Completely renovated a new home recently. We always had issues with the water pressure, so the builder put in a new line from the city (1.5 inch) down a long driveway. We also got a tankless water heater (Navien NPE-210A).
Testing the cold water pressure I get about 100 psi, but as soon as anyone opens up a tap it drops to about 40 psi. After turning the tap off, it slowly climbs back up. I know the tankless water heater has a recirculation feature, but I turned the water off to it to do this test and it didn't make a difference.
The issue is turning on two taps of cold water drops the pressure even lower, making it impossible for more than one person to take a shower at a time. Any ideas of what to test or fix?