What new Zero Emission Buildings can bring for HVAC?

Optimal indoor environmental quality (IEQ) and measuring and control devices for monitoring and regulation of indoor air quality (IAQ) may play an important role in HVAC development. Because of complex technical nature the essence of these requirements may be difficult to understand for national regulators struggling with many other new items to be implemented. Therefore, contribution by national experts and industry can be crucial to ensure adequate implementation quality that is evident precondition for improving and optimising IEQ in future buildings.

Keywords: ZEB, IEQ, EPBD, HVAC, smart control, CO₂ sensor, DCV

EPBD implementation to national regulation should currently run in high speed because Member States shall bring into force the laws and regulations by 29 May 2026. Zero-Emission Buildings (ZEB) requirements must apply in new buildings owned by public bodies from 1 January 2028 and in all new buildings from 1 January 2030.

Capacity to react to external signals

ZEB requirements set in EPBD Article 11 have mostly indirect impact on HVAC, because energy effective systems help to achieve primary energy and operational CO₂ thresholds. However, there is a specific requirement that ZEB shall offer the capacity to react to external signals and adapt its energy use, generation or storage. Together with fossil fuel ban, this is an important opportunity for heating and cooling generation with heat pumps where the most natural external signal is the electricity price. Therefore, price-based control of heat pumps, but maybe also ventilation systems can be important to satisfy this requirement. Another option would be the battery storage that will fulfil this requirement. Depending on national implementation, HVAC may play a big role, because HVAC is always present in buildings while electric battery investments may not be so easily justified if on-site electricity generation will be self-used in the building, i.e. if there will no surplus electricity that is typical situation in non-residential buildings.

Monitoring and regulation of IAQ

Another requirement stressing the importance of smart control of HVAC is in Article 13 for Technical Building Systems. It says that “Member States shall require non-residential zero-emission buildings to be equipped with measuring and control devices for the monitoring and regulation of indoor air quality. In existing non-residential buildings, the installation of such devices shall be required, where technically and economically feasible, when a building undergoes a major renovation. Member States may require the installation of such devices in residential buildings.” While must in non-residential buildings it is important to understand which ventilation and air conditioning systems are needed to satisfy this requirement. EPBD does not specify at which level, i.e. single room, larger zone or ventilation system the regulation and monitoring should be applied. Technically it would be the easiest to add one CO₂ sensor to air handling unit extract air and adjust the fan speed of supply and extract fans accordingly. It is well known that this control can compromise IAQ in occupied zones, especially if these are located at the longer end of the ductwork. Therefore, it would be important to avoid such interpretation possibility in national regulation. This can be done by experts by referring to optimal IEQ requirement in Article 5 that states “those requirements shall take account of optimal indoor environmental quality, in order to avoid possible negative effects such as inadequate ventilation…”. Thus, when looking Article 13 and Article 5 together, it should be clear that central control would not be an option for IAQ regulation and monitoring. Instead, in technical language it should be said that ZEB shall be equipped with measuring and control devices as a part of a demand-controlled ventilation (DCV) system. Here we understand DCV as a general expression for any variable air volume (VAV) ventilation or more simple air flow control satisfying occupant needs.

At which level the control needs to implemented remains a question to be decided at national level. It is evident that air flow control is not cost effective in small rooms occupied by 1-2 person or for instance having lower nominal supply air flow rate than 50 L/s. In continuously occupied rooms with higher number of persons the air flow control would be natural and can be implemented with many DCV systems which may be classified as pressure-independent and pressure-dependent systems depending on the control logic. Thus, it may be expected that implementation of Article 13 will make DCV systems standard solution in non-residential buildings. However, it is well known that these dedicated systems have had many problems in practice. In many cases ductwork design with pressure loss calculations has not been complete, and there have been issues by contractors in installation and commissioning. Also, the maintenance need has been much more than expected. Therefore, it would be an issue for HVAC industry to develop more robust and reliable DCV systems that are easy to design, install, commission and operate. New thinking is needed, because current systems with high-end reputation are to be changed to mainstream standard solutions. Damper-optimized pressure dependent systems with stepless air flow control are perfect, but a lot can be done already with simple two airflow steps especially in the case of low-pressure design. Some good ideas can be found from recent REHVA publication[1], Figure 1. Low pressure design for the final pressure drop is described in REHVA GB No 17.

Figure 1. In pressure independent DCV system with the constant pressure control (AHU1), the fan speed is controlled to keep constant static pressure in the main ventilation duct, at the location of the pressure sensor. Dampers in each zone branch or diffuser adjust the air flow rates based on the room sensors CO₂ (and temperature) readings. Dampers may have continuous or stepwise control that should keep supply and extract air flows in balance with reasonable accuracy. AHU2 (CAV) with time control but not DCV serves toilets and corridors.

In DCV systems such as in Figure 1, supply air diffusers should manage a wide range of airflow rates so that air distribution patterns will not change too much, and draught will not be generated. For instance, active diffusers which have motorized parts for air throw length control can be used. Another option is to design the DCV system with on-off dampers so that all diffusers will have constant airflow rates. In such system the control will be less dedicated as based on switching on or off ductwork branches, Figure 2.

Figure 2. Pressure independent DCV system implemented with on-off dampers in larger rooms. Smaller rooms with one diffuser have constant airflow rates (CAV). Low-pressure design is recommended for main ducts[2] that helps to keep a constant static pressure (AHU 1). The control itself is as robust as possible, room branch dampers are just opened or closed based on the local room sensors CO₂ and temperature readings. AHU2 (CAV) with time control but not DCV serves toilets and corridors.

Operation for optimal IEQ

Operation for optimal IEQ is currently not addressed in standards, however it is included in ongoing revision of EN 16798-1:2019. Therefore, it might not be clear how to select relevant setpoints for CO₂ in different spaces. Here the issue is that common airflow rate sizing depends on number of occupants (7 L/s per person in Category II) and floor area (0.7 L/s per floor area in the case of low-polluting building materials). Therefore, a CO₂ value corresponding to ventilation rate of EN 16798-1:2019 perceived air quality method depends on occupant density. To calculate CO₂ setpoint corresponding for instance to Category II, first the total ventilation rate per person shall be calculated:

where

qsp                  total ventilation rate per person, L/(s person)

qs                   design ventilation rate supplied by air distribution system, L/s

n                    number of the persons in the room corresponding to typical occupancy, -

Note that typical occupancy may be 50-60% of the design value and may be estimated from energy calculation usage schedules. CO₂ concentration setpoint above the outdoor CO₂ concentration can be calculated from metabolic CO₂ generation and CO₂ volume balance:

where

C          CO₂ concentration setpoint value above the outdoor CO₂ concentration, ppm

qCO2    CO₂ generation rate, L/(h person)

     3600 and 10⁶ are unit conversions from hour to second and litre to ppm

Figure 3 illustrates how Category II CO₂ setpoint calculated with these equations depends on occupancy in typical non-residential spaces with 1.2 met activity level.

Figure 3. CO₂ setpoint depending on occupancy in typical non-residential spaces with 1.2 met activity level and 420 ppm outdoor air concentration.

It is also recommended to take the sensor accuracy into account. Thus, about 30-50 ppm should be reduced from values calculated with Equations (1) and (2). For instance, in offices which are designed for 10 m² per person floor area, CO₂ setpoint will change from 820 ppm to 685 ppm if 50% occupancy will be considered. When considering the sensor accuracy, the final setpoint should be about 640 ppm. In classrooms designed for 2 m² per person floor area, 50% occupancy setpoint is 990 ppm, with sensor accuracy consideration 950 ppm.

National implementation

Examples provided in previous figures illustrate technical challenge of IAQ issues. Without HVAC background these topics are not easy to understand for regulators. Therefore, HVAC experts and industry are needed to be in translator role when preparing national regulation. Otherwise, there is a risk that trajectories, minimum energy performance standards, ZEB thresholds and many other EPBD issues capture the most of effort and IEQ issues will not get necessary attention. However, high quality implementation would be a critical precondition to improve and optimise IEQ in future buildings.