What is your building’s HVAC control logic trying to tell you?

As energy engineers looking for potential energy savings during energy audits, retro-commissioning investigations and ongoing commissioning, we typically use trend reviews to evaluate the effectiveness of control logic. Trends offer a window into what the controls accomplished in the past. They need to be interpreted based on context to draw conclusions on the control logic that operates behind the scenes.

Trends allow us to find and diagnose important equipment operation issues. However, trending periods are typically limited based on available building automation system (BAS) storage and based on the duration of a project. So looking at the control logic itself is a much more powerful tool for investigation.

Economizer trends displayed in Johnson Controls metasys

Trend review with scatter plot in Microsoft Excel, showing missed economizer opportunity in mild outside air conditions

Sharing access to the control logic allows owners and operators to get a second pair of eyes on the control sequences that were programmed in their building automation system. Most newly designed buildings use advanced controls sequences that are decided upon, reviewed and discussed by multiple parties (owner, operator, engineer of record, commissioning agent). But too often we find that only one person has worked on the programming of those sequences and only the controls contractor has access to, and can visualize that logic.

Having access to the control logic is a huge benefit. An owner, operator or third-party investigator with access and experience interpreting it may find issues before they manifest in the trends. Additionally, potential issues can be diagnosed more accurately, and recommendations for changes can be more specific. Specific recommendations save time for the person implementing the changes and drastically decrease the likelihood of misunderstandings and unwanted side effects.

One Customer’s Troublesome Setpoints

On a recent project in an office building, we used trended data to investigate several AHUs. We found the economizers were programmed to lock out at outside air temperatures as low as 60°F. This errant control setting resulted in missed opportunities to provide fresh air, improve air quality in the building, and reduce the mechanical cooling load in mild conditions. The lockout setpoints were chosen several years prior and site staff was not sure why they were so low.

The owner provided us access to the control logic for these units. Upon further investigation, we found that the economizer controls only had two modes: a locked-out mode in which the economizer was operating at minimum position, and a modulating mode in which the economizer was controlled to maintain a mixed air temperature setpoint.

Based on our previous experience with similar issues, we concluded that the economizers had most likely been found to cycle too much when the outside air temperature got close to the return air temperature. In these conditions, the mixed air temperature would vary little after a given change in economizer damper position, making noise in the temperature readings the more significant factor. When this occurred the PID loop would not be able to properly determine if it should further open or further close the economizer, which would cause unsteady operation and cycling. Such unwanted cycling, harmful to the damper actuators, had likely led previous site staff to lower the lockout setpoints.

We recommended the addition of a third mode to keep the economizer 100% open when the economizer is not locked out and the outside air temperature is greater than the supply air temperature setpoint. Along with this recommendation, we asked site staff to increase the lockout outside air temperature setpoint back up to a more typical 72°F and remove other lockout conditions (e.g. based on enthalpy) that we found were active in the logic.

Marked-up economizer lockout logic showing the recommended change in OAT lockout setpoint and hysteresis, and the recommended removal of enthalpy conditions and comparative conditions

We provided marked-up logic, which allowed the owner’s controls engineer to make the exact recommended changes quickly. During the follow-up measurement and verification phase, we found that the units operated as expected and now made full use of their economizers.

Ease of access to the control logic varies widely by BAS brand, for many reasons, including historical lack of interest and manufacturers’ worries about proprietary software. We recommend that owners show interest and request to have read-only or full access to their control logic.

It is preferable to have access to the live logic rather than screenshots, as it typically allows engineers to follow values more easily through the various logic blocks and sometimes gives access to additional information such as tuning parameters. If that is not available, screenshots or print-outs are the next best thing and can still allow for useful review.

In-depth control logic investigations require additional time and experience with the specific building automation system at the site, but this experience can prove extremely valuable. We have started seeing owners with large building stocks who choose to perform logic review, in house, prior to new installations. For owners, building managers and operators who are interested in taking their advanced controls to the next level, logic review is a critical step to success. If you’d like help looking into your building’s controls and operations, contact us anytime.


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