Definitions for "Open Circuit"
To transmit a signal, an electrical circuit must be a complete, closed loop. When this loop is broken or not closed, the circuit is said to be open.
To keep two or more parts electrically separate or disconnected. See also short circuit.
A circuit containing a switch, filament, voice coil, etc., which is not intact and current cannot flow through.
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.