20 Most Recent Noritz 7.1 Gpm Propane Lp Gas Tankless Hot Water Heater - Page 3 Questions & Answers

Power supply issue that will sooner or later become permanent.

No, sounds more like you have an electric heating element that has shorted out.

check the power supply

Loud rumbling noises are often caused by either water boiling in the burner or by entrapped air in the system. To help prevent water boiling make sure that the boiler system has a pressure between 15 - 30 psi. Also check for sufficient water flow and remove any sediment from the in-line filter. To remove entrapped air, install an automatic or manual air vent at the highest point in the heating system and also make sure you have a properly working micro-air vent installed. With the unit turned off and cooled down, you can also try flipping the lever on the pressure relief valve to try and flush out any air in the heat exchanger . Hope this helps.

This is a Noritz Tankless. That whoosh is excess gas being ignited behind the normal lead/lag time for G-Valve opening and igniter on. Might be the Igniter but you'll need a new igniter gasket as if you pull the igniter assembly and clean ONLY WITH A GREEN SCOTCHGUARD PAD, you'll shred the gasket and need a new one. NEVER sandpaper an igniter it is coated and sanding destroys it. Also may be a flowmeter signal problem where flow is sensed but ignition signal is slow to igniter but on-time to gas valve opening resulting in excessive gas pocketing in HE before a clean ignition. I'd say Tech Service Issue as this gas can pocket in the extreme enough to blow the unit off the wall with a late or slow ignition spark.

The error is a HIGH LIMIT THERMAL ERROR. That means Calcium buildup has appeared in the unit. It is also called scale. To clear the error you have to power it off. That is done by the circuit breakers. You should descale the unit first though.