Adjustable pressure relief valve

The PRV may not be at fault. If your water service includes a backflow preventer, you need a small expansion tank on the domestic hot water piping to make room for the additional volume of water as it is heated. Most municipal water meters now have a BFP.

The chances are that you have anywhere from 50 to 80 PSI city water pressure.  That is why the original installer selected the adjustable relief valve.  I would select 100 PSI to 120 PSI for a cold side on the tankless coil DHW heater.  Do you know what your city water pressure is under normal conditions?  That would be a determining factor.  As @bburd said about your water meter having a backflow preventer, if that is something that was recently added when the new meter was installed, your system may not be able to handle the expanding hot water in your potable water piping.  That would cause the water in the piping to expand to a point where it might cause a pipe to burst or a relief valve to operate (release water).  A therm-extrol for DHW service may be able to solve the problem.

Hi, Does your valve look something like this? It might have been used so there is no temperature sensitive part. I'd measure the static water pressure and then set the valve about 20 psi higher. It would be good to use a pressure gauge that has the red "lazy hand" so you know what the highest pressure has been over, say a 24 hour period. If you have a pressure reducer or back-flow preventer, you may get pressure spikes when the water heater fires. In no case do you want the system exposed to pressure over 80 psi.

Yours, Larry

Not a T & P just pressure only. Usually a 15-175 lb valve.