The water heater air space is needed as a "cushion" to take up the expansion of the heating water. Or it will find a place to force it's way out, the weakest point in the system. Sounds like your weak point is the pressure relief valve, which is the way it is supposed to work. But you still need to re-establish the the air space.
Turn off city water supply and the water pump. Turn off the water heater until it cools so you don't get scalded. Open a faucet nearest the water heater hot water and leave open. Go out and lift the pressure relief valve tab and let the water drain from the top of the water heater, while the open faucet will allow air in to replace it. When it is done draining (only a minute, give or take) let the pressure relief tab snap back in place, and go in and shut off the faucet. You should have an air cushion back in the water heater.
Note. This air cushion can be lost often, not the world's greatest invention, and may need to be re-established often. I put a small water pressure accumulator from Camping World in the bathroom faucet cold water line supply line of mine to help, and it does. Must be installed in a cold water line.
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles
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