The answer to 'why is my pressure regulator constantly running' is that the pressure regulator is likely faulty or set to the wrong pressure. A pressure regulator is designed to maintain a consistent water pressure in your home, but if it is malfunctioning, it can cause the regulator to run continuously to try to maintain the set pressure. This can lead to higher water bills and potential damage to your plumbing system.
Some common causes of a constantly running pressure regulator include a broken diaphragm, a clogged filter, or the regulator being set to a pressure that is too high for your home's plumbing system. If the regulator is set too high, it will constantly run to try to reduce the pressure, which can put unnecessary strain on the system.