Summary Paper on Controls for HVAC

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Violet Owen
5 years ago
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Summary Paper on Controls for HVAC As one of the big three energy uses in buildings, the HVAC system is a natural target for improvement.

The real savings will need to come from the use of new system configurations, recovery and reuse of energy, and most importantly, from more reliable and sophisticated control.

General Design Principles As with any high performance design, an integrated approach must be used to arrive at a final design.

1 Summary Paper on Controls for HVAC As one of the big three energy uses in buildings, the HVAC system is a natural target for improvement. Interestingly, HVAC equipment has sufficiently increased in efficiency over the years that there are no large savings to be gained from simple equipment swaps. The real savings will need to come from the use of new system configurations, recovery and reuse of energy, and most importantly, from more reliable and sophisticated control. Innovative design for buildings that are approaching and achieving net zero provide a glimpse of what a superefficient HVAC package might look like. General Design Principles As with any high performance design, an integrated approach must be used to arrive at a final design. For HVAC, the steps are: 1) Minimize loads 2) Reduce equipment size accordingly 3) Consider alternative system options, especially for small loads 4) Examine thermal zoning carefully 5) Analyze part load conditions and durations 6) Separate ventilation from conditioning 7) Select equipment based on the lowest life cycle cost, not first cost 8) Technology Trends The USDOE chart below summarizes the near-term potential for saving from a number of current or soon-to-market technologies. Below we addressed a number of these technologies () that have immediate application, particularly in smaller buildings or tenant improvements.

DOAS Dedicated Outside Air Systems allow the building to provide precise air quality without introducing an energy penalty by using an air system simultaneously for the conditioning function,

2 DOAS Dedicated Outside Air Systems allow the building to provide precise air quality without introducing an energy penalty by using an air system simultaneously for the conditioning function, especially when combined with an energy recovery system (see energy recovery equipment below). Air Systems - While variable air volume (VAV) systems have been the standard since the early 1970 s, there are now many more systems available that offer potentially significant advantages over this traditional approach. The following chart, courtesy of Solarc A&E, Eugene, OR, correlates peak building cooling loads with selected system sweet spots as a way of identifying best system fit. 2

3 Peak Cooling Load, SF/ton 2,000 1, Ducted Variable Air Volume Displacement Ventilation Radiant Cooling Natural Ventilation with Thermal Mass Indirect Evaporative Cooling Natural Ventilation Peak Cooling Load, Btu/hr-SF Source: Michael Hatten, Solarc A&E, Eugene, correspondence The dark blue bands represent typical peak cooling load thresholds for various system concepts. Peak loads above the end of the light blue bars will be challenging to meet with the various system types. Thus, load reduction becomes an import element in providing lower energy system options. An excellent discussion of hybrid (passive/active combo) air systems is provided by Erin McConahey, P.E., Member ASHRAE, in the September 2008 ASHRAE Journal (see HVAC Resources list). Underfloor air distribution and displacement ventilation are two active air variants that are gaining popularity as low energy systems due to their greater efficacy and lower fan power. Energy Recovery Equipment - Heat (sensible) and humidity (latent) exchangers, traditionally relegated to only high degree days or very humid climates, are becoming a widespread means of maintaining adequate ventilation without a severe energy penalty for system of all sizes. Evaporative Cooling - Evaporative cooling underwent a similar renaissance as energy prices and ventilation requirements climbed. Standard units are now available in a wide range of sizes and both as direct and indirect systems (the moist air goes through a heat exchanger, not into the building). Evaporative coolers have a particular value for summer peaking utilities, as they become more efficient as the temperature climbs and do not increase the electrical demand the way mechanical air conditioners do. Companies like Idaho Power are investigating the benefits of widespread use of evaporative cooling as a load leveling strategy. Hydronic Systems Because of the greater heat (or coolth) capacity, pumping efficiency and storage potential, hydronic systems have become more common, particularly in high 3

4 performance building applications. When combined with a heat pump, which can provide a multiplier effect, they can be very efficient. In a recent NBI unpublished review of 23 high performance buildings, six of the top eight building used ground source heat pumps. Hydronic systems allow for the use of both radiant and convective heat transfer, using panels, chilled beams or room surfaces (floors, walls and ceilings). Variable Refrigerant Flow While this system has been available internationally for more than 20 years, it is just beginning to make inroads into the U.S. market. Basically, it is a sophisticated multi-split system, meaning that it uses refrigerant to transfer energy rather than water or air and has the ability to provide heating and cooling simultaneously to different zones. It has numerous advantages, including modularity and small pipe sizes for ease of installation and zoning, simplicity of maintenance, high efficiencies (including reduced duct losses), and excellent comfort. On the downside, they tend to be a higher first cost, are less well represented in the service market, and cannot be directly compared to efficiencies of alternative systems because they are not yet AHRI rated. Since they do not use ducted air for conditioning, they require a separate ventilation air system. They appear to be an excellent solution for retrofits where ducts are an issue or where extensive zoning is required. In the tenant improvement (TI) arena, they represent an option for partial building retrofit. Net Zero Buildings An informal review of nine commercial and seven school buildings that have attempted to reach net zero, while not statistically significant, showed the following mix of technologies. Technology Commercial Schools Total Total Number of Buildings High Efficiency Lighting Comprehensive Control Measured Performance (Metered) Daylight Harvesting Ground/Water Source Heat Pumps Energy Recovery Natural Ventilation Passive Solar Design Evaporative Cooling In a similar comparison done in 2006, the daylighting, lighting and controls were in the same position, but the ER and GSHP units were only 30% and 20% of the mix respectively compared to 75% of this sample. If the current trend towards more efficient buildings continues, according to William Fisk, ASHRAE Fellow, in his 2009 paper Climate Change and IEQ, we can expect: 4

5 Smaller capacity HVAC systems with a larger ratio of latent to sensible capacity for buildings with more thermally efficient envelopes and lower internal heat loads; Increased use of low energy cooling systems such as those with evaporative cooling; More and new types of demand controlled ventilation, heat recovery, and gas-phase air cleaning to reduce outdoor air (OA) ventilation energy requirements; More radiant heating and cooling, less air recirculation, and lower pressure drop filtration systems to reduce fan energy; Hybrid HVAC systems, cooling systems with finer spatial and temporal control, and more mechanical ventilation without air conditioning to reduce or eliminate airconditioning energy consumption; Increased resources dedicated to HVAC maintenance and correction of control system problems; Better integration of HVAC components; and Better integration of HVAC with other building systems. Continuous Commissioning - Finally, technology does not guarantee performance. If we compare the performance of CBECS buildings which have an EMS and/or two peripherals commonly linked to an EMS, variable frequency drives and economizers, we find a bimodal pattern exists. In fact, for economizers, there are more buildings in the lowest performing 25% than either the average or top 25%. This finding is reinforced by the results of an NBI study done in 2003 which showed 60% of all economizers five years old or newer were not installed properly or not performing correctly. TECHNOLOGY VS PERFORMANCE PERCENT OF BUILDINGS VFD EMS Economizer CBECS Worst 25% CBECS Average CBECS Best 25% Therefore, it is necessary to meter the building s actual energy use to confirm efficient operation and to provide diagnostic information if its performance is deficient. This process, referred to as fault detection and diagnostics (FDD), needs to become automated where possible. The controls package should provide sufficient feedback on measured performance to allow the onsite or 5

6 remote operator to keep the system operating at an optimum. Providing key performance indicators (KPI) for the operator and the occupants is essential to maintain a low energy profile. HVAC Control Strategies The Control Specification Builder software package ( provides a detailed control specification for most conventional equipment configurations. Specifications are open and non-proprietary and include sequences, schematics, point lists, trends, alarms, and other key requirements in English or metric units. In addition, most high performance designs include well-defined zones separating distinct uses, occupancy (or vacancy) control using occupancy sensors, proportional control using variable frequency drives, Demand Control Ventilation for variable use spaces using CO2 sensing, and seasonally adjusted temperature resets as appropriate. See the accompanying HVAC Resources document for more detail. HVAC Control Trends Research done by NBI and others points to multiple high-performance measures with significant savings (HVAC integration, daylight harvesting, occupancy control, plug control, metering and feedback, system optimization, demand control and FDD), that can fuel the desired rapid increase in efficiency in the U.S. building stock. They all, however, will require control systems that actually provide the integration of building systems operation. There is a new generation of small-scale controls that are digital, flexible, web-based, open protocol, networkable, scalable and highly versatile. They offer increased penetration for universal platforms, including IP, Echelon and BACnet. An example of this class of controls would be the Tritium JACE. Proliferation of similar smart, digital appliances, such as the ipod and iphone, have shown the great traction and usefulness these kinds of devices bring to the market, and offers an alternative view of the market. Rather than a complex, proprietary system, these devices are a platform for application software ( Apps ) that can be mass marketed. And because they are open protocol, applications can be easily built for these devices and widely distributed at relatively low cost. If this model of the market prevails, we can expect significant penetration of the small building and TI markets. Light switches and dumb thermostats will be replaced by simple but powerful controls that can integrate building systems to produce significant efficiency improvements with increased comfort, productivity and personal control. 6

Level II - Whole Building Interval Electric Data with Tenant Plug Load System

Level II - Whole Building Interval Electric Data with Tenant Plug Load System 1. Overview and Scope This specification describes the capabilities and data requirements for equipment or service providers