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 hot 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 hot water heater has a re circulation 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?
Wow.. 1.5 inch should be adequate for an extremely long driveway!
I assume you're monitoring the pressure near where the supply pipe enters the home. If so, I'd try testing the pressure (with water flowing) as near to the customer side of the water meter as possible. That'll immediately reveal whether the flow restriction is in your piping or in the city's system (or meter).