Dr Duncan MacLennan
Max Fordham LLP
Good ventilation is critical to all buildings in order to maintain air quality for occupants and protect the building itself from build-up of moisture and pollutants. In the post-COVID world, maintaining air quality ensuring generous provision of fresh air to occupied spaces is also likely to be one of the key mitigants against virus spread. Efficient ventilation strategies play a key role in the design of low energy buildings and both natural and mechanical ventilation have a part to play.
Mechanical Ventilation with Heat recovery (MVHR) systems are generally accepted as the ‘low energy’ approach to providing ventilation in buildings, particularly in winter in temperate climates. This relies on the assumption that the energy saved by recovering heat, and in some cases coolth, is greater than the energy expended on fan power and other devices.
Natural ventilation can be inherently low carbon as it requires no fan power but, if poorly controlled, incoming air volumes can exceed fresh air demand and during winter can result in additional heating energy demand and other problems like cold draughts. In the UK’s summertime supplying large volumes of fresh air through big openings can provide lots of welcome “free cooling” for our buildings so in many cases natural ventilation can provide a low energy low carbon solution to cooling our buildings.
From an operational energy and carbon perspective, a mixed mode system utilising both MVHR in winter and mixed mode or natural ventilation in summer may be most efficient. However, this can be an expensive approach and the most resource intensive one.
Mechanical Ventilation With Heat Recovery (MVHR)
Where heating is provided via heat pump with a COP of 3, a temperature differential is 10°C requires around 4 kWh of electrical energy for every 3600m3/hr of air delivered to a space via a ventilation system with no heat recovery. A typical MVHR unit would require around 1.8 kWh of fan power to circulate this air and if heat is recovered at even a relatively low efficiency of around 70% the heating power saved through heat recovery is 2.8 kWh. Therefore each hour of operation the energy and carbon savings are around 1 kWh and 0.2 kgCO2 respectively.
In general, many modern air handling units are more efficient both in air delivery and heat recovery so the savings will be greater, particularly at small scale residential applications, so even a temperature differential of only a few degrees can make heat recovery beneficial.
Where Might MVHR Not Be So Useful?
For busy spaces such as an auditorium, it can be expected that for many of its operating hours the air handling unit is used to remove heat from the space. Cool air from outside is ideally used to provide free cooling so the extent of heat recovery needs to be carefully controlled so as not to over-recover heat and require mechanical cooling to be utilised. This starts to become a consideration at around 8-9°C external temperature, a large proportion of the year during daytime hours. Ideally this is solved by ensuring that the air handling unit has facility to vary the extent of heat recovery, for example by introducing a ‘summer bypass’ to allow a proportion of the incoming air to pass through. This does add complexity to the AHUs control routines but is an important factor in ensure heat recovery is used effectively and does not become a hindrance. Installing MVHR systems can be a considerable additional capital expense.
What Are the Potential Issues With MVHR?
The scenarios above assume that systems are well commissioned and well controlled. A 2016 survey by BSRIA of calibration facilities found that less than 10 per cent of the airflow measurements devices in use, by both MEP contractors and commissioning specialists, were UKAS calibrated in 2015 in line with the requirements of Building Regulations Approved Documents. The inevitable upshot of this is that there are many poorly commissioned installations, with confusing controls leaving fans running at full energy and part load. This can be exacerbated by inadequate maintenance contributing to increased energy use (e.g. dirty filters, worn belts etc). The energy penalties that accompany poorly operating MVHR systems can negate the benefits of heat recovery.
Is MVHR the Right Approach to Ventilating Net Zero Buildings?
Where mechanical extract is required for air quality reasons, such as in sanitary accommodation, natural ventilation is not practical and heat recovery mechanical ventilation is likely to be a straightforward choice.
For other building spaces, careful consideration needs to be made of whether MVHR suits the intended use of the space but also the ability of the building operator to maintain the system at optimal performance. Should either of these be impractical, consideration should be made of more effective ways to expend the capital cost of an MVHR system rather than it being installed as a default option. As part of the drive to net zero carbon, this is the kind of decision where good MEP engineers and design teams can really make a difference.