My water heater wont refil with pressure - Bosch AE-9.5 Electric PowerStar PowerStar Electric Tankless Water Heater 9

This important part is right at the end, you have a well!! It sounds like you may have run it dry? Check the well and the pump first. The rest is stop valves and pipes to the tank and beyond and not likely to be causing issues. Your sediment issues and problems with the anode is also due to using well water which is nearly always loaded with minerals..

Without knowing the make and model of the water heater, the overflow pipe would be assumed to be a pressure relief valve. There should be no water coming out of this unless the pressure in the system is too high, the valve is bad or it has been activated and wont seal.

Sounds like the pressure vessel has lost its pressure.

refill the gas bottles and then if that doesn't fix it, get a technician out as it may be the pressure switch not opening the gas so that the pilot light ignites the burner

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3 flashes, failed pressure switch?
Test pressure switch
Gene

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The real question is 'How old is the water heater?'... over time, particularly in areas with hard water, sediment builds up in the water heater. Particularly electric models. typically a TPV (temperature/pressure relief valve) won't cause the symptoms you describe, but mineral deposits around the inlet tube (internal) could, by restricting the flow of water into the water heater.

A TPV is designed to prevent over pressurization which could cause the tank to rupture. I have priced new water heaters as low as $308.00 for a gas powered 40 gallon unit with a 3yr warranty. If you have good soldering (sweat fitting) skills, these are relatively easy to replace yourself. Whether electric or gas, make CERTAIN you shut off the heat source before attempting a replacement.

1) If water flow stops when tap is off, then Sound of water running: could be factory-installed heat trap inserts or nipples: These things cause noise.
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2) If sound of water continues when tap is off, then there is a broken pipe or leak somewhere in plumbing system.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I got a new Rheem heater and it's complete garbage. No water pressure.

I believe you have a high water pressure going to the
tank. Minimize the water volume going to your tank. This is done by fully closing the inlet valve under the tank. Then open about 1/4 to 1/3 of the inlet valve. To do this, count the number of turns to get it full close from full open position. For 1/4, divide the number of turns by 4 and divide it by 3, for 1/3 open. Hope this will solve your problem.

combi or standard if combi pressure too high and will be blowing off your high pressure release valve which would stop your boiler from igniting best of luck