Coordinated air-side control of HVAC system

1. A method for linking control in an heating ventilation and air conditioning (HVAC) system, comprising: receiving a fan speed signal from a fan control loop;

choosing among an independent mode, a first linked mode, and a second linked mode, depending on the fan speed signal;

providing a signal to a pressure control loop to reduce a speed of a supply air fan of an air handling unit, when the first linked mode is chosen;

providing a signal to a temperature control loop to decrease a speed of a heat exchange wheel, to decrease a flow rate of a heat exchange fluid to an evaporator coil, or both, when the second linked mode is chosen; and

allowing the fan control loop to operate independently of the temperature and pressure control loops, when the independent mode is chosen.

2. The method of claim 1 , wherein providing the signal to the pressure control loop when the first linked mode is chosen comprises signaling the pressure control loop to decrease the speed of the supply air fan to substantially zero.

3. The method of claim 1 , further comprising providing a signal to the temperature control loop to decrease the speed of the heat exchange wheel, to decrease the flow rate of the heat exchange fluid, or both, when the first linked mode is chosen.

4. The method of claim 3 , wherein providing the signal to the temperature control loop when the first linked mode is chosen comprises signaling the temperature control loop to power off the heat exchange wheel, close a control valve configured to control the flow rate of the heat exchange fluid, or both.

5. The method of claim 1 , wherein providing the signal to the temperature control loop when the second linked mode is chosen comprises signaling the temperature control loop to adjust a supply air temperature according to a dew point and a dew point threshold.

6. The method of claim 5 , wherein signaling the temperature control loop to adjust the supply air temperature according to a dew point and a dew point threshold comprises:

setting a supply air temperature setpoint to a first value that is above the dew point, when the dew point is below or equal to the threshold; and

setting a supply air temperature setpoint to a second value that is below the dew point, when the dew point is greater than the threshold.

7. The method of claim 1 , wherein choosing among the independent mode, the first linked mode, and the second linked mode comprises:

choosing the first linked mode when the fan speed is below a first threshold;

choosing the second linked mode when the fan speed is between the first threshold and a second threshold; and

choosing the third linked mode when the fan speed is above the second threshold.

8. The method of claim 7 , wherein the first threshold is between about 1% and about 25% of a maximum speed of the fan.

9. The method of claim 8 , wherein the second threshold is between about 10% and about 30% of the maximum speed of the fan.

10. The method of claim 1 , wherein providing the signal to the temperature control loop comprises altering a setpoint signal, an error signal, or both of the temperature control loop.

11. The method of claim 1 , wherein providing the signal to the temperature control loop comprises altering a control signal produced by a controller of the temperature control loop.

12. The method of claim 1 , wherein providing the signal to the pressure control loop comprises altering a setpoint signal, error signal, or both of the pressure control loop.

13. The method of claim 1 , wherein providing the signal to the pressure control loop comprises altering a control signal produced by a controller of the pressure control loop.

14. The method of claim 1 , further comprising receiving one or more additional fan speed signals from one or more additional fan speed control loops, wherein choosing among the independent mode, the first linked mode, or the second linked mode comprises averaging the fan speed signal and each of the one or more additional fan speed signals.

15. The method of claim 1 , wherein the fan speed control loop is configured to control a speed of a chilled beam fan.

16. The method of claim 1 , wherein the fan speed control loop is configured to control a speed of at least one fan of a fan coil unit.

17. An apparatus for controlling an heating ventilation and air conditioning (HVAC) system, comprising:

one or more fan control loops each configured to control a speed of one or more chilled beam fans;

a plant temperature control loop configured to control a speed of a supply air fan of an air handling unit;

a plant pressure control loop configured to control at least one of a speed of a heat exchange wheel of the air handling unit and a position of a control valve configured to control a flow rate of a refrigerant between a heat pump of the air handling unit and an evaporator coil of the air handling unit; and

a linking controller coupled to the one or more fan control loops, the plant temperature control loop, and the plant pressure control loop, the linking controller being configured to receive a signal from the one or more fan control loops indicative of the speed of the one or more chilled beam fans and to choose among an independent mode, a first linked mode, and a second linked mode based on the speed of the one or more chilled beam fans, wherein, in the independent mode, the linking controller allows the plant temperature control loop and the plant pressure control loop to operate independently from the one or more fan control loops, in the first linked mode, the linking controller overrides the plant pressure control loop and causes the speed of the supply air fan to reduce to substantially zero, and in the second linked mode, the linking controller overrides the plant temperature control loop and causes at least one of the speed of the heat exchange wheel to reduce to substantially zero and the control valve to close or allow for a minimum flow rate.

18. The apparatus of claim 17 , wherein, in the first linked mode, the linking controller is configured to override the plant temperature control loop and cause at least one of the speed of the heat exchange wheel to reduce to substantially zero and the control valve to close or allow for a minimum flow rate.

19. The apparatus of claim 17 , wherein the linking controller is configured to choose the first linked mode when the speed of the chilled beam fan is below a first threshold, to choose the second linked mode when the speed of the chilled beam fan is above the first threshold but less than a second threshold, and to choose the independent mode when the speed of the chilled beam fan is above the second threshold.

20. The apparatus of claim 17 , wherein:

at least one of the one or more fan control loops includes a controller and a sensor coupled together, the linking controller being coupled to the fan control loop between the controller and the sensor;

the plant temperature control loop includes a setpoint module, a controller, and a sensor, the linking controller being coupled to the plant temperature control loop at the setpoint module, between the controller and the sensor, or both; and

the plant pressure control loop includes a setpoint module, a controller, and a sensor, the linking controller being coupled to the plant temperature control loop at least at the setpoint module, between the controller and the sensor, or both.