CPD 15 2020: Specifying HVAC Systems for Ventilation

This Daikin sponsored CPD examines how existing regulations and new technologies can address the problem of poor ventilation in both workplaces and homes


In the UK, Public Health England estimates air pollution is responsible for between 28,000 and 36,000 deaths a year, costing the NHS and the private healthcare sector £20bn annually.

With people spending an estimated 90% of their time indoors (and some demographics, such as the elderly, spending even more time than that), indoor air quality is now recognised as a key issue in building design for homes, offices, hospitals, schools and factories.

This CPD assesses the impact of poor ventilation and discusses how existing regulations and new technologies can solve the problem.

The main focus when designing HVAC is on energy use and efficiency, which attract a high weighting in BREEAM assessments.

The cost of poor ventilation

Aside from the long- and short-term physical effects, there is growing evidence that air pollution affects mental health and may be a contributory factor in conditions such as depression and bipolar disorder. It may also have a detrimental effect on children’s learning ability and workforce productivity, as well as patient recovery.

For building owners, poor indoor air quality (and poor indoor environments in general) can also hit their bottom line: demands from leaseholders and tenants can result in having to carry out costly remedial works to both the building fabric and M&E systems, from lighting to climate control. This can lead to higher running costs and potentially affect market and rental values.

Indoor air quality is defined as the quality of the air in and around a building, particularly in relation to the health and comfort of occupants. It is affected by complex and interlinked factors, due to both outside and inside air pollution.

Sources of outdoor air pollution include road traffic, industrial processes, waste incineration and construction and demolition sites. Pollution includes particulate matter, nitrogen dioxide, carbon monoxide and pollen, all of which can be brought into a building through natural or mechanical ventilation and via infiltration through the building fabric. But there are also pollution sources inside a building, including volatile organic compounds (VOCs) given off by wall and floor coverings, furniture and appliances as they age and degrade; dust, damp and mould; emissions from office equipment and industrial machinery and, of course, occupants themselves, who breathe out carbon dioxide and can spread colds and viruses.

Whole building systems

As energy-efficiency standards in building design have risen in recent years, buildings have become more insulated and airtight. This can reduce fresh air circulation, leading to low oxygen levels and increased potential for allergies and odours, as well as the risk of condensation build-up.

Installing heating, ventilation and air-conditioning (HVAC) systems that control temperature, humidity and maintain air quality can address the situation. The main focus when designing and specifying HVAC is typically on energy use and efficiency, not least because they attract the highest weighting of all the factors in BREEAM assessments.

However, BREEAM also rewards the use of HVAC that maintains high indoor air quality by balancing the indoor and outdoor temperature and humidity and preventing ingress of outdoor pollution, while ensuring a supply of fresh air to occupants. Clearly, a balance needs to be struck.

Fundamentally, ventilation aims to remove stale indoor air and replace it with “fresh” outdoor air. HVAC systems are designed to extract water vapour, airborne pollutants and odours, controlling humidity and maintaining good indoor air quality, and to minimise the spread of these impurities to other areas of a building.

Systems must also provide “purge ventilation” to help remove occasionally high concentrations of pollutants and water vapour caused, for example, by food cooking in a kitchen or an accidental water spill.

For larger buildings, ventilation can be supplied by air-handling units linked to the indoor units, controlled centrally or by floor, room or zone. For smaller buildings, heat recovery ventilation units can be integrated with the overall climate control system, to supply tempered fresh air to the indoor units.

Energy efficiency

Whole building ventilation systems incorporating heat recovery deliver high levels of efficiency, using waste heat from cooling and refrigeration to heat different areas of a building.

Manufacturers typically claim Seasonal Energy Efficiency Rating (SEER) figures of 3 and 4 for heat recovery systems. However, it is possible under certain conditions for a system’s efficiency ratio to nearly double, when taking into consideration recovered energy. In practice, a SEER in excess of 6 should be possible to achieve on a fairly frequent basis.

Further energy savings can be achieved using features such as variable refrigerant temperature control. This varies the amount of refrigerant flowing through the system and alters the evaporating and condensing temperatures to match demand. As a result, dramatically less power is needed, and efficiency rises accordingly.

As with any element of HVAC, the design of ventilation, whether as a standalone system or as part of a whole building solution, must meet the requirements of the building’s occupants.

The key factor in designing ventilation is that it must provide sufficient fresh air supply and extraction to minimise moisture build-up (and therefore control mould) and deal with bio‑effluents (body odour), as well as to keep exposure to nitrogen dioxide, carbon monoxide and VOCs to a minimum.

In the UK, ventilation design is controlled by the Building Regulations Approved Document Part F, which sets out criteria for both homes and “non‑domestic” buildings (primarily offices).

Ventilation also has to comply with a number of British Standards covering energy performance, filters and maintenance. And, as part of HVAC systems, ventilation must comply with Part B (fire safety), Part C (site preparation and resistance to contaminants and moisture), Part E (resistance to the passage of sound), Part L (conservation of fuel and power), Part J (combustion appliances and fuel storage systems) and Part P (electrical safety).

There is a wide range of ventilation guidance available for designers, published by industry bodies including BRE, the Chartered Institution of Building Services Engineers (CIBSE), and REVHA (the Federation of European Heating, Ventilation and Air Conditioning Associations). A key source of information on ventilation is CIBSE’s Document B (section 2.3), which builds on the advice given in Part F of the Building Regulations.


Required ventilation rates for homes are based on the number of bedrooms, from 13 litres per second (l/s) for a one-bedroom home, to 29l/s for a five-bedroom property (assuming that two people occupy the main bedroom and one person occupies each of the others). The minimum ventilation rate must not be less than 0.3l/m² for the total internal floor area (all storeys).

Part F of the Building Regulations also gives rates for intermittent or continuous extract ventilation required in kitchens, utility rooms, bathrooms and WCs. Purge ventilation is also required in every habitable room, which can normally be achieved by opening windows and doors.

Typically, control of ventilation in a whole building HVAC system is integrated with heating and cooling


The total air supply and extraction rates for office ventilation (assuming no significant pollutant sources) is 10l/s/person, based on the assumption that the building has an air permeability of 3m³/(h.m³). This is within the range achieved by most modern, well-designed buildings and is negligible infiltration, compared with the ventilation system capacity.

Intermittent extract ventilation is required for specific areas, including WCs and urinals, printing or photocopying rooms, showers and food and beverage preparation areas. Purge ventilation is also required in each office.

The total air supply and extraction rates for office ventilation (assuming no significant pollutant sources) is 10l/s/person, with intermittent extract ventilation also required for specific areas such as photocopying areas

Hospitals and healthcare

Ventilation in healthcare environments must also be designed to Approved Document F. Health Technical Memorandum 03-01: Specialised ventilation for healthcare premises, published by the Department of Health, also gives general guidance and recommendations for HVAC systems in healthcare buildings.

Some healthcare environments, such as operating theatres, critical-care areas and isolation units, have particular ventilation requirements to prevent the spread of infection, odours and hazardous materials. For example, air recirculation systems are normally used in clean rooms and ultra-clean operating theatres, where the extracted air is significantly cleaner than the outside supply.

Locating air intakes and exhaust outlets

Ventilation intakes must be placed as far away as possible from the main sources of local air pollution. For HVAC systems, this typically means on the roof (unless there are higher-level sources). Alternatively, intakes can be placed on walls, in courtyards and in atriums.

Regardless of position, it is important to avoid cross-contamination from boiler flues and exhaust stacks. In fact, exhaust outlets should be placed as far away as possible, preferably on the roof or at a high level, and downwind of intakes where there is a prevailing wind direction. They should not discharge into courtyards or enclosed spaces and it is recommended that they discharge vertically, to avoid downwash.

Controlling ventilation

Ventilation control is critical for maintaining indoor air quality. Typically, control of ventilation in a whole building HVAC system is integrated with heating and cooling.

Automatic systems are fitted with CO2 sensors, which slow down or switch off the unit if CO2 levels drop below a customer-defined level, typically linked to how many people are in the room. If CO2 levels are perceived to be too high, air quality is maintained by variable air volume. Ventilation can also be controlled using the same infrared sensors used to adjust temperature in climate control systems that detect room occupancy.

Systems also need to be flexible enough to enable some ventilation units to be switched off or to reduce flow rates – for example, during rush hour – with the system relying on units that are remote from the pollution source or using recirculated air for a time.

Ventilation can also be noisy and annoying for both occupants and people outside a building, so noise should be minimised wherever possible.

This can be achieved through design – such as by locating units carefully – or alternatively by choosing indoor and outdoor units that have very low sound power levels and sound insulation, or else by introducing noise-reducing measures.


Filtration is another important element of ventilation. All HVAC units will be fitted with filters, primarily to keep them free of dust, to ensure good operation and to maintain energy efficiency. Filters are also fitted to remove particulate matter (PM) from supply air and, in some cases to remove particles where there is a risk of pollution entering the outside atmosphere.

Filter selection is based on the widely accepted thresholds for PM, published by the World Health Organization in its Air Quality Guidelines – Global Update 2005. The recommended annual limits are: annual mean for PM2.5 < 10g/m3; annual mean for PM10 < 20g/m3.

General filters capture larger, heavier particles, such as dust. Fine filters remove smaller particles, typically the size of bacteria, while HEPA and ULPA filters are used in specialist environments, such as clean rooms and ultra-clean operating theatres.

However, while many guidance documents (for example, those published by CIBSE) still refer to EN 779, this has been superseded by ISO 16980 Air filters for general ventilation. The standard uses a filter efficiency classification system based on the three main size ranges of PM, where ePMx describes the efficiency of an air cleaning device to particles within a range of 0.3m to x (the x means the particle size that is your target minimum level).

Eurovent 4/23 Selection of EN ISO 16890 rated air filter classes for general ventilation applications (2018) gives guidelines on selecting air filters based on the minimum filtration efficiency, depending on outdoor and supply air categories, for a range of buildings, including homes, offices, shopping centres and hospitals. It also gives comparisons between the filter grades in EN 779 and ISO 16890.

Indoor air quality is related to physical and mental health and may influence workforce productivity, children’s learning, and patient recovery

Installation, commissioning and maintenance

It is crucial that HVAC systems are installed according to manufacturers’ recommendations and commissioned to the final design. Installers have been known to make changes on site – varying piping length, for example – which can have a significant impact on performance, energy efficiency and indoor air quality.

Using (and supervising) an installer approved to install a particular manufacturer’s systems is key, as is choosing a company with experience of similar installations.

Regular maintenance of ventilation is obviously important and should form part of the overall servicing regime for a building’s HVAC system. However, cleanliness is of particular importance when dealing with ventilation, as dust and dirt can affect their ability to maintain indoor air quality.

Maintenance should include checking the intake and exhausts for any signs of dirt build-up, pollution or contamination, damage from weather and animals, as well as inspections of ductwork and the indoor units.

BS EN 15780: 2011: Ventilation for buildings – ductwork – cleanliness of ventilation systems specifies acceptable cleanliness levels for supply, recirculation and extract air, grouped into three classes – low, medium and high – depending on building use. So, for example, rooms with intermittent occupancy are classed as low, whereas a treatment room in a hospital is classed as high.

Dust should be removed from ductwork, particularly around filters, heating and cooling coils and any change of direction in the ducting. Filters should also be cleaned and replaced as necessary. Indoor units should be cleaned and the dust boxes of those fitted with auto-cleaning systems emptied.


As covid-19 restrictions have become part of our daily reality, concerns have been raised about the role of HVAC in the risk of spreading airborne viruses.

First and foremost, building owners and managers should follow government guidelines. But, as with any airborne contaminants, the risk of potential spread of viruses can be mitigated by ventilation and proper and effective filtration, along with regular cleaning and maintenance of systems.

It is essential that air conditioning systems in buildings where confirmed cases of coronavirus have been diagnosed are cleaned and sterilised according to best practice. It is equally important to do so in buildings with no confirmed cases, as a preventative measure – not only now but as part of ongoing maintenance.

REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, published guidance in April 2020 on how to operate and use building services in order to prevent the spread of coronavirus in workplaces.

The guide gives a number of recommendations for buildings that have mechanical heating and ventilation systems. Primarily, the advice for building owners and managers is to supply as much outdoor air as possible, as coronavirus particles can remain suspended in the air for a long time.

While in urban areas, where air pollution is likely to be higher, this may mean more pollution ingress to a building, the view is that mitigating the risk of coronavirus is more important during the pandemic.

This means extending ventilation operation times (and, where possible, keeping ventilation running 24/7), increasing ventilation rates when the building is occupied and only reducing them during off-peak times such as nights or weekends.

Where ventilation relies on air-handing units (AHUs), as opposed to where an AC unit is used in conjunction with a fresh air AHU, REHVA recommends switching air recirculation features off and says it is crucial to ensure systems are properly maintained and leaks are dealt with, to prevent virus particles in extracted air re-entering a building via the air supply, particularly in heat recovery systems.

However, it says there is no need to change filters more regularly, nor is additional cleaning thought to help reduce the risk of room-to-room transmission. Normal maintenance procedures can be used, with adequate PPE for service engineers and appropriate safety procedures in place, including switching off systems during filter changes.

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