You're welcome to do that, and it's something some off grid systems use, but the energy storage in that sort of thing is fairly poor. Go do the math on it, but

You're welcome to do that, and it's something some off grid systems use, but the energy storage in that sort of thing is fairly poor. Go do the math on it, but you'll find a 55 gallon drum of water, from "room temperature" to "really darn hot" holds 15-20kWh of energy. It's useful, but not a staggeringly large amount for a typical house, that can easily use more than that on a single day's heating. Thermal mass storage requires a lot of mass to do anything useful, or the ability to run exceedingly high temperatures. Or a phase change. Molten salt storage can store a lot of energy, but you're at "very hazardously hot" temperatures with "stuff that's mind-bogglingly corrosive when hazardously hot." It's not the sort of thing most people would want to mess with at home. Myself included, and my tolerance for experimentation is pretty darn high.