a building inspection, note it is normal for PTR valves to leak during a heating cycle due to the increase in pressure during the heating cycle.
If the valve continues to run, there may simply be an issue with the seal.
Opening and closing the valve may re-seal
the washer by blowing out any small pieces of grit that may be causing the issue, or the valve washer may require replacement.
The other reason for a continuous leak is the hot water system thermostat is not operating correctly. I will test the water temperature during an inspection and if the thermostat temperature is lower than 60-65 degrees it would appear to be functioning .
If the thermostat is damaged then at 99 degrees the valve will open.
I would recommend that the buyer should firstly try resetting the PTR valve by opening and closing the valve, as this should be done manually every 6 months. If this is not successful then engage a licensed plumber to assess and rectify by replacing a washer or the entire valve.