Manual Adjustment of Control Throttling Range for Tighter Control of Space Temperature and Humidity
Whereas space temperature controls for maintaining human thermal comfort typically have a deadband of several degrees between heating and cooling setpoints, plus an allowance for control of airflow, water, coils, etc. as the space temperature strays outside of the band defined by the setpoints, projects such as museum archives and laboratories may require considerably tighter controls.
- While there are exceptions for buildings where tight control is critical (labs, museums, certain health-care applications, etc.) ASHRAE standard 90.1 requires at least 5°F between the heating and cooling setpoints to allow for a deadband and to avoid wasting energy on overly tight control (assuming this is simply for human thermal comfort). It is also important to maintain some control bandwidth so that there is a range of temperature over which controls can modulate thermal inputs to the space. In publication DA28 Building Management and Control Systems from AIRAH, the Australian equivalent of ASHRAE, the recommendation for VAV applications is a minimum of 1K actual (truly neutral) deadband between any control actions, plus control ramp bandwidths of at least 1K each for both heating and cooling controls. With setpoints occurring in the middle of the two control bands, this translates to heating and cooling setpoints separated by at least 2K.
- Among the four most common errors listed for VAV system control in actual building, AIRAH includes “Small or non-existent deadbands lead to excessive instability.” The one case where AIRAH suggests there might be less difference between heating and cooling set points is with PID controllers, which can provide tighter control without excessive energy consumption but are expensive to set up and thus less common outside of process applications. PID controls can only be approximated by proportional controls modeling, and even with PID controls and high-quality sensors, the actual control system will need some bandwidth over which to control dampers, coils, etc. Furthermore, this tendency toward instability for very tight controls is exaggerated in simulation, as reality has an unlimited number of time steps over which to make tiny corrections whereas a whole-building simulation will use time steps somewhere between 1 minute and 60 minutes (typically 6 to 10 minutes for the VE).
The pre-defined controls and HVAC profiles require a minimum of 4°F (2.22 K) between the heating and cooling setpoints to avoid overlap of controls, and using this minimum value would provide zero actual deadband (one or the other control would always be active).
The graphic above from the ApacheHVAC user guide section 8.3.15 VAV airflow controls shows the relationship of the standard pre-defined control midbands and bandwidth relative to zone heating and cooling setpoints. In this example, the heating and cooling setpoints are separated by 6°F, and it can be seen that the ramps for cooling airflow and heating coil control will overlap if this difference is reduced by more than 2°F.
If you require tighter control than is allowed by the pre-defined controls, you must either 1) modify the HVAC control bandwidths and/or 2) modify the HVAC control profiles to move the control midband values closer to the setpoint, depending on the intended outcome. Appropriate controls sequencing—i.e., without overlap of controls such as airflow modulation and coil LAT modulation—usually requires that the bandwidth are reduced whenever the midband offset reduced. These operations are described below.