Why does my noritz nr-98sv flash code 20 after a power outage

SOURCE: Noritz not staying hot

How many faucets do you have on when this happens? Turn on more than 1 and if it still does the samt thing than I would check to see if the inlet water screen has debris in it. Valve off the unit and look at the inlet water side and theres a screeen that you should be able to remove and clean undeneith some running water and then replace. Good Luck

SOURCE: Noritz tankless water heater not producing hot water

I called Noritz error code 16 is a overheat problem. Noritz say's to flush your coil with vinegar or CLR.
Use a small pump to recirculate the CLR or vinegar through the system for 45 minutes and flush with fresh water. This worked for me.

SOURCE: I have a noritz water tankless heater and hot

Three flashes typically indicates the unit is overheating due to scale. I'd try a descale initially and see if that remedies the problem.

SOURCE: We hada power outage. The light on the power

I'd first try cutting electricity to the unit and then powering it back up; however, there's a good chance you unit was damaged from a voltage surge following the power return. If you have a volt meter, here are a few things to check:
-voltage across the gfi - the gfi is located on the bottom of the unit; it is a black box with a blue, white, green, black, and white wires coming into it. There should be about 100VAC across the blue/white and black/white pairs.
-If there's no voltage across either pair, your transformer is fried and needs to be replaced.
-If there is voltage across the blue/white but not the black/white, look for moisture in the bottom of the unit. If there's no visible moisture (indicating a leak), you may need to replace the gfi.
-If there's voltage on both sides of the gfi, measure voltage at the remote terminals (location depends on model). You should have 15VDC across the terminals. If there's no voltage, your circuit board is dead.

Good Luck!

SOURCE: We had a power outage.

I'd first try cutting electricity to the unit and then powering it back up; however, there's a good chance you unit was damaged from a voltage surge following the power return. If you have a volt meter, here are a few things to check:
-voltage across the gfi - the gfi is located on the bottom of the unit; it is a black box with a blue, white, green, black, and white wires coming into it. There should be about 100VAC across the blue/white and black/white pairs.
-If there's no voltage across either pair, your transformer is fried and needs to be replaced.
-If there is voltage across the blue/white but not the black/white, look for moisture in the bottom of the unit. If there's no visible moisture (indicating a leak), you may need to replace the gfi.
-If there's voltage on both sides of the gfi, measure voltage at the remote terminals (location depends on model). You should have 15VDC across the terminals. If there's no voltage, your circuit board is dead.

Good Luck!