Plumbers and HVAC

I have an outdoor tankless gas hot water heater and sometimes it doesn't heat like it should.After checking it out and talking to a shop,he said it was the pressure regulator and they could replace it.I bought the valve and ask if it was preset from the factory and all he would say is we could install for you.
Does anybody know if there preset and how to set them if there not.I hope I don't have to hire someone because the prices I got were crazy.
Any help would be great,thanks.

If it is the pressure relief valve it is preset, if it is a pressure regulator it could be either.

Gas pressure regulator or water pressure regulator? Both of these are adjustable within a "maximum" or "minimum" tolerance. I would not trust either of them to be set to your needs out of the box--even if they say they are. Water pressure regulators are easy to adjust with a screwdriver assuming you have a reliable gauge on the water line.

Water pressure relief valves as mentioned already are set to a given PSI and are not adjustable.

How old is the tankless and have you ever flushed the coil?

Thanks for the replies.The heater is about 4 yrs.old and I had it flushed last Aug.