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Control Freak: Do Adjustable Temperature Set Points Lead to Wasted Energy?

Too Much Control?

By Rebecca T. Ellis, P.E.

The idea for this month’s topic was hatched during a retro-commissioning project, but it offers lessons to be applied to new construction commissioning. In short, there is such a thing as “too much control” when it comes to offering the building operator opportunities to adjust set points via the central BAS graphics.

I imagine readers are almost universally familiar with the BAS sequence of operations specification language that defines an initial set point value for a control point and then parenthetically makes it clear that the set point is “adjustable.” For example, “The space temperature set point shall be 72°F (adjustable).” This typically implies that the building operator shall have the capability of changing that space temperature set point as deemed appropriate during post construction operation. The means for providing this adjustment is, at a minimum, an editable object on the BAS graphic screen for the conditioned space.

Giving the building operator the freedom to individually adjust both of those set points is a risky use of the power of a BAS system. If, for some reason, such flexibility is deemed necessary or desirable, it’s critical that the graphic screen also include instructions about how to make smart choices in those adjustments and the potential implications of making bad choices.

A very simple example of this would be a single-zone AHU graphic that has a dual space temperature set point, i.e., one set point for cooling and one for heating. The cooling set point presumably controls the cooling valve and economizer dampers, while the heating set point controls the heating valve. When both of these set points are adjustable, there are significant opportunities for wasted energy due to operator error or misunderstanding.

What happens when the set points are as follows?

Heating set point = 74°F
Cooling set point = 72°F

The heating valve will open fully, the cooling valve will open fully, and the space temperature will be unlikely to equal either of the set points. This would be a huge energy conservation fail but would be caught relatively quickly due to uncomfortable space conditions.

What happens when the set points are as follows?

Heating set point = 72°F
Cooling set point = 72°F

The two valves will continuously fight each other, cycling open/closed. As the heating coil overshoots the common set point, it will close, and the cooling valve will open to compensate. Once the cooling valve opens, it will undershoot the set point, and the cycle will start again. This should allow the space temperature to maintain its common set point but at the cost of excessive energy and wear and tear on the valves.

The designer’s intent with dual temperature set points is almost always to keep a reasonable distance between the two set points to avoid such fighting or hunting. Old-fashioned stand-alone dual temperature thermostats actually have a built-in mechanism to prevent the lower heating set point adjustment arm from coming within about 4°F of the higher cooling set point adjustment arm.

Luckily, for the simple example noted earlier, most BAS systems and programmers handle dual temperature set points with embedded dead bands (i.e., adjustable by reprogramming but not via the BAS) for the space set point. For example, the operator has the ability to adjust a single space temperature set point via the BAS; however, the BAS calculates the heating set point to be 1°F lower than the operator-selected value and the cooling set point to be 1°F higher than the operator-selected value. The BAS graphic can still display the heating set point and cooling set point values, but those values are not adjustable via the user interface.

Although it may be clear to many readers, I am writing this because I recently saw this exact example on a real and relatively new BAS. Next month, I will discuss how the concept of “too much control” can be harmful in less obvious sequences of operation.