It is easy to diagnose. Turn off the valve in the supply line to the boiler; if the PRV is the problem, you will no longer see the pressure coming up. If it still is increasing, the next step is to shut off the isolation valves for the zone supplying the coil of the indirect water heater (if you have an indirect) or the cold-water shutoff valve to the tankless coil (if so equipped. If the coil (tankless or indirect) is the problem, you will see the pressure rise stop in its tracks.

The other cause applies to boilers outfitted to provide domestic hot water, either via a “tankless coil” inserted into the boiler or via an “inderect” water heater containing a coil supplied with hot water from the boiler. Should either of these coils develop pinholes or leaks, plumbing system pressure will backfeed into the boiler, raising its pressure past the relief valve setting.