Can water siphon backwards through the house pressure regulator? My house is above the main water line in the street. If the main line pressure goes to zero, will the house water siphon backwards through the pressure regulator and down into the main line?
If you have a backflow preventer, aka a check valve, installed in the system, then the water will not siphon back. This could be installed near the pressure regulator or main shutoff inside your home. It's also possible for the municipality to install one near the meter.
A standard pressure reducing valve closes when the water pressure inside reaches a threshold. Once the inside pressure drops slightly, the valve opens, at which point a backflow is possible.
Sure it will, though not as fast as it would going in the intended direction.