The firebox is surrounded by water. When the water temperature drops below 160° F, the air injection system kicks in; air is blown on the fire. This causes the fire to instantly go from a dormant stage to an intense hot burn. In minutes the temperature of the water in the system will reach 180° F, at which time the injection air will turn off. The fire, now starved of air will return to a dormant stage, burning very little wood. Meanwhile, the hot water in the system is being pumped into the building 24 hours a day. The furnace water does not enter the tank but heats by convection. The hot water now moves on to heat the building, which can be done by several methods. With hot water systems the furnace can be connected directly into the existing system using a water heat exchanger. If the building has forced air heat, a water-to-air exchanger is used in the plenum. Other choices could be water baseboard heaters; in-floor piping or combining blower fan/exchangers. Every set-up uses a thermostat system to ensure a constant temperature in the building. The original heating method is not disconnected but now acts as a backup system. After transferring its heat to the building, the cooled water is returned to the outdoor furnace.