Advanced DOAS Technologies and Features, Part 2
This article is an update to the article “Advanced DOAS Technologies and Features,” published in Engineered Systems’ September 2017 issue, which discusses best practices learned from data accumulated from more than 800 networked DOAS units since that time.
Proper building ventilation has been a basic need in modern building performance for decades, yet, somehow, it remains one of the greatest difficulties for sustainable modern building design. Given the advancements and availability of state-of-the-art dedicated outdoor air systems (DOAS), this is no longer the case. In order to provide proper building ventilation and humidity control in a sustainable manner, mechanical design needs to satisfy two fundamental criteria: The HVAC design must incorporate a high-performance DOAS strategy, and the long-term operation and efficiency of the equipment must be optimized through connected commissioning.
DOAS FOR INTRODUCTION OF 100% VENTILATION AIR
Even with recent advancements in DOAS technology, engineers often portion small amounts of the required ventilation air across a number of packaged RTUs throughout the building. Taking even small percentages of outdoor air across these units often results in improperly balanced buildings, poor indoor environmental quality (IEQ) with temperature and humidity comfort complaints, poor indoor air quality (IAQ) from inadequate filtration, and excessive energy usage.
Reviewing typical RTU designs, including their performance testing, it’s clear they are specifically designed to process 100% recirculated indoor air and are meant to deal only with sensible heating and cooling needs. With narrow coils, inadequate filtration, and low-cost damper assemblies, typical RTUs often cause building pressure issues, poor IAQ, component failures, and a variety of temperature and humidity comfort complaints.
Sometimes, there is reluctance to specify DOAS equipment because of assumptions of cost, difficulty of maintenance, and a sizable demand on energy consumption for building owners and operators. With the advancement of variable-speed components, these assumptions are outdated. Still, not all DOASs are created equal, and engineers need to specify key technologies to deliver high-performance units that can reach a targeted 20-year lifespan.
Components such as variable-speed, inverter-duty scroll compressors allow for load matching to the outdoor air conditions while operating at the highest possible efficiency. Fully modulating hot gas reheat allows for state-of-the-art systems to achieve the exact dew points required to properly dehumidify incoming airstreams and then reheat the air back to the exact discharge temperatures needed to maintain space thermal comfort.
The use of variable-speed, ECM outdoor fans allows for condensing coil capacity modulation while lowering energy usage. Class 1A, low-leakage, robust outdoor air dampers can ensure the exact amount of air needed is being delivered, easing the testing and balancing process and facilitating proper building pressurization. When paired with remote monitoring, this becomes the only way to ensure a proper, positive building balance.
Most importantly, wider, staggered, multi-row coil designs maximize the residence time, or amount of time air spends travelling through the coil, therefore increasing the unit’s ability to remove moisture. Figure 1 shows how much larger a seven-row, staggered DOAS coil is than a typical RTU single-row coil design. The use of innovative DOAS technology is the only way to decouple outdoor air latent loads from the sensible loads of the building and avoid all previously discussed pitfalls.
Conventional HVAC commissioning involves the verification of the unit’s performance during the initial startup and sometimes during periodic checkups; however, when equipment is only evaluated at periodic times, this creates risks because these systems are dynamic. Elements such as seasonality, occupant feedback, changes to space operations, and the natural complexity of these systems all factor in to how the buildings perform. To ensure proper long-term IEQ, a more continuous approach to oversight on the system needs to be implemented to allow for sustained, fine-tuned adjustment and feedback.
Connected commissioning is the process of consistently gathering component, sensor, and alarm data and transmitting this information through a hardwired or cellular-based internet connection to a centralized platform. This information is cross-checked with unit set points and factory settings before being linked in real time to manufacturer engineering and technical support teams. This differs from traditional building management systems (BMS) in that it ties the unit performance back to the manufacturer, which has a vested interest in the outcome as well as the technical information to properly support their products.
An optional cellular connectivity to DOAS units is strongly recommended, as it allows for a higher rate of success for connection; provides enhanced security measures to protect data; and avoids pitfalls, such as additional field wiring and access to an internal network on-site.
For superior humidity control, a connected commissioning program, ideally included as part of a manufacturer’s preventive maintenance initiative, can ensure space comfort is maintained throughout the life of the equipment (tracking space dew points); assess the impact and economics of dehumidification to optimize energy usage while maintaining space IEQ; assess the impact of set points and make necessary changes remotely, avoiding costly service visits; report any reading anomalies, allowing for a proactive preventive service approach versus reactive emergency service calls; and verify proper building balance, given occupancy and system state.
INTEGRATED SENSORS FOR OPTIMIZED PERFORMANCE/ENERGY SAVINGS
The use of integrated sensors throughout a modern DOAS is required to provide the highest level of performance and efficiency. One example of this is the use of fully modulating hot gas reheat, which is imperative in the active dehumidification process to ensure precise humidity and temperature control. The process of dehumidification is an energy-intensive process, as it requires cooling of the incoming airstream below the dry bulb temperatures required for space comfort. Likewise, the air is cooled beyond saturation, as needed, until a target dew point or humidity ratio is achieved. It is usual practice for DOAS logic to target a fixed universal target dew point temperature upon any call for dehumidification across various applications. This is typically set in the low 40s, at times exceeding what might be needed for many applications. Once the airstream is dehumidified and cooled, the waste heat that would otherwise be rejected to the outdoor condensing coil can now be redirected through a modulating reheat valve back to a reheat coil to sensibly warm the air back to comfort. Even though the process of reheating utilizes waste heat within the system, the initial cooling of the airstream is energy intensive. Every space has a unique sensible heat ratio, and, as such, state-of-the-art DOAS equipment must have the ability to adjust the target dew point to meet the needs of those varying conditions.
Being able to monitor and validate the exact required leaving dew point to maintain a space set point is powerful and necessary information for optimizing any system; however, simply having the data available is only the first step. The data only becomes useful for energy savings if the DOAS logic actually has the ability to adjust leaving coil temperatures. Figure 2 shows a site that is maintaining a space temperature set point but is overshooting the target space dew point due to default dehumidification settings. Importantly, without this monitoring data, the site would continue to operate inefficiently whenever the outdoor conditions were humid enough to activate a call for dehumidification despite the IEQ being satisfactory.
PREDICTIVE FAULTS AND PREVENTION OF DOWNTIME
DOAS units have a much higher level of control over the delivery conditions of the airstream, however, in order to achieve this, they also incorporate a much wider array of temperature, pressure, and humidity sensors throughout the system. Mechanical components and sensors might fail or their accuracy may drift. A minor inaccuracy in a given reading may lead to minor inefficiencies of the system, while severe inaccuracies could potentially cause a system to fail. The latest manufacturer-developed monitoring systems are actually able to implement artificial intelligence (AI) to help review aggregated data to determine the moment these sensors begin to lose accuracy. In turn, this results in preventive alerting to service personnel to allow a proactive service visit to address these types of issues before they become more serious.
This is a superior methodology compared to the historic reactive approach of waiting until a failure occurs. Figure 3 represents a suction line thermistor on an HVAC unit that began to drift from accurate readings, which could have resulted in an improperly calculated superheat and compressor damage. In this event, a service tech was immediately alerted by the system, which allowed for a sensor replacement prior to the point of equipment failure and, ultimately, facility and environmental performance issues.
Certain third-party BMS systems may allow for common remote set point adjustments; however, manufacturer-based, remote-controlled systems often allow a greater level of control for specific logic operations, which give a much higher level of precision. Figure 4 identifies a site that was overheating due to an improperly adjusted intake activation set point. The setting was fixed remotely on a weekend by a technician in a matter of minutes, saving the customer the cost of a site visit, providing immediate comfort improvements, and allowing them to continue site operation.
ENGINEERING ANALYSIS AND NATIONWIDE SERVICE NETWORK
With the advancement of factory-provided remote monitoring and control options, it is imperative the manufacturer also have in-house engineers available to review and interpret this data to be able to take full advantage of performance, energy, and controls benefits. If a site visit is required for any repair work, a nationwide, factory-trained service network with a local presence is the only way to guarantee adjustments or repairs are accomplished correctly the first time and in a timely manner. With a factory service team working alongside this engineering team, they can efficiently and effectively troubleshoot equipment (locally and remotely), ensure the proper parts (as needed) are in hand for the repair, and remotely verify that all repairs have been properly executed — keeping costly site work and repeat visits to an absolute minimum.
Engineering state-of-the-art DOAS, both from a physical and controls perspective, provides superior solutions for accurate air pressure, temperature, and humidity control. Through the usage of high-quality system components and system modulation, DOAS can remove excessive moisture content from the outdoor airstream without overcooling the space and causing excessive cycling of compressors.
Through remote monitoring by technical support and engineering teams, remote control capabilities, and real-time alarm response protocols, connected commissioning helps fully realize the value proposition of DOAS and easily will pay for itself throughout the life of the system, allowing DOAS to provide a sustainable solution to building comfort and humidity control at a lower overall expense than conventional systems.