Why are canals at the peak of emissions, and what can be done?

To meet national and international climate goals, these emissions must be reduced rapidly and significantly, and ducts should be one of the focus areas for the HVAC industry.

The greenhouse gas calculations carried out in the Green HVAC research project show that ventilation ducts are typically one of the three largest individual contributions from HVAC, along with sprinkler pipes and ventilation units. Fortunately, there are many opportunities to reduce emissions, for example through reuse, system selection, design and layout, or the right choice of materials.

We know that it is urgent to cut emissions, but progress is slow. The industry should therefore pursue as many avenues as possible and work to scale the right solutions. Optimizing ventilation ducts is a concrete avenue with great potential.

Decisive decisions are made in the early stages

The most important function of the ventilation system is to provide a good indoor climate for the users of buildings, and thus facilitate good health and a good working environment. The amount of technical installations and ducts depends strongly on the intended use of the building and consequently how much air is needed.

The air volume requirement depends on many factors such as occupancy, activity, processes and surface materials. In particular, the use of carpeted floors results in increased air volume requirements according to the Norwegian Labour Inspection Authority's guidance on Indoor climate and air quality in the workplace. Cooling requirements can often also be a governing dimensioning criterion for ducts, as many buildings are cooled with air. The cooling requirement is then influenced by a number of interdisciplinary design decisions made in the early phase, such as the design of the climate shell, the placement of glass, the type of glass and solar shading, the use of the building and internal loads.

On top of the current needs, a reserve capacity is often added, in the form of oversizing, to make the facility flexible for changes. The strong correlation between early-phase choices and the need for HVAC installations implies that HVAC professionals should be involved early in projects as setting the premises, and clarify the effect of the choices made.

Location of units and ductwork is likely to be of great importance

The location of technical rooms can have a major impact on the amount of ductwork. In general, it will be advantageous to have short distances to where the air volume requirement is greatest. The division into shafts and their location also has an impact on the number of linear metres of ductwork. A well-thought-out location of technical rooms, shafts and main ductwork can also reduce the need for interventions and replacements throughout the life of the building.

The location of ventilation units follows two main principles in a somewhat simplified way: centrally with large units, or decentrally with several small ones. In general, location high in the building provides shorter ductwork for intake and exhaust. And location low in the building will tend to have longer ductwork. It is therefore obvious that the choice of solution affects material use, but this seems project-specific and should therefore be assessed in each case.

Industry practices should be reassessed

Optimal dimensioning of ductwork can represent a great potential for reduced material use. Correct sizing should take into account pressure loss, air velocity and reserve capacity. It is important that the actual need for reserve capacity is assessed in each individual project and that it is separated into different parts of the duct network, against expected use and which future changes are realistic. If this is done correctly, sufficient adaptability can be achieved while keeping energy consumption in operation low and material use limited.

Another obvious possibility for optimization is to scale down ducts as needed, and to move away from current industry practice, which is often that large duct dimensions are routed all the way out in a straight line. The main reason why ducts are often not built this way in practice is that it is faster to install long duct runs in the same dimension, and it is simpler logistically. It also has a functional significance for maintaining static pressure. The environmental perspective is an argument for considering whether current practice should be changed.

Balancing emissions from materials and energy use

For ventilation ducts, it is important to simultaneously maintain energy efficiency in the facilities. Larger ducts provide lower pressure drop and thus lower SFP and energy use for fans. Energy use also increases with the length of the duct network, changes in direction, bends and branches. Energy use during the operating period is also included in greenhouse gas calculations for buildings, and accounts for large emissions over the life of the building. Here, one should therefore optimize for both material use and energy use to find the solution with the lowest greenhouse gas emissions overall.

This is a paradigm shift in duct design, but the optimal design in terms of life cycle emissions is found at the intersection of material and energy-related emissions. This may lead to a reassessment of current duct design practices. Perhaps more use of 45-degree bends and guides that run diagonally through the axis network should be considered. This could result in both shorter duct lengths and lower pressure losses, and is, in the authors' opinion, an underestimated measure, even though it may be perceived as somewhat aesthetically unfamiliar and untidy.

Assessment of alternative duct qualities

Choosing alternative duct qualities can be effective measures for reducing climate impact, and there may be good reasons to challenge established practice, which is dominated by "spiro ducts" in galvanized steel, with an undocumented proportion of recycled metal.

In figures 1 and 2, we have compared circular and rectangular duct qualities that are relevant for use in Norway and only those for which a valid EPD exists. The diagram shows greenhouse gas emissions related to the extraction and production of the materials (life cycle phase A1-A3). For galvanized steel ducts, there are EPDs for several products with different greenhouse gas loads, which are illustrated with columns for the lowest (min) and highest (max) emission levels. Manufacturer names have been deliberately omitted, since our focus is on material quality.

Textile ducts are not included in the comparison since they are rarely used as supply ducts in buildings. Since the duct itself leaks air, it also has no pressure class. The product is designed to function as a supply air unit in a room, and is therefore more comparable to supply air valves and plenum chambers, and most often falls under 364 Air distribution equipment. Textile ducts are, however, made of light organic materials and can be interesting for their use from an environmental perspective.

Plastic ducts may be an alternative, but are omitted because no EPDs have been found for larger dimensions. Large plastic ducts are rarely used in buildings and are significantly more expensive than spiro. There are solutions with small dimensions that are used in some housing projects, but these are not included here.

Sandwich ducts are also not included due to the lack of a valid EPD. "Insulation ducts" are in this context a different product, with aluminum foil on the sides instead of steel, as described in a separate section further down. The EPD for circular insulation duct had too little information to scale and is only included for Ø125.

“The only alternative to steel is better steel”

This was the somewhat pointed conclusion of master’s student Eyyubi Paltaci, who wrote his thesis “Comparative study of environmentally friendly solutions and materials for ventilation ducts” for Grønn VVS and Oslo Met in 2023. He interviewed 18 industry players and found, among other things, that fire safety is a major obstacle to the use of alternative materials. In retrospect, several alternative steel grades have come onto the market, with lower greenhouse gas emissions documented in the EPD.

Reuse is a well-known effective measure for emission reduction, which is also reflected in the EPD with emissions close to zero. Increased proportion of recycled material is another measure that is now also documented in the EPD for ducts made of 100 percent recycled material, which results in a 52-75 percent reduction, compared to virgin steel. Some manufacturers also work with products made of steel from factories that do not use fossil fuels in production.

Use of rectangular ducts should be limited

Spiro ducts are advantageous over rectangular ducts for several reasons, including the most efficient use of materials and price. Spiro ducts have the great advantage that standard parts are delivered ready-made from the supplier. Parts for spiro ducts are well designed in terms of flow technology, which results in less pressure drop and risk of noise. Rectangular ducts, on the other hand, are made in workshops, often to special order and are therefore assumed to have greater waste in production. The format and variation in quality further increase the risk of increased pressure drop and noise. Multiconsult therefore generally recommends using circular ducts where possible

Ducts in technical rooms are extra emission-intensive

Studies carried out in collaboration between the R&D projects Grønn VVS and Hybridene indicate that the large rectangular ducts in the intake, exhaust and in the technical room can account for as much as 40-50 percent of the emissions from ducts in an office floor, and should therefore be given extra focus. The finding surprised the researchers, but is related to the fact that the material thickness is greater for large ducts than for smaller ducts in the distribution network. This emphasizes the importance of working systematically with greenhouse gas calculations and identifying the largest drivers and consequently the potential for emission reductions.

Sandwich and insulated ducts are a little-explored alternative

Ducts in shafts and technical rooms are part of the facilities where the use of ducts in insulating material is more widespread, often with products referred to as sandwich elements. These can consist of either foam or mineral wool, with a thin layer of steel on each side. Sandwich ducts with mineral wool have a fire classification and are therefore often preferred. These are typically used in large intake and exhaust chambers. We only found one expired EPD for sandwich ducts, which is consequently not included in the diagram. Simple calculations indicate that sandwich has somewhat higher emissions than galvanized steel ducts, but since the product is fully insulated, it has an additional function.

Circular ducts made of insulating material have a low material weight and significantly lower greenhouse gas emissions than traditional spiral ducts. The ducts are currently little used, but from an environmental perspective they appear to be an interesting alternative. It is important to note that insulating ducts have a number of properties that differ from spiral ducts, and that will entail some changes in design. The various factors that are most central are discussed in the following section.

The alternatives attenuate sound better than metal

Different duct types have different properties when it comes to sound. In general, straight steel ducts have so little attenuation that it is common to ignore sound attenuation in sound calculations. Spiral ducts attenuate less noise than rectangular ducts, which have somewhat better attenuation properties, but transmit more sound to the surroundings. Insulating ducts have better sound attenuation properties than steel ducts and can be supplied with a fabric on the inside. Sound attenuation in such ducts (with fabric) can be so good that the number of sound traps in the duct system can be reduced. Plastic ducts also have better sound attenuation than ducts made of metal.

Ready-made insulated products, or insulation installation?

Most types of ventilation ducts, except for insulation ducts, must be insulated after installation. The type of insulation depends on whether you insulate for reasons of heat loss, condensation or fire protection. Insulation ducts are naturally thermally insulated in principle, and have the advantage of less weight and material use than other types of ventilation ducts.

Spiro and insulation ducts allegedly equally tight

Tightness in duct networks is classified according to European standards. The standards specify tightness classes from A to D, where D indicates the greatest tightness in the duct network. Spiro ducts can maintain tightness class D with good installation, and should maintain a minimum tightness class C. Rectangular ducts are more difficult to keep tight and should, with similarly good installation, achieve tightness class C, with B as a minimum. Manufacturers of insulation ducts state tightness class D, while circular plastic ducts can achieve class C, and rectangular plastic ducts tightness class B or A. As with other ventilation ducts, the production method accounts for a lot for which class is achieved, in addition to the quality of the installation.

Fire safety limits freedom of choice

Today's industry standards and regulations for fire protection place some restrictions on choices. Spiro ducts and rectangular ducts made of metal can currently be used in fire compartments and through building parts with fire compartment restrictions in both the exhaust and shut-in function for ventilation. The pull-out strategy requires fire insulation of exhaust ducts, but this can be waived in fully sprinklered buildings (requires waiver processing by a fire consultant).

Ventilation ducts made of metal, plastic and insulation ducts can achieve the same fire classification A2-s1,d0. The difference in fire safety between different types of ducts is the requirement for additional protection when implemented in fire cell-limiting structures. Current industry practice can be considered conservative. Current regulations contain a scope for possibilities that may be worth exploring. This applies, among other things, to classification requirements for insulation that can be used.

Frequent remodeling is the main cause of short service life

EPDs include service lives, but it is our experience that these are not a reliable source of service lives to any great extent, as there are very large differences for seemingly very similar products. Current sources for service lives are few and of varying quality and coverage. Multiconsult developed a new set of lifetimes for HVAC components, which will be included as an appendix to the upcoming update of the standard for greenhouse gas calculations in buildings, NS 3720. And in this set of lifetimes, a distinction is made between lifetimes for central components (main ducts) and in distribution networks, as well as by building category.

In GVVS, we have seen that replacement can account for 50 percent of emissions related to materials over an analysis period of 50 years. Choosing products with a long lifespan and carrying out repairs and maintenance to extend the lifespan will generally be effective measures to reduce greenhouse gas emissions. Insulated ducts have lower greenhouse gas emissions during production than steel ducts, but it seems reasonable to assume that they can withstand less external stress. However, it is uncertain how much ducts that hang high up, or are hidden behind a ceiling, are actually exposed to mechanical stress. It may appear more likely that steel ducts are more suitable for dismantling and cleaning with a view to reuse.

Today, the main reason for replacing HVAC components is remodeling, changing needs and the use of buildings. In this respect, the choice of duct quality is of less importance, since HVAC components are rarely used until they are worn out or exhausted. For example, as a result of tenant replacements, HVAC components are replaced more frequently in office buildings, while in residential buildings, HVAC installations are typically kept for a longer period of time.

Need incentives to make it work

Incentives are needed to make the less emission-intensive alternatives more widely used. Currently, most of the alternatives are more expensive, which may also be related to low volume. An argument for choosing the lightest duct quality is that installation time can be reduced, and it also makes transport and logistics easier, for example with insulation ducts that are delivered flat-clamped. A driver that could really boost demand for such products would be if there were a requirement to include HVAC in greenhouse gas calculations in TEK, as well as setting limit values, so that projects are forced to make emission cuts. Therefore, Multiconsult, together with FutureBuilt and Reduzer, has developed a requirement level for HVAC for use in the Zero methodology for buildings.

Hope suppliers will compete to cut emissions

Experience from other product categories is that requesting EPDs and greenhouse gas emissions leads to a reduction in emissions in the specific project, without necessarily having any other costs than the request. Furthermore, this drives suppliers to improve their products, if they are de-selected due to higher emissions than their competitors, which in the long term will result in lower emissions for all projects.

Nemitek has previously written about how the work with EPD led to Novema changing steel supplier. Ventistål has also launched a new EPD based on 100 percent recycled steel, as well as an EPD for recycled ducts. Emission values ​​from the EPD for recycled ducts are 99.6-99.8 percent lower than the minimum and maximum values ​​for new steel ducts. This shows the importance of increasing sales of recycled goods. Several suppliers, including both Lindab and Swegon, are also working on fossil-free steel and circularity, among other things. There are products that projects can choose to reduce emissions already now.

In this article, we have compared greenhouse gas emissions for the EPDs that are currently available. The authors hope that this will be an inspiration for others, and that manufacturers will compete to have the lowest emissions on their sold products. This work therefore aims to create a knowledge base, and thus contribute to suppliers updating their standard products.

A joint industry lift is needed!

It is not known whether it is possible to get ventilation ducts down from the emission peak for HVAC. But the record can be reduced significantly with some simple and some more comprehensive measures. On a general basis, the lowest possible greenhouse gas emissions from materials are achieved by reducing the amount of materials as much as possible. Since CO2 accumulates in the atmosphere and has a long decomposition time, emissions that occur today are more important than emissions in the future. Cuts in material use are therefore considered relatively certain emission cuts.

When the premises for a building have been laid, this determines the need for ventilation ducts. And since replacement accounts for such a large proportion of greenhouse gas emissions, the customer must be made aware of strategies for less frequent replacement of users, and consequently HVAC components. Reuse is an effective environmental measure and several suppliers and reuse centers now offer reusable products. There is also great potential in optimal duct sizing, and it is important to ensure good assessments of reserve capacity. With the knowledge we have today about greenhouse gas emissions, industry practices for phasing out ducts should also be considered for change. It is an industry responsibility to clarify the consequences of poor choices.

Steel is the most established product on the market and the safest to use in terms of fire requirements, but that does not mean that this should not be challenged. Alternative duct qualities have a number of advantages, have an obvious place in our toolbox and should be considered for use in more projects. When steel ducts are used, they should be reused, with a high proportion of recycled metal, or manufactured from fossil-free steel.

Ultimately, the industry needs regulatory incentives, where low-emission solutions are given an advantage, in TEK, functional descriptions, public procurement, BREEAM and FutureBuilt projects. Then suppliers must do what they can to cut emissions in production.

The entire industry must step up to reduce greenhouse gas emissions associated with ducts. This way, we can not only challenge the emission peak in construction projects, but also do our part to limit climate change.

Further information about Green HVAC

Green Plumbing is an Innovation Project in the business sector, supported by the Research Council of Norway. The project was carried out in the period September 2021 to March 2025.

Project objectives: Building competence, developing new services and sharing knowledge. Showing the way to a 50% reduction in greenhouse gas emissions from HVAC installations in model projects

The project manager is Multiconsult. Partners in the project are Höegh Eiendom, GK, Armaturjonsson, Swegon, KLP Eiendom and Pipelife. R&D supplier is OsloMet. The reference group consists of VKE and FutureBuilt.