Control Freak: Don’t let too much Temperature Control Interfere with Efficiency

Last month’s column introduced the concept of giving “too much control” to the building operators through the BAS graphic. I used a simple and fairly obvious example of an air handler with dual temperature set points. This month, I want to share another equally real-life example that is much more subtle.

On a VAV air-handling system, the BAS graphics included the following parameters, all of which were easily adjustable through the user interface:

For the central AHU

- Unoccupied setup temperature set point; and

- Unoccupied setback temperature set point.

For each VAV zone

- Occupied cooling temperature set point;

- Occupied heating temperature set point;

- Unoccupied setup temperature offset set point; and

- Unoccupied setback temperature offset set point.

The AHU was programmed to start in its unoccupied cooling mode during unoccupied hours whenever the warmest VAV zone temperature rose above the unoccupied setup set point. Similarly, the AHU would start in its unoccupied heating mode during unoccupied hours whenever the coldest VAV zone temperature dropped below the unoccupied setback set point. That’s all very normal these days, including having those two set points adjustable by the building operator.

The challenge in this system was the fact that each individual VAV zone had its own setup and setback temperature set points; however, those set points were not clearly identified on the BAS graphic. Instead, the parameters displayed on the graphic were unoccupied temperature “offsets.” In order to know the VAV zone unoccupied setup temperature set point (i.e., the space cooling set point to which the VAV box would control if the AHU was operational during unoccupied hours), the building operator needed to do the following math in his/her head:

Unoccupied Cooling Temperature Set Point =

Occupied Cooling Temperature Set Point + Unoccupied Setup Temperature Offset Set Point

The same was true for the VAV zone unoccupied heating set point:

Unoccupied Heating Temperature Set Point =

Occupied Heating Temperature Set Point - Unoccupied Setup Temperature Offset Set Point

With both VAV occupied set points adjustable and both VAV unoccupied offset set points adjustable, the unoccupied space heating and cooling temperature set points were all over the board and not explicitly displayed in the BAS graphic.

This is not only confusing and hard to manage, it could also result in excess energy consumption. For example, let’s consider the following situation:

The AHU unoccupied setback temperature set point = 65°F; and
The coldest VAV zone’s unoccupied heating temperature set point = 60°.

When the coldest VAV zone temperature drops just below 65°, the AHU will start circulating conditioned air to all of its VAV zones. All of the zones will control their individual calculated unoccupied heating temperature set points, including the coldest zone, whose VAV/reheat will control to maintain its space at a minimum 60°. This coldest space will not actually start warming up until its temperature drops below 60°, and then it will modulate its reheat valve to maintain a constant 60°. It will never raise the coldest zone temperature to the 65° AHU setback temperature set point, thus causing the AHU to run continuously for the rest of the scheduled unoccupied period. This is clearly not a very efficient way of maintaining desired minimum building temperatures.

Seeing this much adjustability in the BAS user interface brings to mind one of my favorite axioms, “Just because you can, doesn’t mean you should.” As noted in last month’s column, it might be best to standardize on pop-up instructions on how to make smart choices in those adjustments and the potential implications of making bad choices. It would be even better to incorporate programming that would prevent the operator from adjusting such interrelated set points to inappropriate values relative to each other.