COMMENT Some were slow to appreciate the role that ventilation played in reducing the transmission of coronavirus, but it is now influencing proposed policy, including the design of buildings. Recently published government proposals stipulate the need for increased ventilation capacity in a wide range of building types, including commercial offices.
The proposals contain key changes to current practices. They include: the ability to boost fresh air rates by 50%; additional ventilation requirements for spaces such as corridors and lift lobbies; a purge ventilation requirement of four air changes per hour for all spaces; and a means of in-use air quality.
Along with protecting against the current and future pandemics, there’s potential for these measures to improve occupants’ comfort during the summer months as well as their overall enjoyment of buildings.
Delivering these proposals requires careful design to avoid excess capital cost, energy use that is not compatible with the climate emergency, and discomfort from over-ventilation in winter.
So what will the proposals mean in practice?
1) Equipment will get bigger
Ventilation units capable of moving more air around are larger, so boost rates from a mechanical system will impact space planning within buildings. This applies to ventilation equipment, and to heating and cooling systems – more air in summer or winter means more heat flow and higher peak demands, particularly if the boost rate is from natural ventilation without heat recovery.
2) Ductwork gets bigger
To hit current ventilation performance targets (specific fan power), the ductwork throughout buildings where the boost rate is from a mechanical system will need to be larger. This will impact space planning in buildings.
3) Ventilation strategy may need reviewing
In low-energy buildings it is relatively common to use a cascade approach, where fresh air is supplied to occupied rooms, transferred through circulation spaces, and extracted from toilets, showers, stores and so on. The consultation documents steer designs away from this or require additional filtration or air purification.
4) Energy consumption in normal use will change
This is not so clear cut. Assuming the boost capacity is designed into a mechanical system, fan power is likely to reduce: bigger ductwork means lower pressure losses, so less power to move the same amount of air around. However, there is a limit to how far fans can be turned down, so increasing the system capacity also increases the minimum flow rate. This also brings the risk of over-ventilating spaces, particularly at reduced occupancies.
5) Summer energy use could decline
Night air in summer can often be used to provide some pre-cooling to buildings, particularly where they have thermal mass and intermittent occupancy. Increasing ventilation capacity could improve this and reduce reliance on air conditioning in warm weather even though it won’t help as much in really hot weather, and is likely to increase the cooling plant peak capacity.
6) Purge ventilation and system sizing will change
The consultation version of Approved Document Part F requires the ability to purge ventilate offices to at least four air changes per hour, using air directly from outside, without any recirculation. For buildings without openable windows, that can mean an even higher ventilation rate than the 50% boost.
7) Risk of low winter humidity
In winter, outside air is often dry in absolute terms, even if it has a high relative humidity. This is because cold air can’t hold much water, for example 100% relative humidity at -4°C is equivalent to 18% relative humidity at 20°C, well below comfort and health targets. Increased ventilation rates in winter bring the risk of either reducing internal relative humidity or increasing energy use through humidification.
Buildings and their systems often fail to achieve the predicted performance in practice owing to a lack of ongoing monitoring and fine-tuning. Greater emphasis on in-use monitoring is likely to help inform clients, consultants and contractors, and lead to better outcomes overall. We hope that a new emphasis on air quality monitoring becomes a useful tool for managing buildings to make sure they are healthy and low-energy.
Gwilym Still is Passivhaus leader at Max Fordham
