Why The Setpoint Is Not The Zone Temp

by Phoenix Energy Technologies on Mar 9, 2020

setpoint_advice

There are some fundamental assumptions and misunderstandings around what setpoint and zone temperatures are. Confusion around the two leads retail and commercial energy users to waste huge amounts of energy. This post delves into those issues.

We have three problems here.

1. Room Temperature

Some in the retail industry have somehow redefined the definition of "Room Temperature". Room Temperature is 72 degrees. NOT 74, 75, or especially not the 76 degrees a lot of retailers set to save on energy costs.

"Room Temperature" is so well defined that medicine and other perishable goods state on the label: "Store at ROOM temperature". It doesn't define what room temperature is. It doesn't have to. The majority of people already know what room temperature is.

Thermostats and BAS setpoints come "out the box" set to 72 degrees F. There's a reason for this. 95% of people are MOST comfortable at precisely 72.5 degrees F.

That's a fact.

2. Setpoints As Levers

The setpoint has become a "lever" that's "pulled" to save energy. This was initially due to demand response initiatives.

I was there in the early days of demand response (which used to be called "peak curtailment"). In fact, I invented the first, early DR curtailment systems at Silicon Energy Corp. 1999.

Back then, in order to shave peak load in an emergency situation, building managers would TEMPORARILY reset the zone temperature from the normal 72 degrees to the higher 76/78 degrees in order to "turn the HVAC OFF".

The customers, building occupants, and employees would endure a temporary uptick in temperature to "do their part" to conserve energy. (This is something retailers do for lighting as well. They turn off every other light fixture during the "crisis".)

Think about this. Setting your setpoint to 76 degrees would turn it off. Always. Guaranteed.

How was this possible?

The only answer must've been that the building temperature setpoint control was set to what it had always been set to since the late 1800s.

72 degrees.

Then around 2008, some folks in the industry got the (not so) bright idea of "Hey! If we can do this during a DR event, why not do it all the time!"

In my blog Is Your Building Running a Fever, I refer to this as the "DR Effect". Energy Management had become mainstream. Everyone was thinking how easy it was. Anyone can do it.

Our industry started doing it. Our customers started doing it.

Adaptive Energy Management - How to Use Your Buildings as a Battery

3. Accuracy

Building and temperature control systems are NOT accurate nor precise. This is important to understand. There's a difference here.

Accuracy

Zone temperature sensors in the retail store industry are a lower GRADE in both quality and accuracy. They are inexpensive. Commercial and Industrial system temperature sensors typically have higher accuracy. A typical Novar room temperature sensor is +/- one degree over its temperature range (20 to 127 degrees).

Precision

Retail grade control systems are not precise either. Meaning that they do not hold precisely at setpoint. They vary between one and three degrees. This lack of accuracy has to do with the type of local loop proportional control algorithm the zone controls employs.

Control loop theory, which we covered in our blog Understanding Closed Loop Control, Proportional (P) control is not nearly as precise as full Proportional Integral Derivative (PID) control.

"PID" loops are often used in industrial control mission-critical applications like oil platforms, nuclear power plants, etc. (Things that have a tendency to blow up...as I like to put it).

It is a big mistake to assume that a (literal) "big box" is going to precisely control to a given setpoint.

An identical store placed in a different orientation, elevation, climate zone, etc. is not going to hold the same setpoint as another store in a different environment. This seems almost self-evident, yet the myth that retail control systems are precise still exists.

Even if they were precise, you still have the sensor accuracy problem.

Fast forward to the present state of affairs, and we can now understand just how big a problem we create when we permanently keep our setpoints too high in the summer and too low in the winter.

Wasted Energy

Let's "split the difference" here as far as accuracy and precision go and understand that IF we set our zone setpoints to 72 degrees, what that is typically going to translate to in the real world is 70-74+ degrees.

This lack of precision and accuracy, coupled with acceptance of unreasonable setpoints in a futile attempt to save energy, costs our industry untold dollars in lost sales and loss of employee productivity (which is estimated to be in the millions of dollars and worse).

More energy is wasted because the zone temperatures are never satisfied (because the unoccupied setpoints are also unreasonable), and so the units never turn off. They run sixteen hours per day in a futile attempt to make a zone temperature setpoint that is impossible to make. Unoccupied setpoints are supposed to be within 4 - 5 degrees max of the occupied set-point. Great starting points are Summer 72/76 (Cooling Occupied/Unoccupied) and Winter 68/64 (Heating Occupied/Unoccupied), respectively.

Why are setpoints not zone temperatures?

Because looking at the setpoint only completely ignores the performance of your building. The setpoint is the GOAL. The actual zone temperature is reality. And you have to measure whether or not you are achieving a goal? You cannot assume you are always meeting temperature goals in buildings without measuring the actual temperature.

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