re: Weird plumbing issue in brand new house with tankless water heater

In addition to the other tips, you should also attach one of those water pressure gauges to an outside spigot for 24 hours to make sure your pressure regulator is set correctly. Most are set to approximately 55 PSI. If for example your water pressure maxes at 45 PSI, you should adjust your regulator to increase the pressure.

Also, if you haven't replaced your pressure regulator in 5 years, its time to replace it as it may not be working properly.

Are you sure that you want your tankless water heater set to 140 degrees. We have ours set to the standard 120 degrees. It gets plenty hot but not too hot. If ours was set higher than 120 degrees it would make it harder to regulate the temperature.

We also have low water flow in our new (20 year old) rural house. It also has water leaking in multiple places outside (over 500' of 3/4" PVC pipe). I am going to run new 1" PEX A pipe from the meter all the way into the house and use 1" PEX C as the main line inside the house feeding the initial branch lines. The larger water line will facilitate better water flow. Not saying you have water flow issues, but if your water pressure PSI is okay but you have water flow issues even after cleaning faucet filters, etc., you might have undersized plumbing pipe sizes.