A water heating system uses a heated fluid storage tank to deliver a continuous supply of water heated to a desired temperature, such as between 100°-130° F. The system also includes a furnace with altitude-sensitive control circuitry to provide multiple sources of heat for the heating system in the most effective way given the altitude at which the system is located. The system also includes a micro-controller that adjusts certain system components in response to changes in atmospheric pressure conditions that are measured by an atmospheric-pressure sensor component. There is also an automatic-air-bleeder subsystem with an optical or ultrasonic sensor mounted adjacent a suitable air accumulator. Also included is an air-release solenoid and a fuel return line.