Method and system for supply-air re-circulation

1. A heating, ventilation, and air conditioning (HVAC) system comprising:

an evaporator coil disposed between a supply air duct and a return air duct;

a re-circulation duct, wherein the re-circulation duct fluidly couples the supply air duct and the return air duct;

a damper disposed in the re-circulation duct, the damper being moveable between an open position and a closed position;

a variable-speed circulation fan for circulating air around the evaporator coil;

a variable-speed compressor fluidly coupled to the evaporator coil;

a controller operatively coupled to the variable-speed compressor, the variable-speed circulation fan, and the damper;

a pressure sensor disposed in a suction line between the evaporator coil and the variable-speed compressor, the pressure sensor being configured to measure a refrigerant pressure in the suction line;

wherein the controller is configured to:

determine if the variable-speed circulation fan is operating at a minimum speed and if a suction pressure measured by the pressure sensor is above a pre-determined threshold;

responsive to a determination that the variable-speed circulation fan is operating at the minimum speed and the suction pressure is above the pre-determined threshold, signaling the damper to move to the open position; and

responsive to a determination that the variable-speed circulation fan is not operating at the minimum speed or the suction pressure is below the pre-determined threshold, signaling the damper to move to the closed position.

2. The HVAC system of claim 1 , wherein the speed of the variable-speed circulation fan is modulated responsive to a speed of the variable-speed compressor.

3. The HVAC system of claim 1 , wherein moving the damper to the open position reduces a volume of air supplied via the supply air duct.

4. The HVAC system of claim 1 , wherein the HVAC system operates in at least one of a cooling mode and a dehumidification mode.

5. The HVAC system of claim 4 , wherein the pre-determined threshold is in the range of approximately 130 psi to approximately 150 psi when the HVAC system is operating in the cooling mode.

6. The HVAC system of claim 4 , wherein the pre-determined threshold is in the range of approximately 110 psi to approximately 130 psi when the HVAC system is operating in the dehumidification mode.

7. The HVAC system of claim 1 , wherein moving the damper to the open position increases latent capacity of the HVAC system.

8. A heating, ventilation, and air conditioning (HVAC) system comprising:

an evaporator coil disposed between a supply air duct and a return air duct;

a re-circulation duct that fluidly couples the supply air duct and the return air duct;

a damper disposed in the re-circulation duct, the damper being moveable between an open position and a closed position;

a variable-speed circulation fan for circulating air around the evaporator coil;

a variable-speed compressor fluidly coupled to the evaporator coil;

a controller operatively coupled to the variable-speed compressor, the variable-speed circulation fan, and the damper;

wherein the controller is configured to:

determine if the variable-speed circulation fan is operating at a minimum speed;

monitor an operating parameter of the HVAC system;

determine if the operating parameter of the HVAC system exceeds a pre-determined threshold;

responsive to a determination that the variable-speed circulation fan is operating at the minimum speed and the operating parameter exceeds the pre-determined threshold, signaling the damper to move to the open position; and

wherein the operating parameter is a refrigerant pressure in a suction line.

9. The HVAC system of claim 8 , wherein the speed of the variable-speed circulation fan is modulated responsive to a speed of the variable-speed compressor.

10. The HVAC system of claim 8 , wherein moving the damper to the open position reduces a volume of air supplied via the supply air duct.

11. The HVAC system of claim 8 , wherein the HVAC system operates in at least one of a cooling mode and a dehumidification mode.

12. The HVAC system of claim 8 , wherein a pressure sensor is configured to monitor the operating parameter in the suction line.

13. A method of utilizing a re-circulation duct, the method comprising:

determining with an HVAC controller if a variable-speed circulation fan is operating at a minimum speed;

monitoring an operating parameter of an HVAC system;

determining if the operating parameter of the HVAC system exceeds a pre-determined threshold;

responsive to a determination by the HVAC controller that the variable-speed circulation fan is operating at the minimum speed and the operating parameter exceeds the pre-determined threshold, signaling with the HVAC controller a damper disposed in a re-circulation duct that fluidly couples a supply air duct and a return air duct to move to an open position; and

wherein the operating parameter is a refrigerant pressure in a suction line.

14. The method of claim 13 , wherein the monitoring the operating parameter includes monitoring the refrigerant pressure in the suction line with a pressure sensor.

15. The method of claim 13 , wherein the operating parameter is an air temperature in the supply air duct.

16. The method of claim 15 , wherein the monitoring the operating parameter includes monitoring the air temperature in the supply air duct with a temperature sensor.

17. The method of claim 13 , comprising diverting air from the supply air duct to the return air duct via the re-circulation duct responsive to opening the damper.