Re-tuning focuses on a number of commonly occurring operational problems in buildings. These guides use examples to provide details on how to detect good (normal) and bad (abnormal) operations. The charts used in the examples, are created using ECAM (Energy Charting and Metrics Tool). The guides also provide a list of points to trend, ECAM charts to generate, and suggested actions for the building or facilities manager to implement to improve the operational efficiency of the building.

Air-Side Economizer Operation

The purpose of the air-side economizer control guide is to show, through use of examples of good and bad operation, how air-side economizers should be utilized and controlled. An air-side economizer is a duct/damper arrangement in an air-handling unit (AHU) along with automatic controls that allow an AHU to use outdoor-air to reduce or eliminate the need for mechanical cooling. When there is a need for cooling and if the outdoor-air conditions are favorable for economizing (outdoor-air temperature is less than return-air temperature), unconditioned outdoor-air can be used to meet all of the cooling energy needs or supplement mechanical cooling. In a properly configured economizer control sequence, the outdoor-, return- and exhaust-air dampers sequence together to mix and balance the air-flow streams to meet the AHU discharge-air temperature set point.

In humid climates, the use of dry-bulb temperature based economizers is not recommended. However, if used, the outdoor-air temperature should be 5 to 10oF lower than the return-air temperature. There are times when economizing should not be used. This includes during building warm-up periods and cool-down periods, when the outdoor conditions are not favorable for economizing, or during unoccupied periods, when the supply fan is operating (unless introduction of outdoor-air is advantageous to the unoccupied or cool-down period).

When an economizer is not controlled correctly, it may go unnoticed because mechanical cooling will compensate to maintain the discharge-air at the desired discharge-air set point. This may include periods of time where too much outdoor-air is being introduced to the AHU (when the economizer control is attempting to maintain a minimum outdoor-air set point) or when there is not enough outdoor-air being introduced to the AHU (when the economizer control is attempting to bring in the maximum amount of outdoor-air). Failure to correct/mitigate this situation, in all likelihood will lead to increased fan, cooling and heating energy consumption.

Air-Handling Unit (AHU) Static Pressure Control

The purpose of the air-handling unit (AHU) static pressure control guide is to show, through use of examples of good and bad operations, how the static pressure can be controlled.

When a building's supply fan(s) system is operational, the supply fan's static pressure set point can be automatically adjusted to load conditions that will allow the supply fan to operate more efficiently. When the set point values are consistently at the same values (constant) for a long period of time and during load conditions which otherwise would be advantageous for the set points to change, these conditions should be detected by the reviewing the graphs for further investigation. Failure to investigate or correct/mitigate, in all likelihood will lead to increased fan, heating and cooling energy consumption.

Air-Handling Unit (AHU) Discharge-Air Temperature Control

The purpose of the air-handling unit (AHU) discharge-air temperature control guide is to show, through use of examples of good and bad operations, how the discharge-air temperature can be controlled.

When a building's supply fan(s) system is operational, the discharge-air temperature set point value should be automatically adjusting to internal/external conditions that will allow the supply fan to operate more efficiently. When the set point values are consistently at the same values (nearly constant) for a long period of time and during load conditions which otherwise would be advantageous for set points to change, these conditions should be detected by reviewing the graphs for further investigation. Failure to investigate or correct/mitigate, in all likelihood will lead to increased fan, cooling and heating energy consumption.

Occupancy Scheduling: Night and Weekend Temperature Set back and Supply Fan Cycling during Unoccupied Hours

The purpose of the occupancy scheduling control guide is to show, through use of examples of good and bad operation, how occupancy scheduling should be utilized and controlled.

If building systems are properly controlled during unoccupied mode, it can lead to significant cost reductions in commercial buildings. It is very simple to implement, simple to track, and simple to administer. The goal is to shut off systems whenever possible or whenever they are not needed, and refrain from starting up the system for an occasional night-time user or weekend user. Many times the night-time operation can be the most costly, when roughly 5-10% of staff are working with all of the heating, ventilating, and air conditioning (HVAC) equipment running, all fresh air open and lights on. The goal is to have significant consumption reduction for nights and weekends. The difference in consumption between the base load and the peak load should be at least 30% and as much as 80% with aggressive set backs on nights and weekends (will be discussed in detail below). When supply fans operate 24/7 for buildings that have unoccupied periods, these conditions should be detected by reviewing the graphs for further investigation. Failure to investigate or correct/mitigate this situation, in all likelihood will lead to increased fan, heating and cooling energy consumption.

Zone Heating and Cooling Control

The purpose of the zone heating and cooling control guide is to show, through use of examples of good and bad operations, how the heating and cooling at the zone level can be controlled.

Zone set points drive the system and have a ripple effect all the way to the meter. For example, if the air-handler discharge-air set point in cooling mode is too low, excessive reheat can occur at the zone level. Also, if the zone set point is too high, energy will be wasted providing additional heat to the zones that is not needed. Also, if the minimum air flow setting in the terminal box is set high, it may result in significant simultaneous heating and cooling, extra fan power consumption, and higher energy consumption for most of the year.

Improper controls design or operator overrides can also lead to excessive reheat at the zone level. All of these situations can be corrected, if detected, resulting in significant savings in both thermal energy and electricity consumption. In most cases, however, it is difficult to detect excessive reheat because it does not impact zone comfort. Failure to investigate or correct/mitigate this situation, in all likelihood will lead to increased fan, cooling and heating energy consumption.

Central Utility Plant Cooling Control

The purpose of the central utility plant (CUP) cooling control guide is to show, through examples of good and bad operations, how CUP cooling can be efficiently controlled.

Chillers come in many design configurations including air-cooled and water-cooled. The smallest chillers are air-cooled, but may use a refrigerant instead of water as the final cooling medium at the air-handling unit (AHU) cooling coil. Direct expansion (DX) cooling systems use one or more motor-driven compressors to compress a refrigerant gas, which is then routed to a condenser or cooler mechanism located outside. The compressor(s) can be located inside or outside near the condenser coil(s). Piping between the compressor(s) and condenser coil(s) provides the means for the refrigerant gas to be moved between components. If the piping distance or elevation difference between the compressor(s) and condenser mechanism is significant, this can cause problems during moderate cooling load periods. One or more condenser fans draw outdoor air through the condenser coil(s), removing most compression heat and causing the compressed gas to cool to a liquid. As the valve opens at the cooling coil in the AHU, liquid refrigerant begins to flow, causing a pressure drop. This pressure drop results in reduced temperatures below ambient. The cooled liquid then removes heat from the air by passing over the cooling coil and experiences a phase change back to a gas, then goes back to the compressor, where the compression cycle starts again.

Air-cooled water chillers use water as the final cooling medium and rely upon the refrigerant cycle previously described, to cool the water that is circulated through the chiller’s evaporator where heat is removed by the refrigerant gas, causing it to “flash,” or phase change from a liquid to a gas. This heated gas is routed to the compressor(s) and then to the condenser coils that have multiple fans that stage on or off as required, to reject the heat to the outdoors. Because air-cooled water chillers have their condenser coils and fans mounted on the same unit as the compressor(s) and evaporator, this design requires that an air-cooled water chiller be mounted outside of the building (or in a part of the building that can accept heat rejection).

More complex chiller systems are water-cooled, where condenser water is routed from a chiller located inside the building to one or more cooling towers that are located outside. The cooling towers are designed to transfer the compression heat from the chiller to the outdoor air, and can use any number of designs to provide evaporative cooling of the condenser water. However, because multiple chillers, cooling towers, and pumps are interconnected via piping and control valves, the configuration and complexity of the control system grows exponentially.

This guide will focus on water-cooled chillers, and their operations related to the chilled water supply set point and loop differential pressure set point. The temperature set point of the chilled water supply should be automatically reset, based upon the loads in the building (the average of the AHU cooling-coil valve commands) or the outdoor-air temperature. Additionally, the design delta-T between the chilled water supply and return temperatures should be met at all times to ensure optimal efficiency of the chilled water use. Failure to correct/mitigate this situation, in all likelihood, will lead to increased fan, heating and cooling energy consumption.

Central Utility Plant Heating Control

The purpose of the central utility plant (CUP) heating control guide is to show, through examples of good and bad operations, how CUP heating can be efficiently controlled.

This guide will focus on hot water boilers and their operations. Hot water boilers are closed systems that simply heat water up to a desired temperature that does not involve a phase change from liquid to gas. This simplifies the design process, but requires the addition of pumps to move the water in the piping loop.

Standard hot water boilers are not designed to allow the return water temperatures to be lower than 140°F to 180°F (verify with specific boiler manufacturer’s design recommendations). If the return water temperatures drop too low, the boiler may experience combustion gases condensing inside the boiler stack or inside the boiler firing chamber, which can cause harm to the boiler in the form of corrosion (aggressive sulfuric acids) etc. Newer condensing boilers, however, are designed to operate with return temperatures below 140°F, and to handle the combustion gases condensing inside the boiler’s components (stack and firing chamber). For newer condensing boilers, achieving lower return temperatures should be the target to extract as much heat as possible from the combustion gases before they exit the boiler and boiler flue stack (stack losses). These losses are typically between 15% and 30%. Thus, the overall efficiency of conventional boilers is between 65% and 75%, while high efficiency condensing boilers range from 85% to 95%.

A direct effect on boiler efficiency (standard or high efficiency condensing boilers) is the excess air required for complete combustion. Typically, around 4% excess air with an O2 trim is required for complete combustion, but too much excess air can cause higher stack temperatures, and can decrease boiler efficiencies by as much as 20%.

The temperature of the hot water plant should be reset automatically, based upon the loads in the building (the average of the heating-coil-valve commands) or based upon the outdoor-air temperature. If the boilers are only used for comfort heating, they should be shut down (if possible) during the summer months. Failure to shut down the boilers during warm summer months, or neglecting reset opportunities for the hot water supply temperature, in all likelihood will lead to increased fan, heating and cooling energy consumption.

AHU Minimum Outdoor-Air Operation

The purpose of the air-handling unit (AHU) minimum outdoor-air operation control guide is to show, through examples of good and bad operations, how AHU outdoor-air operations can be efficiently controlled.

As specified in American Society of Heating, Ventilation and Air Conditioning Engineers (ASHRAE) Standard 62.1, there is a minimum amount of ventilation that must be supplied to the space being served by the AHU. This will usually be reported as a cubic foot per minute (CFM) per square foot (sf) value and CFM per person in the space. The outdoor-air fraction (OAF) is the ratio of the outdoor-air intake and the total supply air flow rate. It can be used to determine the percentage of outdoor air being brought into the building and also can be used to diagnose over- or under-ventilation when the AHU is not in economizer mode, and failures of the economizer mode (i.e., the AHU is in economizer mode but the OAF shows a smaller fraction of outdoor air than expected). Because the outdoor-air intake air flow rate is hard to measure, the OAF can be calculated as a ratio of the difference between the mixed-air temperature (MAT) and return-air temperature (RAT) and the difference between outdoor-air temperature (OAT) and RAT: OAF = [(MAT - RAT) / (OAT - RAT)]

This calculation, however, is only meaningful when the OAT is significantly (i.e., ± 5°F) different than the RAT. A common misconception is that the building automation system’s (BAS’s) outdoor-air damper position signal (% open) corresponds to the outdoor-air fraction, when in fact this is rarely the case. If there are no sensor errors, the OAF is the only true indicator of the percentage of outdoor air entering the building. When AHU minimum outdoor-air operation is not correctly controlled, it may go unnoticed (masked) because of the AHU’s ability to heat and/or cool the air stream to meet the discharge-air temperature set point before entering the space. Failure to correct/mitigate this situation, in all likelihood, will lead to increased fan, heating and cooling energy consumption and may also lead to poor ventilation.

AHU Heating and Cooling Control

The purpose of the air-handling unit (AHU) heating and cooling control guide is to show, through examples of good and bad operations, how AHU heating and cooling can be efficiently controlled.

AHU heating and cooling coils are generally located in the discharge-air stream, immediately downstream of the mixing plenum and filters, to provide heating or cooling. On some AHUs, the cooling coil will also be used to de-humidify the air stream. This may require the heating coil to be located downstream of the cooling coil for climates where de-humidification is required. It is also possible that a secondary heating coil in the AHU is located in the downstream side of the supply fan or cooling coil. The purpose of this secondary heating coil is to add additional heat to the air stream in cold climate zones, or climate zones where de-humidification is required. By monitoring the outdoor-air temperature and heating- and cooling-coil-valve signals over time, the building operator can identify times when the AHU is simultaneously heating and cooling the discharge-air stream, and make adjustments to AHU operation. If the outdoor-air temperature heating lockout set point is lower than the outdoor-air cooling lockout set point, this may be indicative of a leaking heating and/or cooling-coil valve.

Inefficient operation of heating and cooling coils, such as leaky valves, incorrect outdoor-air temperature lockout set points for heating and cooling, poor heating and cooling control such as hunting and overheating and overcooling, and simultaneous heating and cooling, if not corrected, in all likelihood will lead to increased fan, heating and cooling energy consumption.