Apparatus and method for furnace combustion control

Abstract

Furnace combustion in a multi-stage or variable input furnace is controlled by operating the furnace in a high fire mode at furnace start-up until the temperature of the products of combustion, or a temperature corresponding to the products of combustion, equals or exceeds a selected temperature. A temperature sensor, preferably a thermostat, is located downstream of the furnace zone of combustion (e.g., in a flue collector box at an outlet from the heat exchanger). The thermostat provides an indication of the products of combustion temperature and has a set point corresponding to the selected temperature. When the products of combustion temperature is less than the selected temperature, the thermostat contacts remain closed. The furnace is operated in the high fire mode while the thermostat contacts remain closed in response to a demand for heating, irrespective of whether the demand is for first stage or second stage heating. When the selected temperature is reached, the thermostat contacts open, thereby allowing the furnace to be operated in either the high fire or in a low fire mode, depending on the level of demand for heating, as indicated by an external room thermostat. By operating the furnace in the high fire mode until the products of combustion temperature reaches a selected temperature (i.e., during cold start-up operation) condensate formation is reduced and room comfort is enhanced by inhibiting air from being supplied to the room at less than a desirable temperature.