Rooftop high-efficiency gas furnace control with condensate management

1 . A method of controlling an efficiency and/or a wet/dry transition area of a condensing furnace heat exchanger, comprising the steps of:

establishing an air/fuel relationship empirically to modulate the condensing furnace heat exchanger throughout a modulation range;

positioning a temperature sensor in an air stream entering a heat exchanger section to detect an average bulk temperature of the air stream downstream with respect to a mixer for sensing a mixed air temperature when an outdoor air is mixed with a recirculated air;

sensing an incoming air temperature of the air stream entering the heat exchanger section to detect a temperature change in the air stream;

establishing a relationship between the incoming air temperature and an air/fuel mixture supplied to a burner for maintaining an exchanger temperature of a primary heat exchanger of the heat exchanger section which is below a dew point temperature of combustion products that form condensation on a condensate-corrosive surface of the heat exchanger section;

adjusting the air/fuel mixture in the burner to increase excess air and lower a dew point of the combustion products as the sensed incoming air temperature decreases; and

adjusting the air/fuel mixture to the burner dynamically at predetermined increments as a function of the temperature change in the air stream.

2 . The method of claim 1 wherein the air/fuel mixture is automatically adjusted.

3 . The method of claim 1 further including establishing a relationship between the incoming air temperature and a minimum modulation firing rate of the condensing furnace in which unwanted condensation forms on heat exchanger surfaces and the condensing furnace modulates at a minimum acceptable rate;

adjusting a minimum modulation firing rate of the burner to increase a surface temperature of the heat exchanger section to reduce condensation on the condensate-corrosive surface of the heat exchanger section; and

adjusting the minimum modulation firing rate of the burner dynamically at predetermined increments as a function of the sensed incoming air temperature.

4 . The method of claim 3 wherein the minimum modulation firing rate is automatically adjusted.

5 . The method of claim 1 further including positioning a second temperature sensor to sense a temperature of the heat exchanger surface at a critical area of the heat exchanger section to reduce the condensation on the condensate-corrosive surface of the heat exchanger section;

adjusting the air/fuel mixture in the burner to increase an amount of excess air in the burner and lower a dew point temperature of the combustion products as the sensed heat exchanger surface temperature decreases; and

adjusting the air/fuel mixture to the burner dynamically at predetermined increments as a function of the sensed heat exchanger surface temperature.

6 . The method of claim 5 wherein the air/fuel mixture is automatically adjusted.

7 . The method of claim 5 further including establishing a relationship between the sensed heat exchanger surface temperature and the minimum modulation firing rate to reduce condensation forming on the condensate-corrosive surface of the heat exchanger section and to maintain the minimum modulation firing rate;

adjusting the minimum modulation firing rate of the burner to increase heat exchanger surface temperatures to reduce condensation on the condensate-corrosive surface of the heat exchanger section; and

adjusting the minimum modulation firing rate of the burner dynamically at predetermined increments as a function of the sensed heat exchanger surface temperature.

8 . The method of claim 7 wherein the minimum modulation firing rate is automatically adjusted.

9 . A method of controlling an efficiency and/or a wet/dry transition area of a condensing furnace heat exchanger, comprising the steps of:

establishing a minimum average bulk temperature of an air stream entering a heat exchanger section to maintain a heat exchanger temperature above a minimum temperature;

positioning a temperature sensor in the air stream entering the heat exchanger section to detect an average bulk temperature of the air stream downstream of a mixer for mixing an outdoor air with any recirculated air and sensing any mixed air temperature;

sensing an incoming air temperature of the airstream entering the heat exchanger section to detect a temperature change in the air stream;

changing a damper position of a fresh air damper to reduce an amount of the outdoor air mixed with the recirculated air to maintain a minimum air temperature of the mixed air stream; and

adjusting the damper position dynamically at predetermined increments as a function of the sensed incoming air temperature.

10 . The method of claim 9 wherein the damper position is automatically adjusted.

11 . The method of claim 9 further including establishing a relationship between the sensed incoming air temperature and the damper position as a function of unwanted condensation forming on heat exchanger surfaces and the minimum temperature at which the furnace operates;

adjusting a minimum fresh air damper position to increase surface temperatures of the heat exchanger to reduce condensation on condensate-corrosive surfaces of the heat exchanger; and

adjusting the fresh air damper position dynamically at predetermined increments as a function of an increase or a decrease of the sensed incoming air temperature.

12 . The method of claim 11 wherein the fresh air damper position is automatically adjusted.

13 . The method of claim 9 operated in a mid-efficiency, non-condensing furnace.

14 . A method of maintaining minimum off-cycle ambient temperatures in or near critical drain components including a reservoir, a drain valve or an other critical component critical to a condensation management function of the furnace, comprising the steps of:

monitoring a temperature sensor located in, on or near a critical condensate management component and sensing when a sensed temperature drops below an established threshold;

igniting furnace burners with no active call for heat;

operating the furnace burners at a predetermined firing rate of the sensed temperature;

operating the furnace burners until a time that a sensed ambient temperature exceeds a predetermined threshold and then terminating the furnace burners operation.

15 . The method of claim 14 further including limiting the furnace burners operation to a predetermined maximum run time and then terminating the furnace burners operation when reaching a maximum run time.

16 . The method of claim 14 further including monitoring a second temperature sensor located on or near a critical heat exchanger surface to limit a design temperature and terminating the furnace burners operation when a maximum temperature threshold is sensed by the second temperature sensor.

17 . A method of purging a liquid condensate from a drain system of a condensing furnace condensate disposal system, comprising the steps of:

controlling an electrically operated drain valve pneumatically connected on a first side to a combustion air blower to supply a positive pressure from the blower and pneumatically connected on a second side to a condensate disposal system that a drain valve and an other critical drain component are positioned downstream from a valve connection point;

opening the drain valve after a burner cycle terminates and energizing the combustion air blower to create the positive pressure in the drain components to force residual liquid condensate from the drain components into a furnace drain line; and

closing the drain valve and de-energizing the combustion air blower after a predetermined time necessary to expel a residual liquid condensate.

18 . The method recited in claim 17 further including monitoring an ambient air temperature sensor and opening the drain valve and energizing the combustion air blower only when sensing an ambient air temperature below a predetermined threshold.

19 . A method of maintaining minimum off-cycle ambient temperatures in or near critical drain components including a reservoir, a drain valve or an other critical component critical to a condensation management function of the furnace, comprising the steps of:

monitoring a temperature sensor located in, on or near a discharge of a condensate sump and sensing when a sensed temperature drops below an established threshold;

emitting a signal of the sensed temperature to a condensate controller and/or a furnace controller;

activating a circulating air blower with no active call for heat;

operating the circulating air blower at a predetermined blower speed based on the sensed temperature; and

operating the circulating air blower until a time that a sensed ambient temperature exceeds a predetermined threshold and then terminating operation of the circulating air blower.

20 . The method of claim 19 further including limiting the furnace burners operation to a predetermined maximum run time and then terminating the circulating air blower operation when reaching a maximum run time.

21 . The method of claim 19 further including limiting the furnace burners operation to a predetermined maximum run time and then limiting the circulating air blower operation to a predetermined maximum run time of the circulating air blower.

22 . The method of claim 19 wherein the drain valve is in communication with the discharge of the condensate sump.