SylvanLMP
HI Dan, Lucky you asked the right heating expert in hydronics (ME) 
OK the probable causes of excessive pressure in a hot water heating system are the following
1- The expansion tank is flooded
2-The expansion bladder gave up either the ghost Or the bladder lost some of its charge
3- The automatic feeder NORMALLY set 12-15 psi is malfunctioning allowing street pressure in to the system
4- Someone opened by pass OR the fast fill feature of the automatic feeder
5- If you have a tankless coil this coil sprung a leak and is allowing FULL main pressure into this system
6- This boiler has excessive air in the system that is allowing this air to compress and thus build up pressure (air can be compressed and water cant)
7- Your Aqua stat controller (high limit) is not working and allowing excessive pressure/temperature relationship.
Most lightly it is the expansion tank OR tankless coil is applicable
