New concepts, Standards and guidelines for variable flow hydronic systems

1. Variable flow hydronic systems: main features for emitters

According to European Commission [1], buildings in the European Union (EU) account for 40% of total energy consumption and 36% of greenhouse gas emissions. In this scenario, air conditioning accounts for around half of the energy used in buildings [2], thus playing a key role in the decarbonisation process. The building sector decarbonisation, a key driver of energy transition in Europe, involves reducing greenhouse gas emissions by replacing the use of fossil fuels such as oil, coal and natural gas with electricity generated from renewable sources [3]. At the same time, the new F-Gas Regulation (EU) 2024/573 [4], amending Directive (EU) 2019/1937 and repealing Regulation (EU) No 517/2014 [5], entered into force on 11 March 2024 to contribute to the EU climate change targets by reducing emissions of greenhouse gases (such as fluorinated gases, or F-gases, used in a wide range of equipment, including refrigeration and air conditioning). The new regulation aims to further reduce emissions in Europe and to help limit global warming by transitioning to refrigerants with a lower global warming potential (GWP). In this context, variable flow hydronic systems are the core technology for decarbonising the building sector. The hydronic systems use water as heat transfer fluid. In addition to the advantage of being able to heat and cool the indoor environment with a single system, the use of water enables even uniform distribution and precise temperature control, contributing to improve indoor comfort. Specifically, control valves and balancing systems play a fundamental role in controlling the temperature of the heat transfer fluid and correctly distributing flow rates to emitters. Variable flow hydronic systems offer several options in terms of emitters:

• fan coils: they allow rooms to be quickly heated and cooled using a single emitter. In addition, the presence of a control panel allows users to adjust the indoor temperature according to their needs.
• hydronic cassettes: they are “cassette” type fan coils, properly designed to be integrated installation in suspended ceilings.
• surface embedded radiant systems: they are characterised by plastic coils in which water circulates for heating (around 30-35°C) and cooling (around 15-16°C). These systems can be installed under the floor, in the walls or in the ceiling and are suitable for new buildings, but are also widely used in renovations of existing buildings. Furthermore, one of the main advantages of hydronic radiant systems is that they can be optimally combined with high-efficiency generation systems (such as heat pumps) or renewable energy sources, reducing the environmental impact and operational costs associated with energy consumption. Finally, using mainly radiant heat exchange, radiant systems provide uniform indoor heating and cooling without the need for air movement.
• radiant ceilings: they are an effective solution for space heating and cooling, capable of distributing heat uniformly. In particular, radiant ceilings are ideal solutions for both new buildings and renovations, as they can be installed directly on the ceiling or integrated into the suspended ceiling. Moreover, their versatility makes them suitable for different applications (residential, commercial, health care). There are different types of radiant ceilings: metal radiant ceilings, which are characterised by their ability to quickly reach the optimal temperature in the room, and plasterboard radiant ceilings, which have the advantage of being invisible in the room.

2. National, international and European reference and design standards for hydronic systems

The new Standard EN 14336:2023 [6] provides basic support for the design, installation, testing and commissioning of hydronic heating, cooling and hot water production systems. In particular, Annex A of the new European standard introduces commissioning as an integral part of the activities. The annex offers a flexible scheme designed to facilitate the documentation phase and provide the operator with the necessary confidence to adopt the optimum commissioning procedure for the system.

The main Standards for hydronic radiant systems are listed and described below:

• EN 1264:2021 [7]. The European Standard was implemented by all Member States participating in the European Committee for Standardisation (CEN) and was published in Italy as UNI EN 1264 (“Sistemi radianti alimentati ad acqua per il riscaldamento e il raffrescamento integrati nelle strutture”), in Germany as DIN EN 1264 (“Raumflächenintegrierte Heiz- und Kühlsysteme mit Wasserdurchströmung”), in France as NF EN 1264 (“Systèmes de surfaces chauffantes et rafraîchissantes hydrauliques intégrées”), etc. In Italy, the new UNI EN 1264:2021, which went into force on 1 July 2021 as a revision of the previous UNI EN 1264:2011, provides guidelines for surface embedded (wall, floor and ceiling) heating and cooling systems installed in buildings, residential and non-residential. The aim of the new edition is to update the text to include all new technological solutions, to clarify the scope of application and to provide guidelines for the design and installation of the systems. The standard is divided into five parts: (i) “Definitions and symbols”; (ii) “Floor heating: methods for the determination of the thermal output”; (iii) “Dimensioning”; (iv) “Installation”; (v) “Determination of the thermal output for wall and ceiling heating and for floor, wall and ceiling cooling”.
• EN 14037:2016 [8]. The European Standard, developed by CEN, was subsequently implemented by all Member States and it was published in Italy as UNI EN 14037 (“Pannelli radianti sospesi a soffitto per riscaldamento e raffrescamento alimentati con acqua a temperatura minore di 120°C”), in Germany as DIN EN 10437 (“An der Decke frei abgehängte Heiz- und Kühlflächen für Wasser mit einer Temperatur unter 120°C”), in France as NF EN 14037 (“Panneaux rayonnants de chauffage et de rafraîchissement alimentés avec une eau à une température inférieure à 120°C”), etc. Specifically, in Italy, the UNI EN 14037:2016, which came into force on 10 November 2016 as a revision of the previous UNI EN 14037:2005, defines the technical features and requirements for the installation of pre-fabricated ceiling panels supplied with water at a temperature below 120°C. The standard also specifies additional information to be provided to installers for proper installation and usage. It consists of five parts: (i) “Pre-fabricated ceiling mounted radiant panels for space heating - Technical specifications and requirements”; (ii) “Pre-fabricated ceiling mounted radiant panels for space heating - Test method for thermal output”; (iii) “Pre-fabricated ceiling mounted radiant panels for space heating - Rating method and evaluation of radiant thermal output”; (iv) “Pre-fabricated ceiling mounted radiant panels for space heating - Test method for cooling capacity;”; (v) “Open or closed heated ceiling surfaces - Test method for thermal output”.
• EN 14240:2005 [9]. The European Standard drawn up by CEN was implemented in Italy as UNI EN 14240:2005 ("Ventilazione degli edifici - Soffitti freddi - Prove e valutazione (rating)"). The Standard, which came into force on 1 March 2005 as a revision of the previous UNI EN 14240:2004, specifies test conditions and methods for the determination of the cooling capacity of chilled ceilings and other extended chilled surfaces.

Finally, the most important international references that are relevant to design practice are listed and described below:

• ASHRAE Handbook 2023: HVAC Applications [10]. In the field of radiant systems, Chapter 55 (Radiant Heating and Cooling) of the ASHRAE Handbook is essential. In particular, this chapter delves into the principles and applications of radiant heating and cooling systems. A further fundamental aspect for such systems concerns the initial testing, regulation and balancing requirements for the commissioning of the radiant system. In this area, Chapter 39 (“Testing, adjusting and balancing”) deals with the procedures required to assess and optimise heating, ventilation and air-conditioning (HVAC) system performance and to ensure correct adjustment to maintain comfort and energy efficiency in buildings.

• ASHRAE Handbook 2020: HVAC Systems and equipment [11]. Chapter 5 (“Indoor Terminal Systems”) of the ASHRAE Handbook focuses on indoor terminal systems, providing details on the different types, design considerations, installation, maintenance, control and integration with other HVAC systems. As previously mentioned, such systems are fundamental to the regulation of temperature and the improvement of occupant comfort in the environment. Chapter 6 (“Radiant heating and cooling”) deals with radiant heating and cooling systems. In particular, this chapter highlights the benefits in terms of comfort and energy efficiency by describing the operating principles, system components, and design and installation considerations.
• ISO 18566:2017 [12]. The international standard is also a valid reference in Italy for the use of radiant ceilings for space heating and cooling.

3. UNI EN ISO 52120-1:2022: hydronic systems-BACS integration

The UNI EN ISO 52120-1:2022 [13] on buildings energy performance and contribution of building automation, controls and management came into force on 4 November 2022, replacing the previous UNI EN 15232-1:2017 [14]. Among the innovative aspects, the most important one is the introduction of the dynamic balancing function of hydronic systems for heating (function 1.4a) and cooling (function 3.4a). Therefore, dynamic balancing becomes an integral part of Building Automation and Controls Systems (BACS) and a key tool in improving the overall energy efficiency. Dynamic balancing systems are particularly important in large buildings, as they allow the precise control of water flow rates and differential pressures at different sections of the system, ensuring the correct operation of emitters and achieving huge energy savings. In this context, dynamic balancing technologies are an innovative and indispensable element that can be applied to systems in new buildings as well as in existing buildings that are undergoing energy efficiency upgrades.

Moreover, the last version of the Energy Performance of Buildings Directive (EPBD) 2024/1275 [15] (Green Homes Directive) confirms the possibility of using a single, easy-to-read indicator common to all EU countries to assess the readiness of buildings for smart technology (Article 15). This index, known as “Smart Readiness Indicator” (SRI), aims to classify the "smartness" of a building not only on the basis of the BACS energy efficiency classes introduced by UNI EN 15232-1:2017 (Class A: "high energy performance", Class B: "advanced", Class C: "standard", Class D: "non-energy efficient"), but also through a scale from 0 to 100%. It is therefore clear that both the SRI indicator and BACS play a key role in the decarbonisation process towards the efficient and conscious use of energy.

4. References

• European Commission. European Green Deal: Commission proposes to boost renovation and decarbonisation of buildings. 15 December 2021, Brussels.
• Eurovent Certified Performance. HVAC e decarbonizzazione degli edifici. 9 agosto 2023
• Widuto, A. Energy Transition in the EU, 2023.
•  Regulation (Eu) No 2024/573 of The European Parliament and of the Council of 7 February 2024 on fluorinated greenhouse gases, amending Directive (EU) 2019/1937 and repealing Regulation (EU) No 517/2014. Official Journal of the European Union. 20 February 2024.
• Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases and repealing Regulation (EC) No 842/2006. Official Journal of the European Union. L 150/195. 25 May 2014.
• EN 14336:2023. “Installazione e commissioning degli impianti idronici di riscaldamento, raffrescamento e preparazione dell’acqua calda sanitaria”. 
• UNI EN 1264:2021. “Sistemi radianti alimentati ad acqua per il riscaldamento e il raffrescamento integrati nelle strutture”.
•  UNI EN 14037:2016. “Pannelli radianti sospesi a soffitto per riscaldamento e raffrescamento alimentati con acqua a temperatura minore di 120°C”.
• UNI EN 14240:2005. “Ventilazione degli edifici – Soffitti freddi – Prove e valutazione (rating)”.
• American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). ASHRAE Handbook 2023: HVAC Applications, America, 2023.
• American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).  ASHRAE Handbook 2020: HVAC Systems and Equipment, America, 2020.
•  ISO 18566:2017. “Building environment design - Design, test methods and control of hydronic radiant heating and cooling panel systems”.
• UNI EN ISO 52120-1:2022. “Prestazione energetica degli edifici - Contributo dell'automazione, del controllo e della gestione tecnica degli edifici - Parte 1: Quadro generale e procedure”. 
• UNI EN 15232-1:2017. “Prestazione energetica degli edifici - Parte 1: Impatto dell'automazione, del controllo e della gestione tecnica degli edifici - Moduli M10-4,5,6,7,8,9,10”.
•  Parlamento Europeo. Direttiva (UE) 2024/1275 del Parlamento Europeo e del Consiglio del 24 aprile 2024 sulla prestazione energetica nell’edilizia. Gazzetta Ufficiale dell’Unione Europea, Serie L 08.05.2024.