Rheem tankless water heater not heating after deliming.

[N08LE]

Hello. We just delimed our Rheem RETEX-18 tankless water heater. After hooking it back up, it doesn't heat water at all, just pushes cold water through. We tried powering it down, even shutting off its breaker for 15 min. Still doesn't engage. Thermostat is set. This isn't the first time we delimed it but this hasn't happened before. Water was nice and hot before the annual deliming.

Edit: it lights up like it's sensing flow. Bled the air from the lines, forgot to mention that.

 

[Fitter30]

start by removing the access panel cover and insulation. Locate the red reset button that is typically above the thermostat and press and hold it until you hear a click