HVAC energy-saving strategies

Learning objectives

• Learn about typical building automation system energy–saving strategies. 
• Identify relationships between the various chiller and chilled water valve strategies.  
• Consider an example of energy–saving strategies in health care. 

Many energy–saving strategies are available to building owners by employing a building automation system. These BAS are constantly increasing in capabilities and complexity. This is frequently driven by codes and standards, such as ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings and the International Energy Conservation Code. 

Five common heating, ventilation and air conditioning energy–saving strategies are: 

• Chilled water supply temperature reset. 
• Chilled water pumping differential pressure reset. 
• Air handling unit supply air temperature reset. 
• AHU static pressure reset. 
• Unoccupied HVAC setback.  

CHWST reset control strategy  

One control strategy for reducing chiller power consumption is to increase the CHWST based on cooling load. The input to the BAS is polling of the AHU chilled water valve positions.  

Increasing the CHWST has a significant impact on chiller compressor power consumption. This reduction is consistent for water-cooled chillers independent of chiller size and configuration. With the major chiller manufacturers implementing the latest chiller technology, including falling film evaporators, CHWST reset can result in 2% to 2.5% reduction in compressor kilowatt input per degree Fahrenheit of increased CHWST. Table 1 uses data from actual manufacturer’s chiller selections to illustrate this. 

This strategy requires that all of the chilled water valves be assigned a priority (low, medium or high) and that every chilled-water valve position be polled at regular intervals, typically 30 minutes, via valve position feedback. If all of the high priority valves are open 70% or less and the average open position of all of the medium priority valves is 50% or less, the CHWST setpoint will be adjusted up by an adjustable value of 0.5°F.  

Conversely, if all the high priority valves are open 90% or greater or if the average open position of all of the medium priority valves is 75% or greater, the setpoint will be adjusted down by an adjustable value of 0.5°F.  

A chilled water system may be designed to meet the full (peak) cooling load with 42°F chilled water and could be reset between 42°F and 48°F to optimize chiller energy savings. 

Pumping differential pressure reset  

A control strategy for reducing chilled water plant pump power consumption for variable flow systems is to reset the chilled water system DP setpoint that controls the chilled water pump speeds. This reduction in DP setpoint is based on cooling load (chilled water valve positions). 

The effect of chilled water DP reset on plant pump power consumption varies based on cooling load profile and diversity, plant pump configuration, size and efficiency of the selected pumps. Power consumption of a typical variable flow pumping system at various DP setpoints will approach the ideal pump curve (at zero head). 

This strategy requires that all of the chilled water valves be assigned a priority (low, medium or high) and that every five minutes the chilled-water valves’ position be polled via valve position feedback. If all of the high–priority valves are open 70% or less and the average open position of all of the medium–priority valves is 50% or less, the DP setpoint will be adjusted down by an adjustable value of 0.5 pounds per square inch. Conversely, if all the high priority valves are open 90% or greater or if the average open position of all of the medium–priority valves is 75% or greater, the DP setpoint will be adjusted up by an adjustable value of 0.5 psi.  

Both strategies (CHWST reset and pump DP reset) use cooling load (chilled water valve position) to reset their respective parameters. As a result, they should not be used simultaneously to independently reset both parameters.  

AHU reset control strategies  

A control strategy for reducing terminal reheat, pumping energy, chiller power consumption and the number of hours an AHU can operate in economizer mode (where applicable) is to increase the AHU supply air temperature setpoint. This logic to increase supply air temperature should be based on the zone cooling demand of the area served by an AHU, as determined by the cooling loop output of the terminal boxes fed by the AHU.  

Increasing the AHU supply air temperature can have a significant impact on airside and waterside energy consumption. 

This strategy requires that all of the terminal boxes served by an AHU have their respective cooling loop outputs polled to determine the terminal box with the highest loop output. If the cooling loop output of the terminal box with the highest loop output is less than 80%, the discharge air temperature setpoint is increased by 0.5°F. Conversely, if the terminal box with the highest cooling loop output is higher than 95%, the discharge air temperature setpoint is decreased by 0.5°F.  

A typical AHU may be designed to deliver air temperatures of 55°F for commercial applications and 52°F supply air temperatures for health care applications where maximum room relative humidity ranges are to be maintained.  

One common strategy for reducing supply fan energy is to reset the AHU duct SP setpoint. This decrease in AHU duct SP is based on the zone cooling demand of the of the area served by an AHU. The cooling demand is determined based on damper position of the terminal boxes fed by the AHU.  

Decreasing the AHU duct SP setpoint can have a significant impact on airside fan energy consumption and can be as high as 5% to 7% of the HVAC power consumption of a building depending on building geometry, load profile and diversity, duct configuration and fan efficiencies. 

This strategy requires that all of the terminal boxes served by an AHU have their respective damper positions polled on an adjustable time basis to determine the terminal box with the highest damper position. During test and balance of the air handling system, the minimum and maximum duct SP setpoints are determined by setting all of the terminal boxes fed by the AHU to minimum cooling cubic feet per minute and determining the SP at the transmitter location that results in a terminal box with a damper position of 95% open and setting all terminal boxes to full cooling cubic feet per minute and determining the SP setpoint that results in the terminal box with a damper position of 95% open. If the damper position of the polled terminal boxes is less than 90% open, the SP setpoint is decreased by 0.1 inch water gauge. Conversely, if the damper position of the polled terminal boxes is greater than 95% open, the SP setpoint is increased by 0.1 inch WG. 

Unoccupied HVAC setback  

One common and relatively straightforward strategy is to change HVAC setpoints when the building or spaces are unoccupied. Parameters that are typically reset include: 

• Space temperature. 
• Minimum zone airflow. 
• Outside air quantity. 

Maximum savings is achieved when these three strategies are implemented simultaneously to reduce both fan energy and cooling/heating energy. When total and outside air quantities are setback, care should be taken to ensure that the overall building pressurization is not compromised, assuming that exhaust fan systems continue to operate during unoccupied periods. 

Such systems are normally initiated by a time of day schedule and include a means to override via a manual push-button or occupancy sensors.  

Control strategy relationships  

It is imperative that the relationships of the presented strategies to each other be understood when applying multiple strategies to a cooling system. The relationships can be presented between them most easily when they are broken down between waterside and airside. The waterside strategies include CHWST reset and chilled water pumping differential pressure reset, while the airside strategies include AHU supply air temperature reset and AHU SP reset. 

The relationship between the waterside strategies is important because both use cooling load (chilled water valve position). If implemented simultaneously, the two strategies would compete with one another and create a condition where the CHWST increase also would result in an increase in differential pressure setpoint. As the chilled water temperature is increased, the AHU coils will need a higher flow rate to maintain their respective discharge air temperature setpoints and the differential pressure setpoint will inherently be increased to deliver it. This is because both strategies require the chilled water valve positions to be monitored. As a result, the two strategies should not be used at the same time but rather in succession.  

The energy reduction (in kilowatts) associated with the chiller at increasing supply temperature setpoints including the increase in pump kilowatts due to higher flow rate requirements is greater than energy reduction associated with chilled water pump DP reset and constant chiller discharge water temperature. As a result, the CHWST reset should occur first. Once the CHWST is at its maximum setpoint, the chilled water differential pressure can be reset to allow the additional energy savings associated with lower pump horsepower to be realized.  

The relationship between supply air temperature reset and SP reset can be easily understood when the parameter used to adjust it is reviewed. While both strategies use zone cooling demand to determine the appropriate setpoint value, they each use a different control output to determine the required response. In the case of supply air temperature, zone cooling demand is determined based on the cooling loop output of the terminal box.  

In the case of AHU SP reset, zone cooling demand is determined based on the damper position of the terminal box. The fact that both strategies use different parameters to determine the response allows both to be used simultaneously. This is made even more important because a larger energy saving will be achieved on the airside strategies than the waterside strategies. This is because, as a percentage of the total energy used by a facility, a larger percentage can be attributed to the airside systems than the waterside systems.  

Health care energy savings 

With the control strategies at hand, there are unique aspects related to health care that must be evaluated.  

Many health care applications require that specific room temperature and humidity ranges be maintained. For example, the Texas Health and Human Services Commission hospital licensing rules include a table outlining the temperature and humidity requirements for various hospital spaces. Where spaces exist that are not specifically identified in the licensing rules, the state defers to the requirements listed in ASHRAE Standard 170: Ventilation of Health Care Facilities. Table 2 indicates a representative list of spaces that require specific temperature and humidity ranges. 

The relationship between the code requirements (Texas HHSC and ASHRAE Standard 170) and control strategies dictate that specific temperature and humidity thresholds be used. The maximum supply air condition that can be delivered to a space while maintaining the maximum RH values listed cannot exceed a dewpoint temperature of 53.6°F, which corresponds to a humidity ratio of 61.4 grains of moisture per pound of dry air.  

While the CHWST affects the leaving coil air condition, as long as the discharge air temperature setpoint can be maintained and is not above the maximum humidity ratio, the chilled water temperature can be increased according to the chilled water temperature reset sequence.  

This example allows the spaces to be maintained at the lowest temperature allowed by code while maintaining the highest corresponding RH allowed and is not suggested as the recommended design practice. In determining the design setpoints, allowances should be made for some safety and to allow for the RH to drift within a range, since the system will be dynamic and also relies on sensor calibration, network speed, etc.  

In addition, spaces such as operating rooms, pharmacy compounding and sterile processing decontamination are commonly required by the owner to be designed to lower space temperatures than that listed in Table 2. In these cases, the maximum supply air temperature would be much lower, at a dewpoint to maintain a RH within the allowable range at a lower space temperature.  

For example, to maintain an OR at 62°F and 50% RH, the maximum supply air temperature using a conventional mixed air AHU would be 43.6°F. This is not practical, would greatly increase reheat requirements and would limit the ability to integrate chilled water reset into the plant strategy. Therefore, alternate technologies and design strategies (e.g., desiccant dehumidification or glycol subcooling) should be considered for these spaces to allow the remainder of the system to maximize the benefits of reset strategies. 

Operating room setback 

Most areas in a hospital are occupied 24/7, limiting the opportunities for unoccupied setback. However, ORs are seldom used 24/7 in most hospitals. Due to the high airflow requirements when occupied, airflow setback of ORs when unoccupied can result in significant savings in fan, cooling and reheat energy (see Table 3).  

Facility Guidelines Institute 2018 Guidelines for Design and Construction of Hospitals does not list a minimum to which the airflow can be lowered in unoccupied periods. Some states, however, do dictate a minimum airflow at all times. The design minimum air changes per hour should be determined by the engineer together with the hospital facilities and environment of care groups.  

Regardless of the setback ACH, the positive pressure (0.01 inch water column) relative to adjacent spaces must be maintained at all times. Room pressure monitors must be provided for each OR and strategies such as pressure independent air valves on the supply and return should be considered. 

Many facilities hesitate to incorporate OR setback due a concern of the room being ready for emergency cases. Most often, it is not recommended to change the room temperature setpoint during unoccupied mode, since a rapid change in room temperature can cause the humidity to fall outside of the allowable range before the systems can stabilize. If only airflow is setback, the room can recover after manual override and will stabilize before the surgical team completes prepping for surgery. 

The sophistication of modern building automation presents opportunities for significant energy savings. While multiple strategies make sense on most projects, there is not a “one size fits all” method to implementing these strategies. Maximizing savings while maintaining the operational requirements of the facility requires detailed analyses and engineered solutions for each specific case. When properly designed, programmed and maintained, the optimum savings and return on investment of BAS energy strategies will be realized. 

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