Tankless water heater causes magnets (ceiling fans, induction oven) to hum while actively heating water
The problem is probably that the new unit uses solid-state control to vary the power delivered to the heating elements, which makes it more energy-efficient than the old-fashioned on/off control. Those solid-state controllers are notorious for introducing harmonic &quot;noise&quot; on the mains, and a 13 kW one could easily affect other circuits fed from the same main breaker panel. Resistive loads like heaters and incandescent lights aren't affected much, but loud humming from magnetic/inductive loads is textbook harmonic interference.
The first thing I'd try is complain to Rheem about excessive harmonic noise. An expensive brand-name water heater should have some built-in EMI (Electro-Magnetic Interference) filtering to minimize the effect. If it's making your induction stove sing, it's definitely &quot;excessive&quot; harmonics, and it might possibly even damage sensitive electronics plugged in elsewhere in the house. (That's not very likely, but is technically possible, and if needed it might make a useful line of argument to get Rheem motivated to take you seriously.)
The traditional way to eliminate harmonic interference (used with industrial VFD variable-frequency motor controllers, for example) is to connect the &quot;noisy load&quot; (your water heater) via a harmonic filter or isolation transformer. That's very effective, but I can't say I've ever seen a 13 kW harmonic isolation transformer marketed toward residential use... and the cost may be prohibitive. Industrial electrical suppliers and data-center power-supply companies are the obvious place to start looking if you go that route.