IAQ and energy management
Key energy savings opportunities
Though many different HVAC systems and control strategies exist, the following items can create an impact on energy costs and system efficiency. Review Standard 90.1 and the AEDG for climate zone specific requirements and select the heating and cooling equipment efficiencies based on the climate specific tables in either document as deemed appropriate.
Chilled water systems can be designed for high temperature differentials of 12 to 18 F delta T, low supply water temperatures (38 to 40 F), and variable flow with modulating valves. Selecting a chiller for a higher delta T can reduce equipment cost and energy use when compared to the traditional 10 F delta T. This design strategy can reduce pump energy (lower flow) and piping installation cost (smaller pipe sizes); however, lowered leaving water temperature does use more chiller energy that may not be offset by perceived gains in pumping and fan energy savings. The manufacturer’s minimum chiller flow rate should be maintained when setting the minimum pump flow. The total annual system energy use must be considered.
Another aspect of selecting a lower supply water temperature is that it may increase occupant comfort by allowing for a reduction in the supply air temperature and dew point at zone equipment. Low-temperature chilled water systems allow the supply air temperature to be lowered from the traditional design temperature of 55 to 48 F or lower. This type of air-side design is now called a cold air system. The lower supply air temperature requires less airflow, yielding a smaller fan, duct, coils, etc. Another energy savings opportunity is to implement a chilled water temperature reset schedule. The temperature can be reset based on outdoor air temperature, zone cooling demand, or both. The engineer must take care to avoid “dumping” cold air on the occupants by selecting high aspirating diffusers or using fan-powered terminal units to provide tempered mixed air.
HVAC heating water systems designs should be centered on high-efficiency condensing boilers with design temperature differentials of 30 to 40 F. Condensing boilers achieve higher efficiencies by condensing water vapor in the flue gases and reclaiming this waste heat to preheat the return water. Most condensing boilers require return water temperatures of 140 F or less to achieve efficiency levels above 85% dependent on firing rate. The designer must carefully select the entering supply water temperature to ensure that the return water temperature is correct. Again, this design results in lower pump energy, and lower installation costs. Similar to the chilled water system, the heating water system should use a temperature reset schedule.
DOAS coupled with water- or ground-source heat pumps, fan-coil units, or single-zone VAV systems can reduce energy consumption by removing the ventilation OA conditioning and dehumidification load from the zone heating and cooling loads. A separate DOAS unit will heat, cool, and dehumidify the OA to deliver dry, neutral air to the space that has the added effect of offsetting the space latent load. DOAS configurations may include direct exchange (DX) coils, chilled water coils, indirect gas-fired heating, hot water coils, steam coils, and an energy-recovery device. DOAS can be used in conjunction with single zone or multiple zone systems. A designer can use the following strategies to further reduce DOAS system energy costs.
Consider supplying cold OA, rather than neutral temperature air, directly to the zone. This can reduce reheat energy and partially meet the zone sensible cooling load. The terminal HVAC equipment should then be right-sized to account for the reduced cooling load. Please note that there are many design paths that can be taken and many other factors such as space humidity should also be considered during the design process.
Incorporate demand control ventilation (DCV) with modulating dampers and airflow measuring stations in the DOAS. DCV can use a combination of space carbon dioxide (CO2) sensors in heavily occupied zones and occupancy sensors in normally unoccupied/limited occupancy zones. Moreover, the occupancy sensors can control the lighting and set back the VAV box airflows to minimum and ultimately control the main HVAC system. The VAV boxes can receive building automation inputs including building schedules (see Figures 3 and 4 for VAV box schematics). Standard 62.1 outlines when a system must use DCV. The control sequence of operation can be complex, but a general guide is presented in Standard 62.1. Remember to account for building pressurization; the DCV minimum OA setpoint must be equal to or greater than the total exhaust airflow.
Exhaust air energy recovery is used to recover energy from the exhaust airstream and to the OA stream. This can be achieved with sensible heat exchange devices (sensible energy transfer only) or total energy exchange devices (sensible and latent energy transfer). During cooling conditions, the OA is precooled and/or partially dehumidified. During heating conditions, the OA is preheated and/or partially humidified. Commonly used air-side energy recovery devices are run-around loops, plate heat exchangers, total energy wheels, and heat wheels. Refer to Standard 90.1 Table 18.104.22.168 that provides exact conditions when an HVAC system requires energy recovery. The requirements are based on climate zone, percent OA, and design supply airflow. When an energy recovery device is required, the system must have a minimum 50% effectiveness.
The exhaust and outdoor airflows should be balanced as near as possible to maximize energy transfer and to maintain building pressurization. Bypass dampers must be installed around the energy recovery device when an HVAC system uses an air-side economizer. It is imperative to downsize the heating and cooling equipment based on the adjusted design loads with energy recovery. Right-sizing the heating and cooling equipment will have a cascading energy savings (such as reduced pumping power, downsized chillers, and boilers).
Air-side economizers provide free cooling when outdoor conditions are able to fully meet or partially meet the cooling load. A typical starting point in a cooling predominate climate would be equivalent to the selected design discharge air temperature of the building AHUs such as 55 F, but there may be instances where the designer could select other temperatures to meet the project’s needs. Standard 90.1 does not require economizers in climate zones 1A or 1B because of limited operation hours in these hot, humid climates. All other climate zones require economizers on systems with a cooling capacity greater than or equal to 54,000 Btu/h. In more humid climates, the designer should use an enthalpy-based control sequence to minimize unwanted moisture entrainment.
Control sequences are a key element in achieving energy management and savings. In an effort to standardize control sequences and aid in the design process, ASHRAE developed a set of control sequences for commonly used HVAC systems. These sequences provide a good starting point for the designer to expand ASHRAE’s sequence to suit the particular HVAC system and state codes/standards, and to meet the owner’s requirements. Furthermore, Standard 90.1 requires that spaces be grouped into similar thermostatic control zones controlled by a single thermostat. For example, exterior zones and interior zones cannot be zoned together.
Standard 90.1 requires that building automation systems (BAS) employ time-of-day schedules and have night setback/setup temperature setpoints. This is preferred over programmable thermostats because the occupants cannot override the zone setpoint. The AEDG suggests using optimal start controllers to determine the time required for each zone to meet the occupied temperature setpoint and delay system startup as long as possible. Standard 90.1 requires optimal start controls for individual air systems with a supply air capacity greater than 10,000 cfm. Optimal start controls save energy by reducing the HVAC system run-time hours.
Multiple-zone VAV systems must employ a supply air temperature reset schedule based on OA temperature, zone cooling demand, or a combination. See Figure 5 for supply air temperature reset. For example, the BAS monitors OA temperature and resets the supply air temperature up or down. Overrides are typically included to reset the supply air temperature to the minimum if the zone humidity exceeds an upper limit setpoint. Additionally, interior zones and telecom rooms must be designed to meet their cooling loads at the warmest supply air temperature. Failing to do this will result in undercooling when using outdoor temperature reset or the supply air temperature will never reset when using zone demand control. While this strategy increases fan energy use, it decreases both cooling and reheat energy consumption.
Furthermore, Standard 90.1 requires systems with direct digital control (DDC) of individual zone boxes that report to the central control panel and have a static pressure reset schedule based on the zone requiring the most pressure. Typically, the static pressure is controlled so that one zone damper is 90% open. This requires the fan be equipped with a variable frequency drive (VFD). The VFD will modulate the fan based on the system’s cooling demand and will reduce the building electrical load when the spaces are not fully occupied. The VFD lower limit should be set based on the motor manufacturer’s lower limit.
Providing ventilation air for IAQ and maximizing HVAC system designs for energy savings is interwoven with owner requirements, interdisciplinary coordination, equipment selection, and design iterations. As equipment efficiencies approach the law of diminishing returns, overall system efficiency and building efficiency will become the subject of future standards and codes.
Randy Schrecengost is a project manager/senior mechanical engineer with Stanley Consultants. He has extensive experience in design and project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board. Gayle Davis is a mechanical engineer with Stanley Consultants. He has experience in the design of HVAC systems, boiler plants, compressed air systems, plumbing systems, steam distribution systems, central heating, and cooling plant design. He is also experienced in commissioning and retro-commissioning.