A breath of fresh air
Improving indoor air quality with modern ventilation, by Jon Hill, Technical Manager of Polypipe Ventilation
A
ll dwellings need a supply of fresh outdoor air, not just for the health and comfort of the occupants, but also to con-
trol condensation, disperse indoor pollutants and to ensure the safe and efficient operation of open-flue appliances.
Poor air quality and its effects
Moisture is probably the most significant house- hold pollutant because of the high humidity rates generated by day-to-day activities. A house with five occupants can generate in excess of 5kg of airborne moisture daily through condensa- tion, which can quickly make the household atmosphere dank and musty if not ventilated adequately. It can also lead to ugly staining and mould growth, which can affect the health of the occupants as well as being unsightly. Interstitial (i.e. within the building fabric) condensation is another effect of high humidity levels, which may cause rot and, as a result, unseen structural damage. In addition, the level of pollutants in urban
areas has led to outdoor air quality deteriorat- ing. It is therefore important to minimise the levels of pollutants entering the dwelling; imperative for allergy sufferers or those with breathing difficulties.
Getting the balance right
Traditionally, many UK dwellings have relied on natural air infiltration, such as an open window, air bricks or window trickle vents. However, these very basic ventilation methods can result in excessive ventilation rates. In new or refurbished dwellings the reverse is the case with increased air tightness leading to internal pollutant sources having a greater impact on indoor air quality.
Ensuring compliance
Building Regulations Part L is regularly revised to require ever higher levels of air tightness; in Part F, a permeability figure of <5m³/h per m² is considered airtight and >5 is considered ‘leaky’; limited to a maximum of 10m³/h per m² envelope area at a pressure of 50Pa by Part L. As a result, improved ventilation strategies are required. The 2010 revision of Part L also saw the
introduction of maximum Specific Fan Powers (SFP) for mechanical ventilation units; for example, no higher than 0.5 Watts per litre per second (W/l/s) for an intermittent fan.
The introduction of new regulations also
brought a greater emphasis on the quality of ventilation installations and mechanical ventila- tion is now a notifiable service in most cases. Other guidelines that have had an impact
recently on ventilation in domestic dwellings include revisions to SAP scoring (the Standard Assessment Procedure for the energy rating of dwellings) and the introduction of the NHBC Mechanical Ventilation with Heat Recovery (MVHR) specification. The former has also con- tributed towards the adoption of more controlled forms of ventilation, particularly MVHR.
Energy efficient ventilation options
As regulations for ventilation tighten, so too does the requirement for solutions which meet these standards and help improve indoor air quality.
Intermittent Extract Ventilation
Also referred to as System 1 ventilation in Building Regulations Approved Document F, this basic type of ventilation consists of local extract fans fitted in a dwelling’s wet rooms to provide rapid extraction of moisture and pollutants. They operate intermittently under either occupant or automatic control. This is the simplest, lowest cost form of ventilation and is easy to fit and use, but is also the least energy saving method.
Passive Stack Ventilation (PSV)
A System 2, PSV comprises of inlet grilles located in wet rooms, connected by near-verti- cal ducts to ridge or other roof terminals. Warm, moist air is drawn up the ducts by a combina- tion of the stack effect and wind currents. Highly cost effective – there is no electrical supply and no direct running costs – PSV’s provide silent continuous operation, but their effectiveness is heavily weather dependent.
Continuous Mechanical Extract Ventilation (MEV)
System 3, MEV systems actively extract air from wet rooms via ducting to a central ventilation unit. They are relatively easy to install due to sin- gle external termination, but will require system balancing and commissioning. The systems are typically dual speed, providing low speed con- tinuous trickle ventilation and high speed boost flow. This is a more costly ventilation solution, but also a more effective one for modern homes. A sub-set of System 3 is Decentralised
Mechanical Extract Ventilation (dMEV). dMEV systems are an extremely cost effective and easy
to install ventilation solution. They offer continu- ous low levels of ventilation to a single wet room, coupled with virtually silent operation.
Continuous Mechanical Ventilation with Heat Recovery (MVHR)
Unlike all the other ventilation systems discussed, MVHR systems combine supply and extract ventilation in one system. MVHR systems efficiently pre-warm the fresh
air drawn into the building with waste stale air using a heat exchanger; up to 95 per cent of waste heat can be recovered by this mecha- nism. The filtered, pre-warmed air is distributed around the home, meeting part of the heating load in more airtight properties (ones that have a figure of <3 of the permeability figure of <5m³/h per m²). The incorporation of a highly efficient heat
exchanger will also help lower a property’s DER and therefore provide a higher SAP rating. MVHR systems are extremely effective
at reducing the risk of condensation and cold air draughts and also include built-in air filters that are particularly useful in more polluted urban areas. The range of readily available, energy effi-
cient modern ventilation solutions means that no domestic properties – new or old – should ever have to suffer from the effects of poor ventilation. Breathing clean and fresh air in our homes should be seen as a necessity, not a luxury. We are, after all, quite literally breathing life into a property.
Enq. 122
selfbuilder & homemaker
www.sbhonline.co.uk
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