Housing refurbishment: Ventilation

Issues

• 'Build tight, ventilate right'
• The main task of ventilation is to replace polluted indoor air with fresh outdoor air.
• Around 20% of energy loss from space heating is due to ventilation.
• Traditional air infiltration as a form of ventilation is no longer an option where 'best practice' air tightness is achieved.
• The ideal strategy is to provide a balance between energy efficiency and indoor air quality.
• Moist air causes condensation, mould growth and fabric damage. Humidity should be below 70%.
• The target ventilation rate should be between 0.5 – 1.5 air changes per hour (ach).

Design strategy

A design strategy will include:
• Extract ventilation in ‘wet’ rooms such as kitchens, bathrooms, WCs and utility rooms.
• Whole building ventilation which replaces stale air with fresh air throughout the house.
• Purge ventilation throughout the house to remove built-up pollutants. This kind of ventilation is typically provided by the opening of windows.

Mechanical ventilation systems

• All houses sealed to and above best practice will need mechanical ventilation to some degree.
• The efficiency of a system can be improved by:
• Heat recovery: Units work by using the heat from outgoing air to warm up the incoming air.
• Efficient fan motors: the specific power of a fan is defined as its power consumption divided by the air flow.
• Energy-saving controls: Automatic controls can be included to reduce the level of ventilation when pollution levels are low.

System types

(appropriate for applications where at least 'best practice' air tightness has been achieved)

Assisted Passive stack ventilation (PSV)
	• Provides continuous extract ventilation • Vertical ducting can be difficult to accommodate • None of the heat lost through ventilation is recovered. • Vents are located in 'wet' rooms. • Waste air is drawn up the ducts by a combination of the 'stack effect', 'wind effect' and occasional mechanical ventilation. • 'Assisted' means that the system is augmented by an extract fan only when required. • Replacement air is introduced via background ventilators located in habitable rooms. • Free passage of air is achieved through leaving a gap at the bottom of internal doors. • Care is needed to ensure that ducts are installed vertically, with no more than 2 bends and offset angles of no more than 45° from vertical.
Whole house mechanical ventilation with heat recovery (MVHR)
	• Provides controlled, preheated fresh air throughout the house. • The heat exchanger reduces heating demand. • Can be expensive. • The system needs a high level of air tightness to be effective. • Warm moist air is removed from 'wet' rooms and is passed through the heat exchanger. Fresh outdoor air is brought through the exchanger and preheated before being delivered to habitable rooms. • Air filters can be employed on incoming air. • Controls can be used to boost air removal from 'wet' rooms. • Ducting should be designed to minimise flow resistance. • Specific fan power should be 1W/l/s or less at any setting. • Heat recovery system should be at least 85% efficient.

Downloads

• Energy efficient ventilation in dwellings GPG268, Energy Saving Trust, 2006
• Energy efficient refurbishment of existing housing CE83, Energy Saving Trust, 2004
• Refurbishing dwellings - a summary of best practice CE189, Energy Saving Trust,

Standards and further information

Building Regulations
• Approved Document F: Ventilation 2006 edition

British Standards Institute (BSI)
• BS 5250: 2002 Control of condensation in buildings
• BSEN 13141-7: 2004, Ventilation for builidngs - Performance testing of components/products for residential ventilation - Part 7: Performance testing of a mechanical supply and exhaust ventilation units (including heat recovery) for mechanical ventilation systems intended for single family dwellings.

Chartered Institute of Building Services Engineers (CIBSE)
• Handbook of Domestic Ventilation, Roger Edwards, 2005
• Mixed mode ventilation, 2000
• Improved Life Cycle Performance of Mechanical Ventilation Systems, CIBSE / DTI 2002

Building Research Establishment (BRE)
• Passive stack ventilation systems: design and installation, Stephen et al, 1994
• Digest 297,Surface condensation and mould growth in traditionally-built dwellings. BRE, 1985
• Positive Input Ventilation in Dwellings, Stephen, 2000

Further information links

• The Chartered Institution of Building Services Engineers
• BSRIA, Microclimate centre

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