The HVAC system is usually designed for extreme conditions, but most of the time the requirement is to operate at partial/varying capacity because parameters like solar loads, occupancy, ambient temperatures, equipment and lighting loads keep on changing throughout the day. Operating HVAC systems at the design capacity rather than the required capacity can lead to a greater energy consumption and lead to overheating or overcooling spaces.
HVAC controls are sophisticated systems that regulate various elements of an HVAC system to maintain a comfortable indoor environment while optimizing energy consumption. It includes sensors, thermostats, dampers, actuators, and control panels. These parts communicate as a system, letting users control the temperature in basic models and humidity and air quality in more advanced ones.
- Add Demand Controlled Ventilation (DCV) sensors
- Add Air-Side Economizer
- Add VFD to Air-handler Supply Fans
- Implement Fan Static Pressure Reset
- Implement Supply Air Temperature Reset
- Add Energy Recovery Ventilator to Air-handler
- Lower VAV Box Minimum Flow Setpoints
- dd Variable Frequency Drive (VFD) to Cooling Tower Fan
- Implement Chilled Water Supply Temperature Reset
- Install VSDs on chilled wat
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AIR HANDLING UNIT
1. Add Demand Controlled Ventilation (DCV) Sensors
Building codes usually mandate a minimum fresh air supply to maintain good air quality in a building. Many ventilation systems, when installed, set a fixed rate for fresh air intake regardless of the actual occupancy. This can result in poor indoor air quality (IAQ) and increased energy use for cooling, heating, and dehumidification. To make rational use of energy, the ventilation rate can be lowered when spaces are only partly occupied.
Demand Control Ventilation (DCV) is a strategy that adjusts how much outside air is brought into an area depending on the number of people present in that area. It usually uses a sensor (or sensors) that reacts to occupancy or CO2 level changes in the space. The sensor's output is used by a control system, usually a damper, to change the outdoor air flow in the ventilation system.
For example, if an auditorium in a school is filled up in the mornings for assembly, the CO2 concentrations will increase, a signal will be provided to the HVAC system and outside air volumes will be increased accordingly.
Codes such as ASHRAE 90.1 2019 require a DCV for spaces greater than a certain size (500ft2) and design occupancy for ventilation (>=25 people per 100ft2 of floor area) along with some other criteria and exceptions.
2. Add Air-Side Economizer
Air conditioners and heat pumps typically use a compressor to cool a building. To transfer the heat from the building to the outdoors, the compressor circulates and compresses a refrigerant that absorbs the heat from the indoor air and releases it outside. Air-side (outdoor air) economizer is a mechanical unit that uses outdoor air to cool the building when the outdoor conditions are favorable. It saves energy by supplementing compressor based mechanical cooling with "free cooling" provided by the fresh air when fresh air requires less energy than conditioning the building's return air.
The economizer may be operated based on either the dry bulb temperature or the enthalpy of the outside air, and this requires specific sensors. In locations with higher humidity, using differential enthalpy control for the economizer is often preferred.
The costs associated with adding the required hardware to support economizers will vary depending on the existing system in the building.
3. Add VFD to Air-handler Supply Fans
In the Air Handling Unit of HVAC systems, supply fans are used to pull the outdoor air into the room through the ductwork by air inlet. Traditionally, fans and pumps work at a constant speed despite the fact that building loads can vary since they are designed to operate at peak loads. This leads to energy wastage during lower demand periods. Utilizing Variable Frequency Drives (VFDs) for supply fans in HVAC systems is an effective strategy to improve energy efficiency in buildings.
VFDs allow the motor speed of supply fans to vary, based on the actual demand, rather than running constantly at full speed. This allows the supply fan to modulate supply airflow, which results in significant energy savings for heating, cooling and fan energy use, reduces wear and tear on equipment, and improves overall system performance. Additionally, it contributes to maintaining a comfortable indoor environment while minimizing unnecessary energy usage during periods of lower demand.
For a retrofit, replacing existing VFDs with higher efficiency versions or install VFDs in fan motors without them must be considered for energy efficiency.
4. Implement Fan Static Pressure Reset
HVAC systems transfer conditioned air from one point to another through ductwork using fans. During this process, various factors including the presence of coils, filters and diffusers inhibit the airflow causing static pressure. As a result, fans produce enough pressure to overcome the resistance and transport the air to conditioned spaces. In a variable air volume (VAV) system, when a terminal unit damper begins to close, airflow is decreased and pressure in the duct increases. In these systems, fan speed is typically controlled to maintain a constant static pressure under all conditions. This pressure set point is determined as the minimum pressure necessary to transport the air to the most remote location under design conditions (this is typically when all VAV boxes are fully open). At all other conditions, the fan is supplying greater pressure than necessary and energy is wasted. Some of this waste can be mitigated by implementing a static pressure reset control strategy.
Static pressure reset is a control strategy that continually optimizes the supply duct static pressure setpoint. This enables the supply fans to use minimum necessary energy to maintain comfortable indoor temperature and humidity levels. For systems with direct digital controls of individual zone boxes reporting to the central control panel, the static pressure setpoint should be reset based on the zone requiring the most pressure (i.e., the setpoint is lowered until one zone damper (of many dampers) is nearly wide open).
For example: Consider a space with 3 zones having different requirements for heating. The current static pressure is 2” WC, and the damper positions for each zone are 33%, 50%, 0%. None are fully open at 100%, leading to energy waste. To address this, a static pressure reset using a PID (proportional, integral, derivative) loop is typically applied. In simple terms, the static pressure will be gradually lowered until at least one damper is fully open at 100%.
Adding hardware to implement this control algorithm will reduce fan energy use by preventing over-pressurization in the duct for the given demand conditions.
5. Implement Supply Air Temperature Reset
The air supplied by an HVAC system to condition the space is known as the supply air. Numerous HVAC systems in buildings like offices and manufacturing facilities are configured to deliver a fixed supply air temperature throughout the year (usually at 55°F). This approach aims to ensure comfortable relative humidity in hot summer months and prevent mold. Terminal reheat coils are relied upon to reheat the supply air to maintain desired space temperature setpoint. This may be adequate during the hot humid summer months, but for the remainder of the year this can result in simultaneous heating and cooling and hence can be wasteful.
Supply Air Temperature (SAT) Reset is a control strategy used to minimize VAV reheat. It automatically resets (raises or lowers) the supply-air temperature in response to changes in building loads or outdoor air temperature. This generally saves energy during the shoulder and the heating seasons by reducing reheating of supply air that is otherwise tempered lower than any zone really needs. When controlled based on outside air temperature, these controls would reset the supply air temperature to be higher when the outdoor air temperature is low, similarly the supply air temperature would be set lower if the outdoor air temperature is high.
6. Add Energy Recovery Ventilator (ERV) to Air-handler
In a well-ventilated building, stale indoor air, laden with pollutants and moisture is expelled from the building through exhaust vents and fresh outdoor air is introduced into the building. The outdoor air is filtered and conditioned (heated or cooled) by HVAC system before being distributed to various spaces. Traditionally, the conditioned air that is being exhausted is essentially lost to the environment.
An Energy Recovery Ventilator (ERV) is a ventilation system that addresses this inefficiency and improves indoor air quality while efficiently managing energy consumption. It works by exchanging stale indoor air with fresh outdoor air, but with a unique feature: it also transfers the energy (heat or coolness and moisture) from the outgoing air (i.e. recovers otherwise-expended total energy comprised of heat (sensible energy) and humidity (latent energy)) to the incoming air. Energy transfer is typically done with an energy recovery wheel that rotates between the exhaust air and supply air in an ERV cabinet. This process helps to precondition the incoming air, reducing the load on heating or cooling systems.
ERVs are particularly beneficial in climates with extreme temperatures, as they contribute to energy savings by recovering and reusing the thermal energy present in the building's exhaust air. Moreover, as per Advanced Energy Retrofit Guide (for K-12 schools), an ERV that reduces the HVAC system load also reduces the heating and cooling capacity needed, allowing the school to buy smaller units when it is time to replace the boiler, furnace, or chiller. Additionally, modules like ERVe present in the market have a configuration that allows for easy retrofit.
7. Lower VAV Box Minimum Flow Setpoints
Multi zone Variable Air Volume (VAV) is a commonly used HVAC system in commercial buildings. Unlike most other air distribution systems, multi-zone VAV systems use flow control to efficiently condition each building zone while maintaining required minimum flow rates. In a VAV system, VAV boxes are programmed to operate between a minimum and maximum airflow setpoint based on occupancy, temperature and other control parameters and respond by adjusting the position of the damper to increase or decrease airflow. Traditionally, VAV minimum flow setpoints have been maintained around 40% - 50% of maximum flow.
VAV box minimum airflow setpoints have tremendous energy implications. With VAV systems, reducing the zone supply airflow during periods of low cooling and heating load will result in measurable energy savings at the central equipment. During periods of no heating and cooling, VAV boxes must still deliver air to the zones in order to provide ventilation air for the occupants. Often this minimum airflow rate is set higher than needed. Energy savings can be realized by lowering the minimum airflow rate to a level that still provides adequate ventilation air for the occupants, but will result in reduced fan and reheat energy used by the system. This also reduces the amount of return air that is unnecessarily recirculated throughout the building’s HVAC system.
1. Add Variable Frequency Drive (VFD) to Cooling Tower Fan
Image: A Cooling tower (Source: Berkeley Lab)
Cooling towers are heat exchangers that use water and air to transfer heat from air-conditioning systems to the outdoor environment. Typically, they are used to remove heat from the condenser water leaving a chiller.
Many older cooling towers use constant speed (on/off) or two-speed (high/low/off) fans that cycle to maintain the condenser water supply temperature setpoint. Adding VFDs or VSDs to the cooling tower fans and varying the speed of the fans to maintain the condenser water supply temperature setpoint yields energy savings with no associated pump penalty or sacrifice in performance.
VFDs ensure correct motor rotation, prevent wrong direction rotation, and eliminate the need for mechanical brakes and anti-ratcheting devices. They also avert tower icing in cold weather by adjusting fan speed and allow reversing cooling tower fans for heat retention. Additionally, VFDs enable running fans above 60 Hz on hot days for extra cooling capacity while preventing motor overload. This functionality is not achievable without VFDs, eliminating the need for reversing starters and enhancing overall efficiency.
Apart from energy efficiency, implementing VFDs in cooling towers also result in a reduced utility cost, reduced maintenance requirements which decreases personnel & equipment replacement costs; and process water temperature stabilization.
2. Implement Chilled Water Supply Temperature Reset
Centralized systems for air conditioning, typically used in larger buildings that cannot be served with individual A/C units, utilize chillers to remove the unwanted heat from the building by circulating air or water. A water-cooled chiller will typically be designed to supply water to a building at a predetermined setpoint (temperature at which the water leaves the chiller and supplied to the building), such as 44°F. This is done to simplify the programming and installation process, however, this is not the most efficient way for a chiller to run.
Chilled-water supply temperature reset is a control strategy that improves the efficiency of chillers by adjusting the chilled-water set point, which in turn reduces chiller's energy consumption. Typically, this strategy raises the chilled water supply temperature setpoint when the building's loads are lower than the designed conditions. For example, during colder weather, when the building's cooling load falls below the design load, the chiller can get away with supplying 46°F - 54°F (as opposed to the default setpoint at 44°F) according to the requirement.
Based on ASHRAE 90.1 2022, certain chilled water systems are required to include controls that automatically reset supply water temperatures by representative building loads (including return water temperature) or by outdoor air temperature.
3. Install VSDs on Chilled Water Loop
Chilled water pumping systems generally fall into one of two categories: primary-only, and primary secondary. In primary-only systems, one set of pumps circulates chilled water between the chiller(s) and the air handler(s). These systems can either be constant flow or variable flow. Primary-only variable flow systems use less energy than primary-only constant flow, due to reduced pumping energy usage. However, they are usually more complex to design and operate and generally are better suited for larger facilities with multiple chillers and sophisticated operating staff (Taylor, 2002).
- Taylor, S. T. (2002), “Primary-Only vs. Primary-Secondary Variable Flow Systems,” ASHRAE Journal, February, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.