ASHRAE

Optimizing HVAC controls

Strategies for reducing a building's energy use and increasing occupant comfort
Monday, November 4, 2013
By John Murphy

Optimized heating, ventilation and air conditioning (HVAC) control strategies can save energy, and help identify trouble spots in a building that can result in both excessive energy use and uncomfortable occupants.

Most variable-air-volume (VAV) systems rely on a traditional constant duct pressure control strategy, where static pressure in the main supply duct is measured and the speed of the supply fan is adjusted to maintain a constant setpoint. However, this strategy often requires the system to maintain a higher static pressure in the supply duct than is needed, resulting in unnecessary fan energy use.

It’s possible to optimize this static pressure control function to minimize duct pressure and save fan energy. Using communicating VAV controllers, the building automation system (BAS) continually polls these individual controllers, looking for the VAV terminal with the furthest-open damper. The duct pressure setpoint is then reset to provide just enough pressure so that at least one damper is nearly wide open. This allows the supply fan to operate at a lower static pressure, consuming less energy and generating less noise.

The fan-pressure optimization strategy provides the added benefit of allowing the building operator to identify and address rogue zones. A rogue zone is an area where the HVAC system isn’t working correctly, and can be caused by a number of factors related to system design or installation.

Some examples include: an undersized VAV terminal; a duct restriction that hampers required airflow; a zone temperature setpoint that has been adjusted too low; or a zone sensor that’s incorrectly wired, located in the sunlight or directly above the coffee maker. In these cases, the VAV damper operates at a further-open position, preventing the supply fan from running at a lower, more efficient pressure.

When a rogue zone exists, it’s often difficult to detect. Comfort complaints from occupants is an important feedback mechanism and may be the only way that a building owner or manager learns that there’s a problem. And, in the time it takes them to speak up, valuable operational efficiencies are being lost, reducing potential energy savings while providing sub-optimal comfort.

Tracking damper positions over time can help identify anomalies in system performance. For example, while some zones within a building may require more intensive cooling, a VAV damper that’s running nearly wide open most of the time may indicate a potential rogue zone. The building operator can review this trend periodically, identify any potential rogue zones, and dispatch a technician to investigate and correct problems so that the system is able to reap the expected energy-saving benefits of this optimized control strategy. VAV systems with optimized controls can often deliver an annual energy savings of between 10 and 20 per cent compared to a traditional VAV system control.

In addition to helping building owners save fan energy, optimized VAV system controls can save cooling and heating energy by optimizing when the system starts up or shuts down each day, resetting the supply-air temperature setpoint, and optimizing ventilation through the use of demand-controlled ventilation.

For building owners, choosing to upgrade an older VAV system to implement these optimized control strategies doesn’t have to mean replacing the entire system, which could be costly and disruptive to building occupants. Rather, it’s possible to retrofit an existing system by installing retrofit dampers in the ducts leading to the older VAV terminals.

Some available retrofit dampers already have all the communicating controls required to implement these optimized control strategies. In addition, wireless zone sensors and wireless communications between the VAV controllers and BAS simplify the upgrade by avoiding the need to pull communication wiring, allowing for easier relocation when space layout or use changes in the future.

John Murphy is an applications engineer who has been with Trane since 1993. 

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