Consulting-Specifying Engineer - July 2008 - (Page 34)

the intent to reflect the warmest area in the building. The system then judges whether the building is “too cold” or “too hot” based Reset mechanism Pressure setpoint reset Temperature setpoint reset on the readings of these typical sensors. The Standard 90.1-2004 Required, demand-based, Not required sensors may become bad indicators due to §6.5.3.2.3 churn in the area in question, or due to local 2006 IECC Required, demand-based, Not required load changes like small space heaters that §503.4.2 provide local comfort. 2005 California T24 Required, demand-based, Required, except for systems with Outside air temperature: For buildings §144.(c).2.D variable speed drives, §144.(f) with large internal or solar load components, this may lead to incorrect adjustTable 2: Regulatory requirements for hydronic systems ments. For example, take a building with System type Hydronic high internal loads that are constant Reset mechanism Pressure setpoint reset Temperature setpoint reset throughout the day. Resetting supply air temperature (for energy savings through Standard 90.1-2004 Not required Required, except for variable flow less cooling) at a cooler time of day may not systems, §6.5.4.3 work. Thanks to the high internal loads, we 2006 IECC Not required Required, except for variable flow may end up overheating the spaces served. systems, §503.4.3.4 Because we’re only looking at outside air, 2005 California T24 Not required Required, except for variable flow and not the spaces themselves, this is a poor systems, §144.(j).4 overall indicator of building demand. Source for all tables: Taylor Engineering Good demand indicators for supply air using a floating point actuator with position or large section of the building. As such, temperature reset feedback. The floating point actuator posiwe easily will overlook a number of rooms When an air terminal in cooling meets tion, approximated by summarizing “open” that are not receiving sufficient cooling or the required airflow but fails to achieve and “close” commands over time, is accom- heating, because their operations are lost comfort conditions in the space served, it panied by driving the damper to zero posiin the averaging effect. requires colder supply air. The same is true tion for several seconds at regular intervals Typical zone sensors: This is a technique in reverse for heating. Thus, the output of to eliminate inaccuracy in the approximated often used for retrofits of older pneumatic the cooling or heating PID loop of an air position. systems when installing zone-level EMCS is terminal serves as a good demand indicator Poor demand indicators for supply air too expensive. One sensor may be installed at for supply air temperature. temperature reset the top floor in a north-facing zone with the Poor demand indicators for chilled/hot Return air temperature: Provides an intent to reflect the coldest area in the buildwater pressure reset average of what is occurring throughout ing, while another sensor may be installed in Valve position is generally a poor indicathe entire building, or at least a wing, floor, the southwest corner of the building, with tor of pressure demand, because no flow measurement is available at Table 3: Overview of demand indicators most control valves for coils. System type Air Hydronic Thus, a wide-open valve may Temperature reset Pressure reset Temperature reset Indicator for: Pressure reset receive in excess of design flow Return air Return water and would not be a good indiAir terminal temperature temperature cator for additional pressure. damper endGood demand indicators for switch Typical zone sensors Valve Typical zone sensors Poor chilled/hot water pressure reset indicators position Outside air temperature Approximate Outside air In a hydronic campus heatdamper Valve cooling/heating temperature ing system, each building may position control loop output have a heat exchanger with a Accurate dampBtu meter indicating flow. If this Flow meter er position Air terminal cooling/ is used, the measured flow can Good reading Airflow reading heating control loop be compared to design flow and indicators versus flow output versus airflow used to indicate demand for setpoint setpoint more pressure. Table 1: Regulatory requirements for air systems System type Air 34 Consulting-Specifying Engineer • JULY 2008

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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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