High efficiency modulating gas furnace

A gas furnace comprising:

• a furnace controller;
• an induced draft blower that is coupled to the furnace controller, the induced draft blower comprising an inducer motor that is controlled by the furnace controller and an inducer fan that is coupled to the inducer motor;
• a pressure switch assembly that is coupled to the furnace controller, the pressure switch assembly comprising at least one pressure switch associated with a firing rate of the gas furnace,
• wherein an input contact of the at least one pressure switch is connected to an output port of the furnace controller that supplies power to the at least one pressure switch, and
• wherein an output contact of the at least one pressure switch is connected to an input port of the furnace controller;
• and
• a gas valve that is connected to an electrical relay;
• and
• a backup electrical relay that is connected in series with the electrical relay, wherein an input terminal of the backup electrical relay is connected to the output port of the furnace controller,
• wherein upon receiving a heat call, the furnace controller is configured to operate the induced draft blower at or close to a lowest speed that is needed to keep electrical contacts of the at least one pressure switch closed,
• wherein the lowest speed that is needed to keep electrical contacts of the at least one pressure switch closed is below a make point speed of the induced draft blower at which the electrical contacts of the at least one pressure switch close at a steady-state heating condition of the gas furnace, but above a break point speed of the induced draft blower at which the electrical contacts of the at least one pressure switch open at the steady-state heating condition of the gas furnace,
• wherein when the electrical contacts of the at least one pressure switch remain open for more than a predetermined time period, the furnace controller is configured to increase a speed of the induced draft blower to a new speed to close the electrical contacts of the at least one pressure switch without shutting down the combustion heat cycle, and
• wherein if the electrical contacts of the at least one pressure switch close and remain closed for another predetermined time period when the speed of the induced draft blower is increased to the new speed, the furnace controller is configured to:
• record the new speed as a new make point speed of the induced draft blower at which the electrical contacts of the at least one pressure switch close, and reduce the speed of the induced draft blower from the new make point speed to determine a new break point speed at which the electrical contacts of the at least one pressure switch open.

A system comprising:

• a gas furnace that comprises:
• an induced draft blower that is coupled to a furnace controller;
• and
• a pressure switch assembly that is coupled to the furnace controller, the pressure switch assembly comprising at least one pressure switch associated with a firing rate of the gas furnace,
• wherein an input contact of the at least one pressure switch is connected to an output port of the furnace controller that supplies power to the at least one pressure switch, and
• wherein an output contact of the at least one pressure switch is connected to an input port of the furnace controller;
• and
• wherein the furnace controller is configured to:
• receive a first heat call;
• learn and record at the furnace controller:
• a make point speed at which electrical contacts of the at least one pressure switch close during a combustion heat cycle when the gas furnace is operating at a steady-state heating condition,
• a break point speed at which electrical contacts of the at least one pressure switch open during the combustion heat cycle when the gas furnace is operating at the steady-state heating condition, and
• another make point speed at which the electrical contacts of the at least one pressure switch close during the combustion heat cycle prior to an ignition sequence of a combustion heat cycle;
• responsive to recording the make point speed, the break point speed, and the other make point speed,
• increase a speed of the induced draft blower to a make point speed to close the electrical contacts of the at least one pressure switch, and
• reduce the speed of the induced draft blower below the make point speed such that:
• (a) the induced draft blower operates between the make point speed and the break point speed, and (b) the electrical contacts of the at least one pressure switch remain closed
• receive a second heat call;
• increase the speed of the induced draft blower above the other make point by a predetermined value to start an ignition sequence;
• and
• responsive to reaching a steady-state heating condition after the ignition sequence, reduce the speed of the induced draft blower to operate between the make point speed and the break point speed to satisfy a heat demand associated with the second heat call.

A method of manufacturing a high efficiency gas furnace comprising a furnace controller, an induced draft blower, and at least one pressure switch, the method comprising:

• connecting the induced draft blower to a furnace controller;
• connecting an input contact of the at least one pressure switch to an output port of the furnace controller that supplies power to the at least one pressure switch;
• connecting an output contact of the at least one pressure switch to an input port of the furnace controller;
• connecting the output terminal of an electrical relay to a gas valve;
• connecting the input terminal of the electrical relay to the output terminal of a backup electrical relay such that the electrical relay is in a series electrical circuit with the backup electrical relay;
• and
• connecting the input terminal of the backup electrical relay to the output port of the furnace controller,
• wherein the furnace controller is configured to operate the induced draft blower at or close to a lowest speed that is needed to keep electrical contacts of the at least one pressure switch closed in response to receiving a heat call,
• wherein the lowest speed that is needed to keep the electrical contacts of the at least one pressure switch closed is below a make point speed of the induced draft blower at which the electrical contacts of the at least one pressure switch close when the gas furnace is operating at a steady-state heating condition, but above a break point speed of the induced draft blower at which the electrical contacts of the at least one pressure switch open when the gas furnace is operating at a steady-state heating condition,
• wherein when the electrical contacts of the at least one pressure switch remain open for more than a predetermined time period, the furnace controller is configured to increase a speed of the induced draft blower to a new speed to close the electrical contacts of the at least one pressure switch without shutting down the combustion heat cycle, and
• wherein when a number of attempts to close the electrical contacts of the at least one pressure switch by increasing the speed of the induced draft blower is equal to or greater than a threshold number of attempts, the furnace controller is configured to de-energize the gas valve and shut off the combustion heat cycle by controlling the backup electrical relay and the electrical relay.