MANAGING THE QUALITY OF AIR THROUGH VENTILATION IN THE HOME IS IMPORTANT AND CAN ALSO SAVE HEAT
Having adequate ventilation in your home is important for removing moistre, odours, pollutants and so good health. When choosing a ventilation strategy there are, broadly, two options: natural ventilation or continuous mechanical ventilation.
Natural ventilation - This relies on natural driving forces (e.g. wind, convection currents) to provide general background ventilation, and can be supplemented by intermittent fans in areas of high need (e.g. wet rooms, such as bathrooms and kitchens). Many new and existing homes employ intermittent fans, however this strategy may not be suitable for homes built or retrofitted to be more airtight.
Continuous mechanical ventilation - These systems have either a central fan connected to wet rooms by ducting, or multiple local fans sited in wet rooms. Both types are available with at least two fan speed settings, where the lower speed is the default option to maintain an appropriate level of background ventilation. Higher speed options act as a boost, which can be activated manually or automatically for, e.g. periods of high humidity. More complex systems also provide ‘heat recovery’, recycling the heat in the extracted air to increase efficiency and lower your running costs; these MVHR systems require an additional supply air fan (normally within the same unit as the extract fan) to bring fresh air into your home from outside which is then heated via a heat exchanger. However, heat recovery ventilation systems are only effective in very airtight tested homes and require space for ducting, the compression unit and heat exchanger so installing in an occopied home would be disruptive.
The benefit of continuous mechanical ventilation systems is that they provide continuous air flow in the correct quantity. They are less affected by the variability of natural driving forces compared to natural ventilation systems. They do incur greater running costs – but there is likely to be reduced risk of moisture and pollutant build-up. A further factor to consider with mechanical ventilation is the installation complexity. Ducting is the weakest link in a mechanical system, and air moves efficiently in straight ducts. If you have insufficient space to accommodate ducting correctly (e.g. convoluted runs with lots of bends), then the system is unlikely to be successful in terms of air flow (too low) or noise (higher fan speed to get the right air flow). These systems also need commissioning by a professional to ensure the correct air flow is set.
The main natural ventilation systems available are as follows:
1. Intermittent extract system
2. Passive Stack Ventilation (PSV)
The main continuous mechanical ventilation systems available are as follows:
1. Centralised Mechanical Extract Ventilation (MEV)
2. Decentralised Mechanical Extract Ventilation (dMEV)
3. Whole-house Mechanical Heat Recovery Ventilation (MVHR)
Each system has its pros and cons and will be more or less suitable for your home. With the exception of MVHR, all systems require natural air inlets. Commonly these are fitted as ‘trickle ventilators’ in windows, but wall mounted (e.g. air bricks) may be sufficient. A wide range of proprietary air inlets are available.
Please bear in mind that ventilation systems are classed as ‘controlled service’, and any work to a ventilation installation requires notification to an authorised Building Control Body.
With thanks to Ian Mawditt, householder (2013, 2014) and a building performance expert. See Four Walls
Advice - Centre for Sustainable Energy
Advice - Energy Saving Trust
Trade Association - Federation of Envronmental Trade Associations