This blog is a bit of a departure for me, a change from previous blogs that have all been about homes.
I've come to this issue because of my involvement, through my kids, with schools - seeing what their school is doing with regards to ventilation to try and make school safer during the coronavirus pandemic, and how similar measures have been covered in the media. I'm going to discuss ventilation in relation to schools, but much of what I'm covering applies to workplaces as well.
I'm not an expert on the risks of transmission of Coronavirus, so I'm not going to comment on that, I'm just going to comment on what the current ventilation strategies mean in terms of thermal comfort and energy use for different types of ventilation.
Schools are being asked to keep rooms well ventilated at all times, either through natural ventiation (opening windows) or through increasing mechanical ventilation rates, and dress code requirements have been relaxed in recognition that this will mean rooms may be colder than usual. Beyond this general advice there is more detail on how ventilation systems should be operated to minimise the risk of Coronavirus transmission in this guidance from the Federation of European heating, ventilation and air-conditioning associations (REHVA). This suggests actions to increase the ventilation rate in order to reduce the risk of transmission through aerosols. For mechanical ventilation systems the guidance recommends increasing the ventilation rate to the maximum, with no recirculation of air, for all occupied hours, and for two hours before and after occupancy. For natural ventilation it recommends opening windows much more than usual, even if this results in thermal discomfort. For buildings with mechanical ventilation the guidance recommends that window opening can be used to further increase ventilation rates.
This increase in ventilation rates has caused complaints from parents in some schools who are worried about their children having to learn while being cold (and potentially damp from being outside more than usual), and the impact this will have on both their learning environment and their health. Teachers are also concerned about the impact of opening windows.
There are lots of different types of ventilation, but for the purposes of this commentary they can broadly be separated into mechanical ventilation with heat recovery (MVHR), and ventilation without heat recovery, which includes both mechanical ventilation without heat recovery and natural ventilation (fresh air supplied and extracted through opening windows or vents).
For MVHR systems the extract air does not mix with the fresh supply air, but there is a transfer of heat from one to the other. This is the type of ventilation that is installed in Passivhaus buildings. 100% of the air being supplied is fresh air, there is no recirculation, only the recovery of heat that would otherwise be lost. This is a good overview of how MVHR systems work.
The vast majority of schools (and workplaces) have ventilation systems without heat recovery. For these schools, the impact of increasing ventilation rates during cold weather will be to decrease comfort levels and increase energy use for heating, dramatically so during cold weather. Thermal comfort is reduced by introducing cold draughts and by increasing air speeds (if you're interested there's a detailed discussion on thermal comfort here). As outdoor temperatures get colder in the winter, schools may also struggle to maintain the target air temperatures, and this will decrease comfort yet further. There is even a chance that during extremely cold weather schools will be unable to maintain temperatures above 16°C. If this happens schools may be required to close due to health and safety regulations.
It's worth starting the discussion on MVHR with noting that schools with well-designed and commissioned MVHR systems typically have better air quality all the time than naturally ventilated schools, due to more effective ventilation, especially in winter, when people in naturally ventilated schools are likely to close windows and vents because of discomfort. This has a big impact on the quality of education provision in normal times. There is a good summary of what is known about the impact of air-quality on learning outcomes for students, and how Passivhaus schools compare to naturally ventilated schools here. So before even considering increasing the ventilation rate these schools are starting from a safer point than their naturally ventilated cousins since they are better ventilated.
What happens when you increase the ventilation rate on MVHR systems? In terms of comfort the impact will be very small - the air speed will increase slightly, but because there is heat recovery the supply air is nearly as warm as the air in the room, so there are no cold draughts. For an illustration of this imagine a cold winter's day, with 0°C air outside and 20°C inside the classrooms. In this situation, with a 90% efficient heat recovery system (this is typical for a Passivhaus MVHR system) the supply air will be delivered at 18°C. Compared to a system with natural ventilation or mechanical ventilation without heat recovery, where the supply air will be at 0°C, the comfort difference is considerable. What about the impact on the heating energy demand? This is also small; in energy terms doubling the ventilation rate with MVHR is the same as increasing the ventilation rate by 10% in a system without heat recovery. There will be an increase in electricity use for the fans but this is also likely to be small compared to the additional heating energy required in a system without heat recovery.
There is an active discussion in Passivhaus circles about whether simply increasing ventilation rates through the MVHR is enough to reduce Coronavirus risk to tolerable levels, and even whether increasing rates considerably is desirable since very high ventilation rates during cold winter weather will lead to very dry indoor air, which carries some of its own problems in terms of increasing the risk of infection. I don't know enough about Coronavirus risk to comment on whether the standard ventilation rates for Passivhaus schools are sufficient, or whether they should be increased through increasing the mechanical ventilation rate, or even further through also opening windows. What is certain, however, is that in all three situations a Passivhaus school has a huge comfort and energy advantage over schools ventilated without heat recovery.
We've already seen that increasing the ventilation rate through the MVHR, even by large amounts, will have only a small impact on energy use and thermal comfort. But what about opening windows?
Opening windows will certainly have an energy and comfort impact, but both of these impacts will be smaller than in 'normal' schools. This is because the very high standards of insulation required of Passivhaus buildings mean that the heat losses are so small that the length of the 'window opening season' (the months in which windows can be open while maintaining comfortable conditions inside) is much longer. Because heat losses through the building fabric are so low, and surface temperatures are close to the internal air tempertature, windows can be opened more of the time without occupants feeling cold. Furthermore, the number of days in which opening the windows means the heating has to come on is lower. In both comfort and energy terms Passivhaus buildings win, even if window opening is required.
Coronavirus should be a wake-up call for the construction industry - we knew before that most new buildings are inadequate in terms of comfort, energy use, climate impacts and various health risks (such as asthma). We now know that they are also inadequate in the face of global pandemics. Coronavirus makes the argument for Passivhaus standard even stronger - for new buildings we should be building to that standard, for existing buildings we should be undertaking deep, whole-building retrofits, where we can to EnerPHit standard (the Passivhaus standard for retrofit), and for health reasons we should be starting with the ventilation system.