We’ve mentioned in previous articles how Life Cycle Assessment (LCA) of buildings is an engineering problem that is becoming more and more relevant. As action for climate change mitigation becomes more urgent, engineers are looking for more sources of energy savings, that is, opportunities for energy efficiency. Heating, Ventilation and Air Conditioning (HVAC) systems are known to be the cause of massive amounts of energy use in buildings, reaching almost 50% of operational use in developed countries . From the perspective of Portugal, it is crucial to understand their impact, since only a small portion (23%) of the households in Portugal counts with a fixed heating equipment and even smaller with air conditioning systems (10%), but in recent years these numbers have shown the tendency of growth . There is no widespread knowledge regarding the embodied carbon of these systems, but early studies are being conducted with more and more detail. It turns out, the impact of their energy use during operation may be very similar to that of their manufacturing and installation, and systematic replacements over a building’s lifetime.
Let’s brush up on the typical life cycle phases of a product (as defined by the European Committee for Standardisation in the ISO 14040 and 14044 standards):
C. End of life
Not all these energy-using phases need to be considered in every single study on the problem. It may turn overly complex, and some phases may not even be worth accounting for. For instance, end of life of HVAC systems is practically irrelevant carbon-wise, due to the relative ease with which they are disassembled and disposed of. So in this article I will analyse only the cradle-to-grave cycle (without Disassembly nor Reuse).
What constitutes an HVAC system? Any piece of active technology that heats, cools, or moves air into, out of, or within a building. Active means that it consumes power (e.g. burns natural gas or uses electricity). The technology needs power because, as opposed to the flows seen in passive strategies, heat or air are forced to flow in the opposite direction in which they naturally would if the building space were left alone. That is, heating a room that is already warmer than the outdoors during wintertime, or the opposite. Think of it as pushing a big rock up a hill with growing steepness instead of letting it roll down to the bottom. Or more accurately: holding it at a specific point on the hill. These scenarios would require you to exert physical power onto the rock. The specific point of the hill can be thought of as the set point of the HVAC system’s thermostat, and the slope’s steepness as the difference in temperature with the outdoors.
A recent experimental study in six small healthcare centres in Extremadura, Spain, showed that the embodied carbon of their HVAC systems amounted to 2.3 times that of the operation phase considering 15 years lifetime for the HVAC systems . We should note that Badajoz in Extremadura suffers average minimums of 4.5ºC in January and average maximums of 33.7ºC in August (with 438 cooling degree days, and 1372 heating degree days). This means that not only are health centres in continuous operation (thus their HVAC systems practically don’t switch off), but both heating and cooling are essential at different points during the year. It’s safe to say, then, that proportion of embodied-to-operational carbon is alarming. Why? Well, when engineers approach energy efficiency in buildings, they usually look at two things: heat flows across the building envelope and efficiency of the selected HVAC technology. In other words, they seek to reduce the operational use (life cycle Phase B) and do not look into phase A. Mining the materials, transporting, manufacturing, and installing are extremely energy-intensive activities, and prone to using carbon-base fuels (e.g.: petrol used in transport or mining), as proven in the mentioned ratio.
To understand how to tackle this issue, we must dive into the details of the Phase A of HVAC systems. Try the following thought exercise: think of a simple packaged air conditioning unit. Think of all the parts that constitute the cooling system: ducts, mechanical ventilation devices, fittings, air terminal, and the list goes on. And now imagine all the work put into creating those parts and bringing them together. This results in embodied carbon for each part, which may then be added up to the total. The graph below shows the results of a study based on Building Information Modelling (BIM) of Siemens’ office building in Zug, Switzerland . It shows not only how energy-demanding maintenance can be (reflected in the Replacement phase) along the studied 60 years, but also shows that the impact of piping must not be overlooked. When annualising, the embodied carbon is 1.32 kgCO2eq/m2 for Fabrication and 1.70 kgCO2eq/m2 for Replacement. Compare this to 1.25 kgCO2eq/m2 for Operations and you will probably agree on the importance of LCA in HVAC.
The numbers above tell us that the issue of human thermal comfort is more complicated than we were aware of. The fact that this discussion is barely starting is worrying. On a personal note, I have not seen significant mention of HVACs’ embodied carbon, even as I sat through a specialisation programme in Sustainable Energy Systems that went through Energy in Buildings thoroughly. In Portugal, wood and coal-powered sources of heat are still very common, but modernisation of the economy and the households are leading to adoption of electrified HVAC systems such as heat pumps or simple ACs (in the South). Each piece of HVAC system is different, the technology is ever evolving, and the climate is changing. This means there can be a significant range of values for embodied carbon, depending on numerous variables. In future articles, we’ll look into some of the specifics of each technology. But for now we can agree on one thing: HVAC systems’ embodied carbon is a pressing issue in today’s and tomorrow’s built environment.
 S.K. Alghoul: A Comparative Study of Energy Consumption for Residential HVAC Systems Using EnergyPlus (2017)
 Vilhena A. et al., O parque habitacional e a sua reabilitação. Análise e evolução 2001–2011, INE (2013)
 Justo García-Sanz-Calcedo et al.: Measurement of embodied carbon and energy of HVAC facilities in healthcare centres (2021). Journal of Cleaner Production.
 Kiamili, C. et al: Detailed Assessment of Embodied Carbon of HVAC Systems for a New Office Building Based on BIM (2020). Sustainability