What type of material will reflect most of the electromagnetic waves coming from a microwave magnetron? What kind of material is the best?
If it affects every fixture in the house than it sounds like the isolation valve is partially closed. The plumber would have shut this off to replace the pipe and probably did not turn it fully back on right away to prevent his fittings from blowing out from water pressure. Check around your hot water heater for a valve on a pipe that feels warm. If it has a lever, the lever must be turned so that it is parallel with the pipe when fully on. If the valve has a knob, turn it counterclockwise until it stops, this is the fully open position. If this problem is isolated to only one fixture, check the stops to the specific fixture to make sure they are fully on. If you have a mixing valve for any of these fixtures, this may need adjusting too. If this doesn't work call the plumber back to repair. It is industry standard to warranty work for at least 1 year.