Rinnai HD70E PCB Control Unit - The Only Genuine Replacement You Should Use The truth is everything that goes wrong on a Rinnai HD70E is usually down to a fault in the electric control unit. The PCB controls the water flow sensor, the solenoids, and both servo motors and it's the bit that keeps the whole system running. When the control unit goes, the effects are instant: the heater may not light, it won't regulate the temperature like it should or you're stuck with error codes that no amount of fiddling with the reset won't clear. The HD70E has a main processor with a built in temperature limiter - and that capability sits inside the control unit. Replacing it with a dodgy second hand part risks throwing your whole system out of calibration, you'll also lose the dip switch settings and any remote temperature controllers you have fitted will stop working. Thi