Methods of controlling fan capacity and coil capacity in hvac systems

A method of controlling a HVAC system with a three-speed discharge fan with a low speed setting, an intermediate speed setting and a high speed setting, comprising:

• maintaining a discharge fan capacity at a low fan capacity setting and varying a coil discharge air temperature between a low discharge air temperature limit and a high discharge air temperature limit;
• and
• when the coil discharge air temperature reaches the low discharge air temperature limit and when the discharge fan capacity required to maintain the space temperature setpoint is higher than the low fan capacity setting, varying a fan speed between the low speed setting and the intermediate speed setting while maintaining the discharge coil temperature at the low discharge air temperature limit,
• when the coil discharge air temperature reaches the low discharge air temperature limit and when the discharge fan capacity required to maintain the space temperature setpoint is higher than the intermediate fan capacity setting, varying the fan speed between the intermediate speed setting and the high speed setting while maintaining the coil discharge air temperature at the low discharge air temperature limit.

A HVAC system, comprising:

• a fan configured to provide a variable fan capacity between a high fan capacity setting and a low fan capacity setting;
• a coil configured to provide a variable discharge air temperature between a high discharge air temperature limit and a low discharge air temperature limit;
• and
• a controller configured to control the variable fan capacity of the fan and the variable discharge air temperature of the coil so as to maintain a space temperature setpoint;
• wherein when the variable discharge air temperature reaches the high discharge air temperature limit, the controller is configured to vary the variable fan capacity of the fan to maintain the space temperature setpoint while maintaining the variable discharge air temperature of the coil at the high discharge air temperature limit;
• when the variable discharge air temperature reaches the low discharge air temperature limit, the controller is configured to vary the variable capacity of the fan to maintain the space temperature setpoint while maintaining the variable discharge air temperature of the coil at the low discharge air temperature limit;
• and
• when the variable discharge air temperature is between the high discharge air temperature limit and the low discharge air temperature limit, the controller is configured to vary the variable discharge air temperature between the high discharge air temperature limit and the low discharge air temperature limit to maintain the space temperature setpoint while maintaining the variable capacity of the fan at the low fan capacity setting.