Monday, June 13, 2011

Control Stategies for ECM Fan Powered VAV's

There are lots of opportunities to take advantage of ECM motors. Assuming you have the ability to custom program a DDC controller on a series (or parallel) fan powered box, the ECM motor can easily be controlled using an analog output on the controller and the analog input control card on the ECM motor.

An ECM motor's efficiency increases, in terms of watts/cfm, as it is turned down. A PSC motor doesn't share this advantage. At full flow, however, a PSC motor may use less real power than an ECM, so the secret is to minimize the fan airflow rate. With a series fan box, however, it is important to keep the fan flow equal to or greater than the primary air flow to avoid over pressurizing the unit and causing primary air to spill into the return air plenum. The ECM motor can be (and typically is) programmed for pressure independent operation, making the fan airflow directly controllable with an analog signal, without the need for a feedback control loop.

There are limits, of course. The main limit is the ability of the supply diffuser to properly provide uniform conditions in the space, which degrades at very low airflows as the cold air supplied at the ceiling falls into the space instead of hugging the ceiling. (Often referred to as "dumping", we have recently learned of alternate terms including "premature ceiling separation" and "failure of coanda". My marketing department prefers the term "horizontally challenged").

The best way to determine a given diffuser's turndown is to use ADPI analysis. I have a couple papers on this process on our website.

We are going to present some ideas on increasing occupant productivity at Greenbuild in Toronto in October. Without giving it all away in advance, occupant productivity has been shown to be greatly affected by ventilation, so one should strive to maximize ventilation. There is a trade off in energy, however. Conditioning outside air is expensive when it is humid, too hot, or too cold. Often, economizer operation has been abused by allowing space temperature and humidities to wander outside acceptable (or optimal) conditions, in an effort to minimize energy use.

Fan boxes are likely required in cold climates to manage perimeter zones when heating from the ceiling, if one is to comply with both Standards 62.1 and 90.1 (and many codes). By utilizing the variability, and pressure independence, of ECM motors, one can provide high ventilation rates when conditions permit, and reduce them when outside conditions are less favorable. Further control options can be realized if the terminal is supplied with a non-condensing (sensible) cooling coil on the induction port and DOAS air provided to the primary air valve. Alternately, a second DOAS inlet can be provided on a series fan box. Controlling wither two inlets or two coils can be a challenge, so Krueger offers a contact closure / 4 position, fresh air, pressure independent actuator option.

We also have a couple engineers using a parallel box with ECM motor where the fan is designed to operate at all times. This allows the induced air to be independent of the primary airflow rate, offering a number of advantages in controlling room conditions.

In my opinion, occupant satisfaction (and productivity) will be optimized when ventilation air is provided at the highest possible rate, outside conditions permitting, and when the system is operated within the limitations of the air distribution system installed in the space. Maximizing ventilation in the morning, then reducing it as the temperature rises during the day, is an excellent strategy. It will require a clever control strategy, taking advantage of both sensors and equipment options.

Authored by: Dan Int-Hout, Chief Engineer Krueger