Written by the BEC Technical Committee
This is the second in a series on the challenges faced by building managers during the COVID-19 pandemic. The first article published on October 22, 2020 provided a survey of the first six months of the pandemic in terms of building occupancy, energy consumption and the operational changes required to adapt buildings to this unprecedented situation. The article also presented the recommendations of the American Society of Heating, Refrigerating and Air Conditioning (ASHRAE) aimed at mitigating the risks of transmitting the corona virus through proper operation of ventilation systems. It has now been a full year since the beginning of the pandemic, and a growing number of employers plan to allow more teleworking in future. Indeed, many are considering splitting employees’ working hours more or less equally between the office and teleworking. This “new normal” will obviously have an impact on how energy consumption is distributed among various traditional uses: lighting, heating, cooling, ventilation, plug loads, etc.
Given that context, building managers are strongly urged to review which plans should be given priority in terms of upgrades and energy efficiency projects that will affect heating, ventilation and air conditioning systems (HVAC) and their controls, taking into account that in addition to the new reality described above, there will be increased emphasis on the quality of indoor air and on mitigating the risks of viral transmission. That is the subject of this second article.
Priorize Upgrades & Energy Efficiency Projects in a COVID/post-COVID Context
Generally speaking, the following upgrades and energy efficiency projects should be given priority in the short and medium term:
- Recommissioning and continuous operation of mechanical systems;
- Improving the efficiency of fresh air conditioning;
- Improving management of fresh air flow;
- Improving system capacity modulation;
- Improving lighting control.
The following is a description of the measures and issues to consider for each type of project aimed at ensuring healthy air quality for occupants, while also managing energy consumption.
Recommissioning and Continuous Operation of Mechanical Systems
Recommissioning of Mechanical Systems (RCx)
The first strategy involves recommissioning mechanical systems, commonly referred to as RCx. This approach allows for improving the comfort of building occupants while also reducing energy costs. RCx is a process that consists of optimizing operations in existing electromechanical systems based on the real needs of indoor spaces. It is thus more relevant than ever, as occupancy conditions have changed significantly. RCx helps to identify low-cost measures that lead to energy savings and to a 5 to 15% reduction in greenhouse gas (GHG) emissions, with a return on investment (ROI) period of generally less than 3 years. The process helps improve comfort and indoor air quality, and to address operational problems in HVAC systems. In this pandemic period, RCx is an opportunity to integrate or improve strategies aimed at mitigating the risks of spreading the virus. A decision-making tool has been developed by Natural Resources Canada to evaluate a building’s potential for recommissioning.(https://www.rncan.gc.ca/sites/www.nrcan.gc.ca/files/canmetenergy/files/RCx%20%20Screening%20Form%20FR.pdf)
Issues/Features to consider: RCx is strongly encouraged and currently subsidized by MERN and Énergir when carried out by accredited agents. In addition to conferring a guarantee of professional quality, accreditation helps improve the cost-effectiveness of the steps taken. It is thus preferable to have RCx done by an accredited agent.
Continuous Commissioning (CxC)
Among the anticipated impacts of the pandemic over the next few years, changes in rental space are expected to be even more frequent. Faced with this new reality, CxC (a supplement to RCx continuous commissioning) will prove to be an essential tool for maintaining good energy performance in buildings and ensuring good air quality. CxC involves carrying out a set of verification, monitoring and operational tests in a periodic and cyclical manner in order to optimize the operations of electromechanical systems based on the evolving needs of building spaces. In addition, fault detection and diagnostic software can be used to ensure systematic monitoring of electromechanical systems. These programs employ user-defined rules to observe deviations in systems control that could affect air quality or energy consumption. The data and reports generated by the software can facilitate the CxC process.
Issues/Features to consider: CxC is generally a collaborative process involving the agent accredited for recommissioning mechanical systems, the building’s management and operational personnel, and the automatic system control contractor. It is best to implement the process with a comprehensiveoptimization approach, or to include it in a master building upgrades plan.
Improving the Efficiency of Fresh Air Conditioning
Many ASHRAE recommendations aimed at mitigating the risks of spreading the COVID-19 virus involve increasing fresh air rates and minimum relative humidity set points (see BEC article of October 22, 2020). Given the Quebec climate, implementing those recommendations might lead to a significant increase in energy costs for ventilation systems and might also create problems for the building envelope. The following measures can help limit such impacts.
Adding Heat/Energy Recovery Equipment
This measure consists of installing a heat exchanger in order to transfer some of the energy from the exhaust air to the fresh air in order to preheat it. Since fresh air is subject to outdoor weather conditions, it needs to be heated, humidified, cooled or dehumidified. It is usually a very cost-effective measure, given that treating fresh air is a major energy consumption item in most commercial buildings. The most frequently used heat recovery systems on the market are: the enthalpic core, the heat wheel, the glycol loop (with or without heat pump), the heat pipe, and the cassette heat recovery system. When a new fresh air system is purchased, the heat recovery unit can be installed by the manufacturer. If there is an existing fresh air system that is not scheduled for replacement, then the heat exchanger can be installed separately from the system.
Issues/Features to consider: Some heat exchangers have the disadvantage of contributing to cross-contamination between exhaust air and fresh air. The level of air quality required therefore influences the choice of the type of energy recovery unit. For example, a glycol loop will not have any cross-contamination while a heat wheel will have some cross-contamination. In addition, the type of contaminants present in the exhaust air will influence the choice of material for the recovery system. It is thus important to analyze the desired application to help determine which heat exchanger to select. Finally, the location of the evacuators in relation to the fresh air system influences the choice of recovery unit. For example, to recover heat from several decentralized evacuators, it is necessary to determine whether it is more cost-effective to use a glycol loop to recover heat from existing evacuators, or to reconfigure the evacuation ducts to centralize them near the fresh air system.
Raising relative humidity set points while increasing fresh air flows poses a challenge, as it can sometimes create an increase in humidification capacity and greater electrical capacity (motive power and cooling capacity), higher energy bills, etc. An adiabatic humidifier sprays fine water droplets into the airflow of a ventilation system. The dry air absorbs the droplets and thus becomes humidified. Unlike the more standard humidifier (steam generator), the adiabatic humidifier injects steam rather than water. Since steam generation for humidification is an energy consuming process, adiabatic humidification can be an efficient solution in some contexts.
Issues/Features to consider: By absorbing water droplets from the adiabatic humidifier, the air is cooled. When a ventilation system is operating in an air conditioning system, the adiabatic humidifier works in the same direction as the needs of the system as the droplet injection cools the air. On the other hand, when a system has low cooling requirements or operates in heating mode, droplet injection may force the air to be reheated. In that case, adiabatic humidification may not be cost-effective. It all depends on the efficiency of the unit and the energy source used to reheat the air. Analysis of its cost-effectiveness can be quite complex, and generally a professional is required to carry out the analysis. The cost of an adiabatic humidifier is high and its installation must therefore be studied in detail. In addition, minimum pipe lengths are required to allow the airflow to absorb the droplets (absorption distance). It is therefore essential that the design be done properly so as to meet these criteria.
Ventilation systems are usually equipped with one or more filters. Filtration efficiency is standardized by ASHRAE based on the MERV scale (minimum efficiency reporting value). With the pandemic, ASHRAE suggests raising filtration levels to a minimum of MERV 13 whenever possible (see BEC article of October 22). Using MERV 13 filters was generally not the norm in commercial buildings before the pandemic. Raising the filtration level increases the work that the fan must do, and therefore increases energy consumption to maintain the same flow rate as before. In order to compensate for this additional energy consumption, the following energy management measures must be analyzed: modulation and control of fresh air flow, variable frequency drives, air balancing, calibration of controls. Otherwise, the main energy efficiency measure for increased filtration is proper filter maintenance. Pressure loss in filters is greater in dirty filters. It is therefore important to adjust routine maintenance schedules for finer filters, and to check that the differential pressure sensors at the filters are functional and send the proper alarms to indicate an anomaly or a need to replace the filters.
Issues/Features to consider: When raising a system’s filtration level, it is important to analyze the impact of a resultant loss of pressure. You might need to recalibrate or replace the system’s fan or replace the fan motor. It is also important to validate space available in the filter section of the system, and the space available for a new motor if it needs to be replaced. The filter section can be rebuilt by a ventilation contractor installing higher filtration level MERV filters, along with pre-filters. You might need to make the joints in the filter section watertight, because the air will take the easiest route and thus might not be filtered. Given all these considerations, it is sometimes impossible to raise the filtration level in an existing system. If your building manager does not have the required expertise, engage a professional to analyze the entire system and the possible impacts.
Improving Management of Fresh Air Flow
Variable occupancy is more and more the norm, and indoor air quality has become a priority issue in the design and operation of workspaces. Those two aspects are strong indications of the importance of improving fresh air management in our buildings. To reconcile these two new realities, the following energy efficiency strategies should be promoted and implemented.
Modulating Fresh Air based on Occupancy
The amount of fresh air supplied to a ventilation system is based on the size of the space served, the use made of the space, the number of occupants and other factors specific to the system and the air distribution network. Current standards and codes allow a portion of the fresh air to be modulated according to actual demand in an occupied space. Demand can be based on readings of carbon dioxide (CO2) concentration, and sometimes simply on a schedule, a known occupancy rate or profile, or on sensors detecting human presence. Presence detection provides additional data on building occupancy, and thus allows for managing fresh air flow according to the actual occupancy of the building. Fresh air modulation is generally done by means of a motorized damper or a variable frequency drive (VFD), and can be done for a room, a rental space or an entire floor. It is a measure that usually has a very short payback period. The demand on which it is based, however, must represent the spaces served.
Issues/Features to consider: One of the main recommendations in a pandemic context is to increase, where possible and reasonably feasible, the rate of air exchange in occupied spaces. While that is easily done, this measure should not be applied in the current pandemic context. In the meantime, the addition of CO2 sensors will allow you to monitor and confirm consistent good indoor air quality.
Adding Flow Measurement Stations
In order to better manage fresh air flow, adding flow measurement stations might help. A flow measurement station allows you to modulate precisely the fresh air supply in a ventilation system. It provides security in the event of deterioration over time of the coupling between the actuator and the flaps. Variable volume multi-zone systems are the main systems targeted for the addition of flow measurement stations. These stations become a requirement under current codes and standards if demand-responsive ventilation control strategies are implemented. They allow for real time measurement of fresh air flow, with downstream zone dampers modulating according to demand.
Issues/Features to consider: Since it involves measuring equipment, flow stations should be calibrated at the time of commissioning and every 5 years thereafter. It is sometimes difficult to integrate such equipment into an existing system that was not designed for that purpose in terms of space and available straight lengths.
In the context of improving fresh air flows, balancing a ventilation network consists of adjusting the supply and exhaust flows for each floor, and adjusting the system fans to supply or discharge total flows. Balancing the aeraulic networks should be carried out for each zone once the renovation or rearrangement of building spaces has been completed. How the spaces are used often changes after a few years of operation, but flow rates are not adjusted, as is the case with low occupancy in office buildings in the current context. In addition, the methods of fresh air control and the associated standards evolve from year to year. An analysis of real occupancy and fresh air flows might be required to verify whether a system rebalance is required, and whether it can be zoned by floor in the current context. That is usually part of a re-commissioning process, although it can also be done separately.
Issues/Features to consider: Given that it is a review of system needs, the approach can lead to an increase in air flows or a decrease. The impact on energy consumption can therefore go up or down. It is important that the balancing report emphasize fresh air flows over the entire range of fan operation if the system is variable speed. Too often, balancing reports mention total flows without being very precise about the specific fresh air flows.
Adding Dedicated Fresh Air Systems
A dedicated fresh air system, also known as a 100% fresh air system, is a system whose sole function is to admit and condition fresh air inside a building. It differs from a typical “H” system in that it does not handle re-circulated airflow or exhaust air. Its main advantages are that it facilitates efficient preconditioning of fresh air (e.g. integrated heat recovery system), measures and controls the flow of fresh air admitted and reduces the driving force associated with ventilation (e.g. local recirculation with fresh air supply).
Issues/Features to consider: For buildings not initially designed with that principle in mind, it can be complex and very costly to consolidate the various fresh air supplies. The technical and economic feasibility of such a measure must be evaluated by an engineering professional. It is something to be considered when designing a new building, or during major renovation work such as adding a new wing to a building.
Improving System Capacity Modulation
Variable Frequency Drives
Motors with variable frequency drives (VFDs) can modulate their speed to meet exacting requirements as they change over time. This may involve a fan motor or a pump motor. There are many ways they can be put to use, for example in a ventilation-air conditioning unit or a hydronic heating or cooling network. VFDs can also be used to modulate the capacity of certain chillier compressors. In an HVAC system, for example, the power of the fans, and therefore the air flow rate, can be modulated according to the occupancy rate, which can be determined by means of CO2 probes installed inside the offices. That will prevent the engines from unnecessarily running at full speed when demand is low, and will increase the efficiency of the system. The CO2 sensors show occupancy in real time. System capacity modulation is not limited to use of VFDs. Modulation with VFDs can be used in conjunction with other modulation strategies such as: modulating the supply temperature of a ventilation system according to demand in building spaces, modulating the supply temperature of a hydronic network by opening valves, etc. As with other control measures, it is a measure that usually has a very short payback period.
Issues/Features to consider: The electrical connection must be analyzed when adding a variable frequency drive to prevent the risk of harmonics. Make sure that the motor to be modulated is compatible with a variable frequency drive. Check whether it needs to be replaced, which will increase costs. Finally, the main issue is often the modulating equipment in the distribution network: VAV boxes and 2-way/3-way valves. The modulation strategy must be compatible with the control equipment in the distribution network. That sometimes requires major changes in regulations governing existing networks.
Improving Monitoring of Ambient Conditions
Adding and Centralizing Room Sensors (temperature, humidity, CO2)
Monitoring HVAC systems can take into account several parameters, among them the outdoor temperature and the temperature in different areas of the office, as well as the humidity level and CO₂ concentration. The latter is directly linked to the occupancy rate of a space. Adding and centralizing sensors provides the control system with more precise and comprehensive information, thereby allowing for optimal functioning of HVAC systems based on real conditions. In turn, that increases the comfort of occupants and the quality of the indoor air.
Issues/Features to consider: The centralized control system must have free points in existing monitoring spots in order to incorporate new sensors. If not, I/O expansions can be installed. Additional probes can also be added to the calibration schedule for measuring equipment.
Over time probes may need to be recalibrated, so it is important to have a preventive maintenance and calibration program for sensors to ensure that the readings are correct and accurate. Refer to the sensor maintenance manual to determine how often they should be calibrated. Generally speaking, temperature probes should be recalibrated every 2 years, and humidity, CO₂ and outdoor sensors every year. Outdoor sensors or weather stations have a substantial impact on energy consumption, given that several start-up sequences for heating and air conditioning equipment may depend on them. It is very important to include them in the calibration program. That is usually part of a recommissioning process, although it can also be done separately.
Issues/Features to consider: The calibration program can be an additional cost in the maintenance budgets of small and medium-sized commercial buildings that do not generally adopt such an approach. Note, however, that the success of most of the energy efficiency measures discussed in this article is based on the use of sensors in good working order, thereby allowing for a fairly precise portrait of needs in real time.
Improving Lighting Controls
Centralizing and Rezoning Automated Lighting Controls
Lighting control and zoning is a simple but quite effective measure to implement. Replacing incandescent or fluorescent lighting with LED (light emitting diode) lighting is a good idea, but it is also very important to properly define and control the various lighting zones so that if only one part of the office is occupied, the other spaces are not affected.
Issues/Features to consider: The configuration of the electrical lighting network greatly influences the complexity of this measure.
Adding Occupancy Sensors
It is also a good idea to install occupancy sensors to ensure that only occupied areas are lit. Several types of occupancy-based lighting control technologies are available. It can be as simple as a sensor built into the light switch. It is also becoming increasingly common to integrate lighting control into the building’s centralized control system to establish schedules (day, evening, weekend) and generate even more savings. Whether or not occupancy sensors are integrated into the centralized control, the savings are significant.
Issues/Features to consider: Complex implementation and installation costs vary considerably depending on the system you choose. If occupancy sensors for lighting control are centralized, then it may be possible to use the data to control other centralized equipment such as HVAC systems.