Systems and methods for controlling a draft inducer for a furnace

A furnace system operable in at least two operating states comprising:

• a combustion chamber producing combustion gases;
• an exhaust outlet for venting exhaust combustion gases from the combustion chamber;
• at least one heat exchanger for extracting heat from exhaust combustion chamber gases and for transferring heat to conditioned air;
• a fan for moving the combustion chamber gases through the exhaust outlet for inducing a draft in the combustion chamber that causes a pressure drop across the heat exchanger wherein the density of the combustion chamber gases flowing across the heat exchanger and the fan differs from a first operating state to a second operating state;
• a motor including a shaft for driving the fan in response to a motor control signal so that different motor speeds result as a function of the density of the combustion chamber gases flowing across the fan;
• a speed circuit providing a speed signal representative of the motor speed;
• a circuit for defining first and second sets of speed/torque curves corresponding to a desired pressure across the heat exchanger;
• and
• a control circuit responsive to the speed signal for generating the motor control signal as a function of the first set of speed/torque curves until the speed signal indicates that the motor speed has reached a predetermined speed and for generating the motor control signal as a function of the second set of speed/torque curves after the speed signal indicates that the motor speed has reached the predetermined speed whereby the motor will operate in accordance with one or more of the first speed/torque curves when the furnace system is in the first operating state and in accordance with one or more of the second speed/torque curves when the furnace system is in the second operating state.

A method of operating a draft inducer apparatus for use with a furnace that includes a combustion chamber producing combustion gases and an exhaust outlet for venting exhaust combustion gases from the furnace and that also includes at least one heat exchanger for extracting heat from exhaust combustion chamber gases and transferring heat to conditioned air, and for use with a fan for moving the combustion chamber gases through the exhaust outlet for inducing a draft in the combustion chamber that causes a pressure drop across the heat exchanger, said furnace being operable in at least two operating states wherein the density of the combustion chamber gases flowing across the heat exchanger and the fan differs from a first operating state to a second operating state, said method comprising the steps of:

• driving the fan with a motor in response to a motor control signal so that different motor speeds result as a function of the density of the combustion chamber gases flowing across the fan;
• providing a speed signal representative of the motor speed;
• defining first and second sets of speed/torque curves corresponding to a desired pressure across the heat exchanger;
• generating the motor control signal in response to the speed signal as a function of the first set of speed/torque curves until the speed signal indicates that the motor speed has reached a predetermined speed;
• and
• generating the motor control signal in response to the speed signal as a function of the second set of speed/torque curves after the speed signal indicates that the motor speed has reached the predetermined speed whereby the motor will operate in accordance with one or more of the first speed/torque curves when the furnace is in the first operating state and in accordance with one or more of the second speed/torque curves when the furnace is in the second operating state.

A method of operating a furnace system in at least two operating states, said furnace system including a combustion chamber producing combustion gases and an exhaust outlet for venting exhaust combustion gases from the furnace system and also including at least one heat exchanger for extracting heat from exhaust combustion chamber gases and transferring heat to conditioned air and a fan for moving the combustion chamber gases through the exhaust outlet for inducing a draft in the combustion chamber that causes a pressure drop across the heat exchanger wherein the density of the combustion chamber gases flowing across the heat exchanger and the fan differs from a first operating state to a second operating state, said method comprising the steps of:

• driving the fan with a motor in response to a motor control signal so that different motor speeds result as a function of the density of the combustion chamber gases flowing across the fan;
• providing a speed signal representative of the motor speed;
• defining first and second sets of speed/torque curves corresponding to a desired pressure across the heat exchanger;
• generating the motor control signal in response to the speed signal as a function of the first set of speed/torque curves until the speed signal indicates that the motor speed has reached a predetermined speed;
• and
• generating the motor control signal in response to the speed signal as a function of the second set of speed/torque curves after the speed signal indicates that the motor speed has reached the predetermined speed whereby the motor will operate in accordance with one or more of the first speed/torque curves when the furnace system is in the first operating state and in accordance with one or more of the second speed/torque curves when the furnace system is in the second operating state.

A draft inducer system for use with a furnace that includes a combustion chamber producing combustion gases and an exhaust outlet for venting exhaust combustion gases from the furnace and that also includes at least one heat exchanger for extracting heat from exhaust combustion chamber gases and transferring heat to conditioned air, said furnace being operable in at least two operating states wherein the density of the combustion chamber gases flowing across the heat exchanger differs from a first operating state to a second operating state, said system comprising:

• a fan for moving the combustion chamber gases through the exhaust outlet for inducing a draft in the combustion chamber that causes a pressure drop across the heat exchanger wherein the density of the combustion chamber gases flowing across the fan differs from the first operating state to the second operating state;
• a motor including a shaft for driving the fan in response to a motor control signal so that different motor speeds result as a function of the density of the combustion chamber gases flowing across the fan;
• a speed circuit providing a speed signal representative of the motor speed;
• a circuit for defining first and second sets of speed/torque curves corresponding to a desired pressure across the heat exchanger;
• and
• a control circuit responsive to the speed signal for generating the motor control signal as a function of the first set of speed/torque curves until the speed signal indicates that the motor speed has reached a predetermined speed and for generating the motor control signal as a function of the second set of speed/torque curves after the speed signal indicates that the motor speed has reached the predetermined speed whereby the motor will operate in accordance with one or more of the first speed/torque curves when the furnace is in the first operating state and in accordance with one or more of the second speed/torque curves when the furnace is in the second operating state.

A draft inducer apparatus for use with a furnace that includes a combustion chamber producing combustion gases and an exhaust outlet for venting exhaust combustion gases from the furnace and that also includes at least one heat exchanger for extracting heat from exhaust combustion chamber gases and transferring heat to conditioned air, and for use with a fan for moving the combustion chamber gases through the combustion chamber for inducing a draft in the combustion chamber that causes a pressure drop across the heat exchanger, said furnace being operable in at least two operating states wherein the density of the combustion chamber gases flowing through the combustion chamber and across the heat exchanger differs from a first operating state to a second operating state, said apparatus comprising:

• a motor including a shaft for driving the fan in response to a motor control signal so that different motor speeds result as a function of the density of the combustion chamber gases flowing through the combustion chamber;
• a speed circuit providing a speed signal representative of the motor speed;
• a circuit for defining first and second sets of speed/torque curves corresponding to a desired pressure across the heat exchanger;
• and
• a control circuit responsive to the speed signal for generating the motor control signal as a function of the first set of speed/torque curves until the speed signal indicates that the motor speed has reached a predetermined speed and for generating the motor control signal as a function of the second set of speed/torque curves after the speed signal indicates that the motor speed has reached the predetermined speed whereby the motor will operate in accordance with one or more of the first speed/torque curves when the furnace is in the first operating state and in accordance with one or more of the second speed/torque curves when the furnace is in the second operating state.

A draft inducer apparatus for use with a furnace that includes a combustion chamber producing combustion gases and an exhaust outlet for venting exhaust combustion gases from the furnace and that also includes at least one heat exchanger for extracting heat from exhaust combustion chamber gases and transferring heat to conditioned air, and for use with a fan for moving the combustion chamber gases through the exhaust outlet for inducing a draft in the combustion chamber that causes a pressure drop across the heat exchanger, said furnace being operable in at least two operating states wherein the density of the combustion chamber gases flowing across the heat exchanger and the fan differs from a first operating state to a second operating state, said apparatus comprising:

• a motor including a shaft for driving the fan in response to a motor control signal so that different motor torques result as a function of the density of the combustion chamber gases flowing across the fan;
• a torque circuit providing a torque signal representative of the motor torque;
• a circuit for defining first and second sets of speed/torque curves corresponding to a desired pressure across the heat exchanger;
• and
• a control circuit responsive to the torque signal for generating the motor control signal as a function of the first set of speed/torque curves until the torque signal indicates that the motor speed has reached a predetermined torque and for generating the motor control signal as a function of the second set of speed/torque curves after the torque signal indicates that the motor torque has reached the predetermined torque whereby the motor will operate in accordance with one or more of the first speed/torque curves when the furnace is in the first operating state and in accordance with one or more of the second speed/torque curves when the furnace is in the second operating state.