@IsraelDelendaEst @wizardmanperson @bronze @grey I looked it up after asking, and people have had this discussion hundreds of times online. Instant hot water heaters can handle it, but you need the stainless steel type that don't get stripped by the extremely low dissolved solids, often acidic, water from RO. It's acidic because of dissolved carbonic acid which returns to solution under heat. So basically it's going to eat my heater alive and not worth it.