The Role Of HVAC Controls

When planning an HVAC upgrade, here are three control strategies to help ensure the new equipment runs as efficiently as possible.

By Julia Ingersoll, P.E.
From the August 2019 Issue

By the time most commercial buildings decide to complete a large-scale air handling unit (AHU), chiller and/or boiler retrofit, their existing equipment has typically been in operation for over 20 years. Much has changed in two decades when it comes to high efficiency systems, driven largely by modern building automation technology. Most states have adopted an energy code—either the International Energy Conservation Code (IECC) or ASHRAE 90.1—mandating control strategies and energy conservation measures for an HVAC upgrade installation.

HVAC upgradeConsider the following commonly-required low-cost/high impact control strategies for your upcoming HVAC infrastructure upgrade.

Optimum Start/Stop Controls. The intent of optimizing start/stop controls is to conserve energy by creating “setback” setpoints for unoccupied times. Because it is a guessing game to determine the right time to change equipment setpoints (and there is often no guarantee how hot or cold each space will reach during that period), energy codes now require a calculated optimum start and stop for new AHUs with direct digital controls (DDC). This calculation may be based on the difference between indoor and outdoor air temperature. Sophisticated building automation systems can “learn” the thermal mass of a building to provide an even more accurate timeline for resetting temperature setpoints while maintaining temperatures during occupied mode.

TIP: A common concern with this strategy is that the BAS system will not be able to “catch up” to setpoint if the programming fails. I recommend:

  1. Waiting to enable this programming in the shoulder season until engineers gain confidence in the system;
  2. Programming alarms to be sent to the engineer’s email/phone to help determine if the calculation start/stop times are out of range; and
  3. Visually confirming outdoor air dampers remain closed during morning warm up/cool down.

Chilled/Hot Water Temperature Reset. By resetting the chilled water, or hot water temperature setpoint, a building is able to shave energy use. The most sophisticated way to determine temperature reset is by how “open” terminal units, or AHU control valves, are. The more “open” a valve, the more building demand there is for cooling or heating. If all valves are partially closed, there is an opportunity to reset the temperature setpoint of the chiller or boiler plant, higher or lower respectively.

TIP: If the temperature reset is significant, the boiler will be offloaded, but may require a significant increase in pump energy to function. Generally speaking, a chiller/boiler is the highest energy consumer. Have a design engineer calculate the cross-over point for your plant and evaluate how much power the boiler will consume at various hot water supply temperatures against the power consumption for the pumps using affinity laws.

Supply Air Temperature/Pressure Reset. Modern variable volume AHUs with DDC controls are often required to have both supply air temperature and pressure resets. Both the pressure and temperature reset may be accomplished by detecting how “open” the terminal unit dampers are, translating directly to how much cooling/pressure a particular zone demands. The intent of the pressure reset is to reduce the fan speed within the AHU, while the temperature reset reduces the demand on the chiller/boiler plant to save energy. Increasing the supply air temperature setpoints while cooling—or decreasing the supply air temperature setpoint during heating—takes a load off the chilled water system/boiler, increasing efficiency and reducing electrical demand.

TIP: When terminal boxes are more than 65% open, both pressure and temperature resets will likely occur. Determine which system consumes the most energy, and which reset provides the most cost savings, so that the controls may cascade the two resets accordingly.

Retrofit rebates help reduce owner’s bottom line. Utility companies may offer incentives for large (and small) scale retrofits. Three important things to know if you plan on utilizing rebates:

  1. Complete a cursory evaluation. The North Carolina Clean Energy Technology Center maintains a database of all state level and utility incentives.
  2. Involve the utility early and often for potential rebates. They can often help to achieve even greater savings with low upfront costs.
  3. Rebates often come at the very end of the project, after the utility has completed an on-site review of the installation. Consider reserving the financing upfront in case there is a hiccup on the utility’s end, as these programs are often first come/first serve.
  4. Check if the contractor will accept the rebate directly from the utility. This allows building owners to avoid paying taxes on the rebate.

One final tip. While local energy codes are updated every few years, some jurisdictions still require older versions. Ask design engineers to consider how the newest requirements may improve efficiencies. Many have an ROI of under five years, while the HVAC infrastructure replacements are sure to be in service for 20+ years.

HVAC upgradeIngersoll, P.E., CDT, is a mechanical engineer with McGuire Engineers, Inc., a design, engineering and consulting firm in Chicago, IL specializing in a full range of in-house engineering services including mechanical, electrical, plumbing, sprinkler, and life safety systems.

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