure increases and then makes the relief valve go off and then the hole cycle starts again.