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Commissioning: Single Zone Air Handling Unit Trend Analysis - Cooling

In this column I will describe the “issue” disclosed through analysis of the trend graph included in my February, 2012 Commissioning column. I will also introduce a new system; show its trend graph; and then leave it up to the reader to discover the operational “issue” illustrated in the graph. The answer will be given at the start of next month’s column.

February 2012 Trend Log Analysis

Last month’s trend graph showed about two weeks of 15 minute interval data from the building automation system for selected points on a single zone, constant volume, mixed air handling unit. The system was equipped with air-side economizer capability, heating hot water coil, and chilled water cooling coil for discharge air temperature control. The discharge air temperature setpoint was reset based on space temperature setpoint.

The trend data was from the fall of 2010 when outside air temperatures varied between 30°F-60°F, a good time to observe economizer operation. What the trend graph tells us is that the air handling unit was providing excellent temperature control. Its discharge air temperature line was tracking nearly identical to its setpoint.

Based on the outside air damper and hot water valve trends, however, the system was not achieving this desired temperature control as efficiently as possible. There were many times when the hot water valve was open as much as 50% or 100% while the outside air damper was not in its minimum position (10%). This means that the hot water was being used to heat more cold outside air than necessary, using more energy than required.

This imprecise control would be nearly impossible to observe without the trend logging capability of the building automation system. It is an example of how the power of direct digital controls can and should change the way building operators manage and evaluate their systems. Correcting the control system programming should be a low cost energy conservation opportunity requiring neither hardware nor installation labor; the challenge is knowing that it needs to be done. That is where proactive trend log analysis comes in handy.

New System

This month’s trend graph is of the same type of single zone, constant volume, mixed air handling unit with air-side economizer, heating hot water coil, and chilled water cooling coil for discharge air temperature control. The discharge air temperature setpoint is reset based on space temperature setpoint. This graph has a selected number of points with two different vertical axes (one on the left and one on the right) to help see and evaluate the pertinent points more easily than with a single shared axis.


The building automation system was used to trend the following sensor and device data at 15 minute intervals for a little more than three weeks in the fall of 2011:

Left Hand Vertical Axis:

  • Mixed air (outside air) damper command (blue)

  • Cooling valve command (magenta)

Right Hand Vertical Axis:

  • Outside air temperature (gray)

  • Mixed air temperature (orange)

  • Discharge air temperature setpoint (purple)

  • Discharge air temperature (green)

Trend Log


Commissioning: Getting It Right

Engineered Systems, March 2012

Rebecca Ellis, PE, LEED AP BD+C, CCP, CPMP, CxA
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318