Consulting-Specifying Engineer - July 2008 - (Page 33)

Using demand-based reset strategies How to clarify the methods used to implement demand-based reset strategies where the EMCS infrastructure allows, and how to make these systems work as intended. BY REINHARD SEIDL, PE, Principal, Taylor Engineering, Alameda, Calif. hanks to the growing application of energy management and control systems (EMCS), it has become possible to monitor equipment much more closely than what was possible with pneumatic and electric control systems. When an EMCS includes controls “at the zone level,” it typically means that controls extend beyond the central mechanical rooms to individual air terminals serving spaces in all parts of the building. For new installations, this is usually the case. For retrofits or upgrades of existing facilities, however, extending an EMCS to the zone level is often too costly, and only individual pieces of equipment or mechanical rooms are equipped with modern controls. This type of retrofit for older systems does not allow use of demand-based reset as described below. A traditional approach to controlling systems is to specify fixed setpoints. A typical example would be to run a rooftop package unit with a fixed supply air temperature setpoint of 55 F and a fixed supply pressure setpoint of 1.5 in. water gauge. This assures stable operation and reduces comfort problems and callbacks. It also assures less than optimal energy efficiency. Since 1999, ASHRAE Standard 90.1 requires the use of a demand-based static pressure setpoint reset for air systems with zone level controls, as does California’s energy code, part 6 of Title 24. Table 1 shows a quick overview of the regulatory landscape. T As shown in Table 1, air systems typically are required to have pressure setpoint reset while not having temperature setpoint reset. Hydronic systems (see Table 2) are the reverse; regulations require the use of temperature setpoint reset while not requiring the use of pressure setpoint reset. For an explanation of this, we’ll begin by looking at indicators used for building demand, and the resulting reset mechanisms that make sense. Building demand To begin, let’s describe how to quantify building demand, because that’s what we have to respond to. The following list describes typical demand indicators: Poor demand indicators for supply air pressure reset Air terminal damper end-switch (in the absence of a flow sensor, on legacy systems): This only indicates that a damper is wide open. The damper may at that point be receiving in excess of the design airflow rate, so this is not a good indicator of required pressure. Approximate damper position: On floating actuators, which receive only “open” or “close” commands, there is no direct indication of damper position. By summarizing the “open” and “close” commands over time, the damper position can be approximated, but usually is not very accurate when the damper is not driven to an autozero position at regular intervals (example: once every 24 hours). Good demand indicators for supply air pressure reset Comparison of airflow to setpoint: In lieu of approximating damper position, a call for pressure can be generated when the airflow of a zone is below setpoint for a certain period of time. This method can be used when damper position is unavailable. Note that this indicator can only be used for the trim-and-respond logic described below, not for the proportional-integralderivative (PID)–based reset logic. Accurate damper position: When pressureindependent terminals (with flow sensor) are used, a fully open damper is an indication of insufficient flow. Note that this is not the case in terminals without flow sensors (see above). The damper position can be sensed with a modulating actuator, or by Editor’s note This article is derived from a paper presented by the author at the 16th National Conference on Building Commissioning (NCBC), April 21–24. NCBC is owned and managed by PECI. For the original paper and others, visit www.peci .org/ncbc. ConsultingSpecifying Engineer is the media sponsor for NCBC. Consulting-Specifying Engineer • JULY 2008 33 http://www.peci.org/ncbc http://www.peci.org/ncbc http://www.peci.org/ncbc

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Consulting-Specifying Engineer - July 2008 Contents Viewpoint Letters News M/E Roundtable 40 Under 40 Using Demand-Based Reset Strategies VFDs and Motors: Making the Right Match Grounding and Bonding Practices for Hazardous Areas Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - July 2008

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