Ashok A Khedkar
• How to design, install, control, maintain, test, commission, tune, optimize, procure and upgrade AHU VAV system in built environment in 195 countries in 2024?
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· How to design, install, control, maintain, test, commission, tune, optimize, procure and upgrade AHU VAV system in built environment in 195 countries in 2024?
· How to design, install, control, maintain, test, commission, tune, optimize, procure and upgrade AHU VAV system in commercial real estate and government buildings in 8 continents in 2024?
· How to design, install, control, maintain, test, commission, tune, optimize, procure and upgrade AHU VAV system in office buildings, hospitals, universities, schools, shopping malls and commercial real estate in 195 countries in 2024?
· How to design, install, control, maintain, test, commission, tune, optimize, procure and upgrade AHU VAV system and HVAC to improve occupant comfort, health, wellbeing, safety, engagement, collaboration and productivity in 195 countries in 2024?
The primary goal of any heating, ventilation, and air conditioning (HVAC) system is to provide comfort to building occupants and maintain healthy and safe air quality and space temperatures. Variable air volume (VAV) systems enable energy-efficient HVAC system distribution by optimizing the amount and temperature of distributed air. Appropriate operations and maintenance (O&M) of VAV systems is necessary to optimize system performance and achieve high efficiency.
VAV systems supply air at a variable temperature and airflow rate from an air handling unit (AHU). Because VAV systems can meet varying heating and cooling needs of different building zones, these systems are found in many commercial buildings. Unlike most other air distribution systems, VAV systems use flow control to efficiently condition each building zone while maintaining required minimum flow rates.
Each VAV box can open or close an integral damper to modulate airflow to satisfy each zone’s temperature setpoints. In some cases, VAV boxes have auxiliary heat/reheat (electric or hot water) where the zone may require more heat, e.g., a perimeter zone with windows.
One of the most significant benefits of VAV systems is their ability to conserve energy by precisely matching airflow to the actual heating and cooling demands of each zone. By modulating airflow and adjusting fan speeds, VAV systems minimize wasted energy associated with over-conditioning spaces, resulting in substantial energy savings and lower utility costs. VAV systems are highly adaptable to changing occupancy patterns, weather conditions, and building usage. Whether it’s accommodating fluctuating occupancy levels or responding to external temperature changes, VAV systems offer unparalleled flexibility to optimize performance and comfort in diverse environments. Terminal units, including diffusers and grilles, are responsible for distributing conditioned air from the VAV box into the occupied space. These units are strategically placed throughout the building to ensure even airflow distribution and optimal comfort levels for occupants.
A sophisticated control system acts as the brains behind the operation of the VAV system. In various zones, it constantly monitors temperature, humidity, and occupancy levels, and sends signals to adjust airflow and temperature. Advanced control algorithms optimize system performance while maximizing energy efficiency and occupant comfort.
The ductwork transports conditioned air from the HVAC equipment to the VAV boxes and terminal units throughout the building. Properly sized and insulated ductwork ensures efficient airflow distribution and minimizes energy losses. Sensors give the control system real-time data, so it can make accurate adjustments to the VAV system. Actuators control the movement of dampers and fans within the VAV boxes, enabling precise modulation of airflow rates and temperature levels.
Some features of a VAV system include the following:
Distribution system provides conditioned air to spaces to meet varied zonal temperature and airflow requirements.
Variable frequency drive-based air distribution system can reduce supply fan energy use.
Supply-air temperature reset capability allows adjustment and reset of the primary delivery temperature with the potential for savings at the chiller or heating source
There are two major classifications of VAV boxes or terminals—pressure dependent and pressure independent.
A VAV box is considered pressure dependent when the flow rate passing through the box varies with the inlet pressure in the supply duct. This form of control is less desirable because the damper in the box is controlled in response to temperature only and can lead to temperature swings and excessive noise. A pressure-independent VAV box uses a flow controller to maintain a constant flow rate regardless of variations in system inlet pressure. This type of box is more common and allows for more even and comfortable space conditioning.
Variable Air Volume (VAV) systems are the best way to zone especially in large buildings.
A typical pressure-independent VAV box; in this case, the box also has a reheat coil. This VAV box has three modes of operation: a cooling mode with variable flow rates designed to meet a temperature setpoint; a dead-band mode whereby the setpoint is satisfied and flow is at a minimum value to meet ventilation requirements; and a reheating mode when the zone requires heat.
The three main types of VAV boxes available are:
Series (Series VAV boxes are fan-powered)
Parallel boxes (Parallel VAV boxes are fan-powered)
Standard VAV without a fan
Either type of box can offer zone comfort and energy savings if the proper controls are installed. Secondly, if they have the appropriate programming for the sequence of operation. Additionally, the system needs to be properly balanced and calibrated.
If the zone temperature is too cold the DDC/BMS backs down on the damper. The damper allows air from the primary source (VAV AHU) to feed the zone. Depending on if the box is parallel or series then the blower fan kicks on. This allows air from the plenum (or above the ceiling) to be redistributed into the zone (this air is usually warmer than the primary air supply). If the zone temp continues to drop, then DDC will close the damper to minimum position (not closed but an engineered rate of CFM minimum flow) maximizing the use of warmer plenum air and activate electric or hydronic heat.
There are VAV boxes that are fan-powered and there are VAV boxes that are not fan-powered. The main purpose of fan-powered VAV boxes is to make use of the plenum air. The air above the ceiling where the fan-powered VAV box is located. This air is usually warmer than the air supplied by the VAV air handler. So in the sequence of operation for a fan-powered VAV box, and a non-fan powered VAV box, would include closing the damper to allow minimal air from the air handler and at the same time energize the fan contactor so that the fan in the VAV box comes on and pulls warmer air from the plenum.
Plenum air in most VAV box applications is also, usually, the return air for the VAV air handling unit. When the fan in the fan-powered VAV kicks on it pulls this plenum air into the VAV box. Furthermore, it mixes with the minimal airflow coming from the VAV air handler. Then it either hits a reheat hot water coil or electric heat strips. The air is warmed further to a desired supply air temperature (SAT) set point.
Each VAV terminal box is connected to a supply air source. This is a ducted connection that provides air from an AHU. Primary components of the AHU include air filters, cooling coils, and supply fans, usually with a variable speed drive (VFD). The pressure sensor measures static pressure in the supply duct that is used to control the VFD fan output, thereby saving energy.
VAV terminal box. The VAV terminal box consists of several individual components, including:
Airflow sensor – measures the airflow at the inlet to the box and adjusts the damper position to maintain a maximum, minimum, or constant flow rate regardless of duct pressure fluctuations.
Damper – modulates the airflow based on airflow sensor and zone temperature requirements.
Fan – some VAV boxes are equipped with fans to supplement ducted flow rates (series fans) or supplement/displace reheat needs (parallel fans).
Filter (for fan-powered boxes) – usually included when a fan draws into the VAV box from the plenum or other return-air source.
Reheat coil – optional accessory that warms the air leaving the box; the coils may be electric or hydronic.
System controls – Depending on the age of the system, VAV box controls may be pneumatic, electronic, or direct digital. An airflow sensor in the box measures airflow. Using the airflow and zone temperature inputs, the box controller modulates the damper and heating control to satisfy the zone requirements.
Zone temperature control. The primary control point for any VAV system is the zone temperature. Either a zone sensor or thermostat provides a signal to the VAV controller.
Keeping VAV systems properly maintained through preventive maintenance will minimize overall maintenance requirements, improve system performance, and protect the asset.
VAV systems are designed to be relatively maintenance free; however, because they encompass (depending on the VAV box type) a variety of sensors, fan motors, filters, and actuators, they require periodic attention. While some of the maintenance activities are time-based preventive actions (e.g., verifying actuator function or checking, cleaning, and changing filters), some can fall into the predictive maintenance category, whereby tending temperature data can be used to identify sensors requiring calibration.
It is important to keep a written log, preferably in electronic form in a Computerized Maintenance Management System (CMMS), of all services performed. This record should include identifying features of the VAV box (e.g. box number, location, and type), functions and diagnostics performed, findings, and corrective actions taken
VAV Box Duct Connections - Check VAV box duct connections for leakage or movement. Verify that hangers and mountings are secure.
VAV Box Zone Temperature Sensor (Thermostat) - Verify function and accuracy (compared to calibrated value). Check signal to controller to verify corresponding control, damper action, and minimum setting.
VAV Box Airflow Sensor - Verify function of flow sensor (compared to calibrated value) and corresponding control of box damper. Clean sensor per manufacturer’s recommendations.
VAV Box Controls - Verify function by technology type and per manufacturer’s recommendations:
· Pneumatic – check for air leaks in hoses and fittings.
· Electronic – check for proper electrical connections.
· Direct Digital Control (DDC) – check for proper connections corresponding to damper action.
· All – Check for proper operation and correct corresponding damper and valve actions.
· VAV Box Damper - Check seals and alignment in duct.
· VAV Box Damper Linkage and Control - Check linkage for tension and position relative to control point. Lubricate per manufacturer’s recommendation. Verify minimum and maximum positions are correct.
· VAV Box Filter (if present) - Check, clean, and/or replace filters on all fan-powered VAV boxes. Change per manufacturer’s recommendations.
· VAV Box Hydronic Reheat (if present) - Check and clean reheat coil. Check control valve and fittings for water leaks, and check coil for cleanliness and fin condition.
· VAV Box Electric Reheat (if present) - Check and clean reheat coil. Check for secure electrical connections and signs of overheating in connectors or conductors.
· Building Automation System (if applicable) - Perform VAV system re-tuning.
· AHU VAV System - Perform appropriate inspections and maintenance of other components and systems including, but not limited to, AHU, return fan, economy dampers, and VFDs.
On the back of the VAV, at the inlet, I find some tubes forming a cross shape. This is an air flow sensor. It is measuring the change in pressure across the device. From that it can calculate the average air velocity and thus the flow rate into the VAV terminal. Basically, there are some small pinholes on the front and also on the back of the tubes. If I imagine the supply air coming in and hitting the front side, then it’s going to impact on this surface and cause pressure. Therefore, the front is going to have a high pressure side and the back side will be a low pressure point. I can find the differential pressure across the device, and if I average that for the size of the duct, and calibrate it, then I can calculate the flow rate of air entering into that box and zone.
The most common option for VAV performance monitoring is using the building automation system (BAS). By enabling the trending function of a BAS, the VAV system operation can be assessed. Key points to trend include:
Static pressure in supply duct and control point for system VFD fan to assure modulation with changing VAV box flow rates.
VAV box damper position versus zone temperature and reheat status to assure damper minimum setting before reheat application.
Reheat valve position versus call for heat.
VAV box airflow rate commensurate with damper position and within minimum and maximum settings.
VAV box delivered air temperature appropriate for zone conditions.
VAV box reheat call appropriate for conditions and corresponding chiller operating point and reset status.
Zone temperature.
Zone occupancy status.
Modern VAV systems are designed to be more efficient and have less overall wear due to reduced system fan speed and pressure versus the on/off cycling of a constant volume system. However, at the zone level, the VAV system can have greater maintenance intensity due to the additional components of dampers, sensors, actuators, and filters, depending on the VAV box type.
Fan-powered VAV systems incorporate a fan within each VAV box to boost airflow and overcome pressure losses in the ductwork. These systems offer improved air distribution and temperature control, particularly in large or high-rise buildings with long duct runs. Fan-powered VAV systems are also more resistant to airflow variations caused by changes in duct static pressure. In a single-duct VAV system, a single duct supplies a variable volume of conditioned air to multiple zones within a building. Each zone is equipped with its own VAV box, allowing for individual temperature control. Single duct systems are cost-effective and widely used in commercial buildings with diverse heating and cooling requirements.
Series fan-powered VAV systems feature multiple fans arranged in series along the main ductwork. This configuration allows for precise control over airflow distribution and pressure regulation, making it suitable for buildings with complex layouts or variable occupancy patterns. Series fan-powered systems excel in maintaining consistent airflow and temperature levels across multiple zones.
Choosing the right VAV system depends on factors like building size, layout, occupancy patterns, and budget. HVAC professionals can optimize energy efficiency and occupant comfort by designing and implementing VAV systems that meet the specific needs of their clients.
In general, when it comes to consuming energy efficiently, buildings do not perform as well as they did when they were first built, and the gradual loss of this efficiency is known as energy drift. Energy drift happens for various reasons, including mechanical wear and tear, malfunctioning equipment, alterations to BMS controls, changing site conditions, building design defects, or human error. To identify and prevent drift, property owners and managers need equipment-level data, both historical and real-time, to assess and resolve specific issues accurately and promptly. 24/7 monitoring of equipment allows you to identify and rectify failures immediately, preventing massive triggers of drift, such as overnight operation of equipment, BMS overrides, schedule overrides, fire alarm failures
Additionally, consider your set points. Typical set-points are 21–22°C in winters and 23–24°C in summers. However, for every 1°C adjustment of the air-conditioning closer to the outside temperature, the energy required for heating and cooling is cut by 5–10%. The feeling of an office being freezing in summer and stuffy and hot in winter means that the set points may not be optimised, and that your building is working a lot harder than it needs to provide those temperatures. Revisiting temperature setpoints and ranges Even as little as a 1°C change can translate into significant efficiency improvements.
Adjusting the temperature of your building just marginally can yield great reductions in energy consumption: even a 1°C change can translate into impressive long-term savings for you and reduced carbon emissions for the environment. a slight adjustment to the temperature dead band – the temperature range deemed suitable for building environments and where HVAC turns off – can greatly affect how much energy a building uses. Typically, temperature dead bands are set to ±2°C of the setpoint, causing many HVAC systems to work inefficiently as small temperature fluctuations cause the system to cycle on and off. To prevent this, I recommend increasing the dead band range to ±3°C of the setpoint, which can save more than a third of HVAC energy use with no drastic effect on occupant comfort.
Accounting for the thermal lag is one possible way to achieve this. Ensure your BMS systems are configured to automatically have HVAC systems shut down in advance of closing, or at least consider having your HVAC switch off an hour before closing and allowing building temperatures to slowly change. The approach has been shown to have a minimal impact on occupants and can yield significant savings over keeping your HVAC active for the whole working day.
Effecting Outside Air Temperature (OA-T) lockouts can help prevent chillers and boilers operating unnecessarily. Optimisation is also generally possible when it comes to cooling towers wet-bulb temperature control, adjusting the chiller’s chilled water (CHW) temperature setpoints, and even chiller cooling & boiler heating calls: for central cooling systems, you want to ensure that chillers kick on at the right time - engaging chillers early increases the risk of energy over-consumption. Similarly, boilers should be enabled when needed, not before.
Another form of optimisation is the ‘economy mode’ operation, or the use of ‘free cooling’ wherein use of outside air is more efficient to cool the building than return air.
Buildings in warmer climates need additional cooling during evenings. In commercial offices unoccupied over weekends, however, building temperatures may soar, placing a massive load on building air conditioning, which can be minimised using a night purge. An important element of upgrading your portfolio is electrification. Removing the dependency of commercial properties on fossil fuels through electrification will allow us to transition buildings from being amongst the largest planet-warming polluters to contributors of a decarbonised world. Under a PPA, the energy vendor sets up and maintains the renewable energy technology—for example, a wind farm or solar energy array—and the energy purchaser agrees to buy the power on a per kWh basis. Roadmap to Net Zero, involves carbon
offsetting. Through carbon offsets, organisations can determine the extent of their emissions, and then attempt to mitigate their activities by helping reduce carbon elsewhere, via the purchase of units to compensate for their emissions. Carbon offsetting projects typically include reforestation, investments in renewable energy in developing nations, or converting waste to energy.