DDC Or Pneumatic?
Energy savings isn’t the only factor when considering a controls retrofit.
Much of our work involves energy studies and retrofits to commercial buildings. We find a variety of systems on these projects, from the original pneumatic systems installed years ago to the latest DDC systems that are part of an integrated system. Because of this, we are often in a position to advise owners about upgrading both pneumatic and older DDC systems. The cost to upgrade these systems is significant, and owners demand to know the benefits of upgrading in terms of reduced energy expense, improved comfort, and better tools for operations.
The challenge is that simply upgrading from pneumatic to DDC will not necessarily save energy; in fact, since DDC controls are more accurate, they can actually use more energy. The solution is to use the new control sequences for systems optimization that can readily result in significant energy efficiency. A few examples of these sequences include improved scheduling, economizer control, static pressure, and discharge air reset.
For VAV terminals, the use of DDC allows for better occupancy control; for reheat boxes it allows the ability to utilize different minimum flow settings for heating and cooling. In addition to saving energy, networked control systems are an essential tool for better building operations, allowing operators to see what is going on in the space from any place where there is an internet connection.
Of course, upgrading the controls on a mechanical system is a lot like renovating an old house. It isn’t so hard to tell when to start making changes but often hard to know when to stop. While there is rarely a question about replacing pneumatic logic, receiver controllers, indicators, and transmitters, on many projects, it is tempting to leave in existing pneumatic valves and damper actuators. We generally recommend replacing pneumatic actuators with new electronic actuators. These devices have become highly reliable and provide accurate feedback for improved control. For control valves, we usually will retrofit butterfly valves with new actuators, but it is generally better to replace older globe valves.
One of the biggest challenges is the decision about what to do with VAV boxes. Leaving the boxes with pneumatic controls makes it impossible to properly optimize systems and have good visibility into what is going on in the space. Retrofitting is expensive, though, since it requires new controllers, power, network connection, new thermostats, control valve, and rebalancing at least at the box.
While some boxes can readily be converted to DDC, we find some older boxes are not able to accept new DDC controls due to the use of integrated actuators, no flow sensing, or the use of mechanical volume regulators. Even boxes that can readily accept a new DDC controller are often in such poor condition that they are problematic. When doing a retrofit, we typically recommend either having an allowance to repair boxes or (ideally) planning to replace the boxes when upgrading the controls.
A new control system is likely to provide numerous benefits for improved operations of an existing building, but a new control system coupled with systems that are redesigned for efficient operations can result in better comfort, operability, and energy savings that can rapidly pay for the investment.