When you hook a HydraTherm up to your in-house water heater, it pushes fresh hot water to your bath tub and through the HydraTherm, doing double duty. Most of the time a residential heater cannot carry you like that. Especially in the morning at shower time. Still, if your heater was big enough, it would have always fresh water coming in and would maintain pressure from the water line. These are easier to plumb because there's no trouble getting the head space taken up by air out of the boiler.