3.3.2023
Position paper by Nordic Ventilation Group (*
Edited by Panu Mustakallio based on discussions in the NVG meetings and presentations at the Nordic Ventilation Forum 2022
Position paper by Nordic Ventilation Group (*
Edited by Panu Mustakallio based on discussions in the NVG meetings and presentations at the Nordic Ventilation Forum 2022
Demand Controlled Ventilation – Current situation, challenges, and needed remedies
This document summarizes the importance of demand controlled ventilation (DCV) systems in buildings and the necessity of correcting current challenges in these systems. Authorities in Nordic countries were concerned about the improper performance of DCV. They were interested to see a summary of the current situation, reasons and remedial measures. This document was made by Nordic Ventilation Group (NVG) based on group´s experience and results of the Nordic Ventilation Forum on 21st September 2022. It applies to all building types with DCV systems. The main focus is on commercial and public buildings where DCV systems are mostly used, but also issues in residential buildings are discussed and similar principles apply to these.
1. Need for DCV systems in buildings and potential advantages
Demand for significant energy saving and the same time for increasing and optimizing ventilation airflow rates in buildings creates the evident need for DCV systems in buildings [1]. Currently, close to 40% of total energy consumption in Europe is used for buildings and the ventilation of buildings generates a significant part of that [2,4]. According to scientific studies, the energy consumption of heating, ventilation and air-conditioning (HVAC) systems can be reduced by 20-50% [1-3] compared to the HVAC systems with fixed ventilation airflow rates. This depends strongly for instance on the room usage/occupancy profiles in the buildings, which vary significantly in different spaces . Typical occupancy ratio is for instance 30%-40% in many office buildings [6].
2. Current systems and their principles
The main principle of the DCV system is to maintain good indoor climate conditions for occupants in buildings by dynamic control of the ventilation supply and exhaust airflow rates depending on occupancy, pollution load and thermal load. These systems are called also variable air volume (VAV) systems. Typically, also water-based heating and cooling room systems (for instance chilled beams, fan-coils or radiant panels) are linked to the DCV system. With these systems, the airflow rate is based on occupancy and excess cooling/heating demand is covered with water-based system. HVAC systems based on fixed ventilation rates (CAV systems) need to be designed for most demanding situations with maximum airflow rate and are not able to reduce ventilation fan energy consumption and cooling/ heating energy consumption for ventilation without DCV operation. Also even with efficient heat recovery from ventilation exhaust excess constant ventilation airflow during the heating season causes heat loss and increases both fan and heating energy consumption. Depending on the building usage and type, and based on life cycle cost (LCC) analysis, DCV or CAV system should always be considered.
Typically in a DCV system, ventilation airflow rates are controlled based on schedule, occupancy detector, temperature sensor and indoor air quality sensors. The most commonly used indoor air quality sensor in DCV systems are carbon dioxide (CO2) sensors. Control of supply airflow rates in rooms or in HVAC terminal units can be modulated between the room minimum and maximum airflow rates, based on step control with specified airflow steps or on/off control with boost/normal airflow rates. Control of airflow rates in DCV systems can be designed for individual rooms, zones or specific modules in open areas like landscape offices.
DCV systems in apartment buildings can control ventilation airflow rates at the apartment level. This is typically done by switching to low airflow rate when the apartment is empty or by having boost airflow mode when the kitchen hood is used. Additional kitchen hood exhaust air should be properly balanced by controlling the supply airflow rate in the DCV system. In small apartments, this can be done by borrowing airflow rates from other apartments, but the balancing of airflow rates should be done carefully when several apartments are boosting at the same time.
For maintaining indoor climate condition, the DCV system should control ventilation airflow rates at room level. Commonly the target for controlling ventilation airflow rates at room level is also to maintain the balance between supply and exhaust. Three most typical concepts for exhaust air flows are:
DCV systems can be defined as pressure-independent and pressure-dependent system categories related to the control of ventilation airflow rates.
Air handling units (AHU) in buildings with DCV systems are using pressure control of supply and exhaust fans to desired pressure level in the ductwork. Variable frequency drives (VFD) are used to control fan speed for avoiding excess pressure losses, and to balance supply and exhaust airflow rates. There can be a fan speed optimization function where the opening of the dampers can be maintained as open as possible for the reduction of fan energy consumption.
3. Problems in the performance of current DCV systems
This chapter presents identified problems and challenges with DCV systems in buildings based on the Nordic Ventilation Forum presentations and discussions. These were based on experiences from real building cases as identified challenges for DCV system usage [4,5,8]. The variety of these problems justifies the need for improving the performance of DCV systems, even if it does not report exactly the extent of these problems in buildings with DCV systems. Most of these problems are related to the different stages of the building process. There can be technical problems involved, but these have not been corrected properly in the building processes. DCV systems are complex, they need more knowledge and include more sensors and actuators than ventilation systems with constant airflow rate. There is not enough knowledge and skills of maintenance staff to manage these systems. Problems appeared at all stages of the building process: Design, installation, commissioning and operation. The findings are listed in the following according to the building process stages:
Design
Installation
Commissioning
Operation
4. What is needed and should be improved for reliable and well-performing DCV systems
Based on the Nordic Ventilation Forum presentations and discussions, the following improvements are suggested.
Needed improvements related to the design, installation and operation of DCV systems:
Needed improvements related to the technology of DCV systems:
One important issue raised during the Nordic Ventilation Forum was the experience from Sweden with mandatory regular ventilation system inspections and significantly fewer reported problems with DCV systems. This would be one important action needed to consider also in other Nordic countries with a specific focus on the inspection of DCV system operation. Another important aspect presented was from Norway, where a thorough commissioning test procedure was developed for the DCV system and tested in practice successfully. The main difference to the commissioning of the ventilation system with constant airflow rates, including a test of only one operation mode, was to test all operating modes of the DCV system and need to consider different control zones in parallel.
These reported improvements are suggested to be implemented with more detailed specifications for achieving reliability and performance of DCV systems in buildings with good indoor climate conditions and energy savings during the building life cycle.
1. Need for DCV systems in buildings and potential advantages
Demand for significant energy saving and the same time for increasing and optimizing ventilation airflow rates in buildings creates the evident need for DCV systems in buildings [1]. Currently, close to 40% of total energy consumption in Europe is used for buildings and the ventilation of buildings generates a significant part of that [2,4]. According to scientific studies, the energy consumption of heating, ventilation and air-conditioning (HVAC) systems can be reduced by 20-50% [1-3] compared to the HVAC systems with fixed ventilation airflow rates. This depends strongly for instance on the room usage/occupancy profiles in the buildings, which vary significantly in different spaces . Typical occupancy ratio is for instance 30%-40% in many office buildings [6].
2. Current systems and their principles
The main principle of the DCV system is to maintain good indoor climate conditions for occupants in buildings by dynamic control of the ventilation supply and exhaust airflow rates depending on occupancy, pollution load and thermal load. These systems are called also variable air volume (VAV) systems. Typically, also water-based heating and cooling room systems (for instance chilled beams, fan-coils or radiant panels) are linked to the DCV system. With these systems, the airflow rate is based on occupancy and excess cooling/heating demand is covered with water-based system. HVAC systems based on fixed ventilation rates (CAV systems) need to be designed for most demanding situations with maximum airflow rate and are not able to reduce ventilation fan energy consumption and cooling/ heating energy consumption for ventilation without DCV operation. Also even with efficient heat recovery from ventilation exhaust excess constant ventilation airflow during the heating season causes heat loss and increases both fan and heating energy consumption. Depending on the building usage and type, and based on life cycle cost (LCC) analysis, DCV or CAV system should always be considered.
Typically in a DCV system, ventilation airflow rates are controlled based on schedule, occupancy detector, temperature sensor and indoor air quality sensors. The most commonly used indoor air quality sensor in DCV systems are carbon dioxide (CO2) sensors. Control of supply airflow rates in rooms or in HVAC terminal units can be modulated between the room minimum and maximum airflow rates, based on step control with specified airflow steps or on/off control with boost/normal airflow rates. Control of airflow rates in DCV systems can be designed for individual rooms, zones or specific modules in open areas like landscape offices.
DCV systems in apartment buildings can control ventilation airflow rates at the apartment level. This is typically done by switching to low airflow rate when the apartment is empty or by having boost airflow mode when the kitchen hood is used. Additional kitchen hood exhaust air should be properly balanced by controlling the supply airflow rate in the DCV system. In small apartments, this can be done by borrowing airflow rates from other apartments, but the balancing of airflow rates should be done carefully when several apartments are boosting at the same time.
For maintaining indoor climate condition, the DCV system should control ventilation airflow rates at room level. Commonly the target for controlling ventilation airflow rates at room level is also to maintain the balance between supply and exhaust. Three most typical concepts for exhaust air flows are:
- Supply and exhaust airflow rates are balanced at room level.
- Constant room exhaust airflow, and when supply airflow is boosted, space is over-pressurized, and boosted airflow is centrally exhausted.
- Only supply air terminal units are installed in rooms and exhaust air is transferred to centralized exhaust.
DCV systems can be defined as pressure-independent and pressure-dependent system categories related to the control of ventilation airflow rates.
- Pressure-independent systems require variable air volume (VAV) control dampers/units at all locations of ventilation ductwork where the ventilation airflow rate is measured and controlled to the desired level. This requires VAV dampers before all room supply and exhaust air terminals for the most flexible DCV system solution. Pressure-independent systems can be used for both supply and exhaust ductwork. It compensates for the variations of the static pressure in the ductwork.
- Pressure-dependent system uses constant static pressure (CSP) control dampers to adjust ventilation air ductwork zone to the desired level. CSP dampers include typically the measurement of airflow rate and static pressure from the specific location of the ductwork zone. The ductwork zone is needed to be designed for maintaining a constant static pressure level by utilizing the static regain principle after room branches. This requires a low initial air velocity of 3-4 m/s with maximum airflow rate and the same main zone duct size in the whole zone ductwork. Pressure-dependent systems can be used to supply air ductwork. Additional VAV dampers with airflow measurements are not needed before all room supply air terminals. Active supply air terminals can be used for adjusting ventilation airflow rates in the room spaces. They have a constant throw pattern with all airflow rates, which reduces the risk of draught.
Air handling units (AHU) in buildings with DCV systems are using pressure control of supply and exhaust fans to desired pressure level in the ductwork. Variable frequency drives (VFD) are used to control fan speed for avoiding excess pressure losses, and to balance supply and exhaust airflow rates. There can be a fan speed optimization function where the opening of the dampers can be maintained as open as possible for the reduction of fan energy consumption.
3. Problems in the performance of current DCV systems
This chapter presents identified problems and challenges with DCV systems in buildings based on the Nordic Ventilation Forum presentations and discussions. These were based on experiences from real building cases as identified challenges for DCV system usage [4,5,8]. The variety of these problems justifies the need for improving the performance of DCV systems, even if it does not report exactly the extent of these problems in buildings with DCV systems. Most of these problems are related to the different stages of the building process. There can be technical problems involved, but these have not been corrected properly in the building processes. DCV systems are complex, they need more knowledge and include more sensors and actuators than ventilation systems with constant airflow rate. There is not enough knowledge and skills of maintenance staff to manage these systems. Problems appeared at all stages of the building process: Design, installation, commissioning and operation. The findings are listed in the following according to the building process stages:
Design
- Too narrow and asymmetrical ductwork for proper DCV system operation
- Ventilation airflow rate range from minimum to maximum is typically very large (1:8) in commercial and public buildings causing measurement and control challenges for DCV system design and operation
- Supply air terminal design not properly done for all VAV conditions and causes draught
- Level of DCV system documentation was not sufficient
- Automation documents were general standard schemes without building specific control strategies and setpoint data
- The medium ventilation airflow rate in the room was not specified in the design documentation
- Control sequence of the DCV system with water-based cooling was not documented (air or water cooling first)
- Too small ventilation airflow rate in room due to undersized AHU or duct system caused by incorrect pressure loss calculations
- Too small ventilation airflow rate in room due to undersized VAV units or air terminals
- Noise problem due to system without zone dampers
- Unbalance (pressure difference between rooms) due to supply and exhaust covering different zones, with no air-transfer
- Unstable operation due to too low flow rate over VAV unit (VAV unit cannot measure flow rate)
- Unstable operation due to unsteady pressure at the sensor caused by turbulence in pressure-controlled systems
Installation
- Actuators and control sensors were installed in the wrong places
- VAV dampers were installed in difficult locations regarding the maintenance
- VAV damper reports wrong air flow rate due to incorrect installation (wrong direction, too close to duct bend/t-branch, not proper safety distances used or found in drawings)
- Some electrical wires were not connected
- Loose, compressed or wrongly installed pressure tube in pressure measurement
- Too small or too high ventilation airflow rate in room due to wrong location of room CO2 sensor [7]
- Noise problem in pressure-controlled systems due to bad location of pressure tap
- Noise problem due to VAV-unit located too close to duct t-branch
- Polarity error in VAV system (24V AC field bus connected with VAV units with different polarities)
Commissioning
- The commissioning for the DCV system was not properly done
- Ventilation airflow rates did not match the design values
- In the control of minimum-medium-maximum ventilation airflow rates in the room, the medium-maximum airflow rates were in the wrong order
- The ratio of supply and exhaust ventilation airflow rate was not correct
- The setpoint for the room air temperature was too low, leading to continuous unnecessary cooling with the maximum airflow rate
- Wrong k-factor for VAV flow-cross in VAV unit program parameters
- Too small ventilation airflow rate in room due to too low pressure setpoint in the duct
- Too small or too high ventilation airflow rate in room due to room CO2 sensor (signal error, wrong calibration or response time)
- Noise problem in pressure-controlled systems due to too high pressure setpoint
- Addressing error in VAV system controllers (VAV unit controlled by the sensor is located in a different room than the supply air terminal unit)
- Too small or too high ventilation airflow rate in room due to incorrectly programmed Vmin and Vmax at VAV unit
- Connections to building management system not correctly and clearly done
Operation
- The DCV systems were not working as designed. Research study of 8 buildings with DCV systems revealed that the DCV system in only one building worked as designed [4]
- Building management personnel did not know how to use automation systems and did not understand the overall operation
- HVAC and automation design documentation was not available in many cases
- Actuators had been stuck
- VAV unit reports 0 m3/h due to dirty VAV flow-cross or broken pressure transducer
- VAV-pressure transducer blocked with dust
- Exhaust airflow measurement devices were dirty and gave the wrong airflow rate
- VAV unit reports 0 m3/h due to oversized VAV units, which cannot measure low flow rates
- Unbalance (pressure difference between rooms) due to soiled VAV airflow in exhaust duct
- Unstable operation due to soiled VAV flow-cross
- Too small ventilation airflow rate in room due to VAV unit stopped due to bus-error
- Noise problem in pressure-controlled systems due to pressure sensor damaged due to pressure or electrical spike
- Unbalance (pressure difference between rooms) due to zero pressure error in pressure transducer due to pressure/electrical spike
- Mechanical fault with VAV damper blade operation or loose orifice plate in incorrect position
4. What is needed and should be improved for reliable and well-performing DCV systems
Based on the Nordic Ventilation Forum presentations and discussions, the following improvements are suggested.
Needed improvements related to the design, installation and operation of DCV systems:
- Existing processes for design, commission and maintenance do not guarantee high performance, these should be improved
- Designers, contractors, and maintenance staff training should be improved
- Properly done design with a focus on requirements
- Updated and property-specific documents
- Commissioning tests before the building is occupied
- Improved commission and maintenance processes/contracts
- Well-designed utilization of BMS for continuous monitoring
- Appreciation and improved motivation of maintenance staff
- Regular inspections and retro-commissioning
Needed improvements related to the technology of DCV systems:
- Large and reliable measurement range of airflow rates in VAV measurement units
- Smart and robust control system is needed
- Utilization of IoT to monitor indoor climate conditions and systems operation
One important issue raised during the Nordic Ventilation Forum was the experience from Sweden with mandatory regular ventilation system inspections and significantly fewer reported problems with DCV systems. This would be one important action needed to consider also in other Nordic countries with a specific focus on the inspection of DCV system operation. Another important aspect presented was from Norway, where a thorough commissioning test procedure was developed for the DCV system and tested in practice successfully. The main difference to the commissioning of the ventilation system with constant airflow rates, including a test of only one operation mode, was to test all operating modes of the DCV system and need to consider different control zones in parallel.
These reported improvements are suggested to be implemented with more detailed specifications for achieving reliability and performance of DCV systems in buildings with good indoor climate conditions and energy savings during the building life cycle.
References
- Li B and Cai W. A novel CO2-based demand-controlled ventilation strategy to limit the spread of COVID-19 in the indoor environment. Build. Environ. 219 (2022) 109232. https://doi.org/10.1016/j.buildenv.2022.109232
- Merema B, Delwati M, Sourbron M and Breesch H. Demand controlled ventilation (DCV) in school and office buildings: Lessons learnt from case studies. Energy Build. 172 (2018) 349–360. https://doi.org/10.1016/j.enbuild.2018.04.065
- Mysen M, Berntsen S, Nafstad P, Schild PG. Occupancy density and benefits of demand-controlled ventilation in Norwegian primary schools. Energy and Buildings 37 (12) (2005), 1234-1240. https://doi.org/10.1016/j.enbuild.2005.01.003
- Zhao W, Kilpeläinen S, Bask W, Lestinen S and Kosonen R. 2022. Operational Challenges of Modern Demand-Control Ventilation Systems: A Field Study. Buildings 12 (2022), no. 3: 378. https://doi.org/10.3390/buildings12030378
- Mysen M, Schild PG, Cablé A. Demand-controlled ventilation - requirements and commissioning. Guidebook on Well-Functioning and Energy-Optimal DCV. 2014.
- Halvarsson J. Occupancy Pattern in Office Buildings: Consequences for HVAC system design and operation. Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Energy and Process Engineering. Doctoral Theses at NTNU, 2012:37. http://hdl.handle.net/11250/234598
- Mylonas A, Kazanci OB, Andersen RK, Olesen BW. Capabilities and limitations of wireless CO2, temperature and relative humidity sensors. Build. Environ. 154 (2019), 362-374. https://doi.org/10.1016/j.buildenv.2019.03.012
- Alanko A. Tarpeenmukaisen ilmanvaihdon käytännön haasteita kenttätyön näkökulmasta. Sisäilmastoseminaari 2020. Sisäilmayhdistys raportti 38, 207-212. https://www.sisailmayhdistys.fi/content/download/4691/30364/
*) The members of the Nordic Ventilation Group:
Alireza Afshari, Professor, Aalborg University
Amar Aganovic, Associate Professor, UiT The Arctic University of Norway
Gyangyu Cao, Professor, NTNU – Norwegian University of Science and Technology
Lars Ekberg, Associate Professor, Chalmers University of Technology
Per Kvols Heiselberg, Professor, Aalborg University
Dennis Johansson, Associate Professor HVAC, Lund University
Risto Kosonen, Professor, Aalto University
Jarek Kurnitski, Professor, TalTech – Tallinn University of Technology
Ivo Martinac, Professor, KTH Royal Institute of Technology
Hans Martin Mathisen, Professor, NTNU – Norwegian University of Science and Technology
Arsen Melikov, Professor, DTU – Technical University of Denmark
Panu Mustakallio, Professor of Practice, Aalto University
Peter V. Nielsen, Professor emeritus, Aalborg University
Bjarne W. Olesen, Professor, DTU – Technical University of Denmark
Thomas Olofsson, Professor, Umeå University
Pertti Pasanen, Director, University of Eastern Finland
Svein Ruud, Tekn. Lic., Senior expert, RISE Research Institutes of Sweden
Sasan Sadrizadeh, Professor, KTH Royal Institute of Technology and Mälardalens University
Peter Schild, Professor, OsloMet – Oslo Metropolitan University
Olli Seppänen, Professor emeritus, Aalto University
Martin Thalfeldt, Professor, TalTech – Tallinn University of Technology
Pawel Wargocki, Associate Professor, DTU – Technical University of Denmark
Manager of the group
Siru Lönnqvist, Secretary general, VVS Föreningen i Finland and SCANVAC
Alireza Afshari, Professor, Aalborg University
Amar Aganovic, Associate Professor, UiT The Arctic University of Norway
Gyangyu Cao, Professor, NTNU – Norwegian University of Science and Technology
Lars Ekberg, Associate Professor, Chalmers University of Technology
Per Kvols Heiselberg, Professor, Aalborg University
Dennis Johansson, Associate Professor HVAC, Lund University
Risto Kosonen, Professor, Aalto University
Jarek Kurnitski, Professor, TalTech – Tallinn University of Technology
Ivo Martinac, Professor, KTH Royal Institute of Technology
Hans Martin Mathisen, Professor, NTNU – Norwegian University of Science and Technology
Arsen Melikov, Professor, DTU – Technical University of Denmark
Panu Mustakallio, Professor of Practice, Aalto University
Peter V. Nielsen, Professor emeritus, Aalborg University
Bjarne W. Olesen, Professor, DTU – Technical University of Denmark
Thomas Olofsson, Professor, Umeå University
Pertti Pasanen, Director, University of Eastern Finland
Svein Ruud, Tekn. Lic., Senior expert, RISE Research Institutes of Sweden
Sasan Sadrizadeh, Professor, KTH Royal Institute of Technology and Mälardalens University
Peter Schild, Professor, OsloMet – Oslo Metropolitan University
Olli Seppänen, Professor emeritus, Aalto University
Martin Thalfeldt, Professor, TalTech – Tallinn University of Technology
Pawel Wargocki, Associate Professor, DTU – Technical University of Denmark
Manager of the group
Siru Lönnqvist, Secretary general, VVS Föreningen i Finland and SCANVAC