The manual states E28 is a Mixing valve initialization failure.
With all failures on this model, make sure your condensate line and check valve are clean and clear. I see many different issues that come up that seem to go away after removing, cleaning and replacing the condensate line.
I received the E27 and took the mixing valve out for inspection. In order to remove it, you'll need turn off the water and gas and electric and drain the tank. Disconnect the large grey flexible piping in front of the valve and move it out of the way it to gain access. There are 2 alligator clips that hold the mixing valve in place and 2 sets of connecting wires that are clipped to it. Remove those and inspect. My valve was clean, so I disconnected the Servo motor from the valve. I cracked open the Servo motor and discovered that one of the plastic gears (there are 3 of them) was worn down and cracked. It needed to be replaced but I couldn't find that part, and I couldn't find a Servo replacement. So, I purchased the entire valve assemble from Rheem - SKU: RTG20298N Brand: Rheem for $95. Replaced the valve, installed the alligator clips, reattached the wire connections and replaced and secured the flexible grey tubing. Turn on the water, allow tank to fill, turn on the gas and electric and E27 went away. I then started getting the E1 error. To fix that, make sure all gas and water connections are open, turn on the gas at your stove top burner to get gas moving, push the off button on your water heater. Then press and hold the on/off button until you feel a slight click. That completes a hard reset and the gas igniter will try to kick in again as the unit turns back on. Mine worked fine and the E1 went away. All is good after that.
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