Mechanical supply ventilation (positive pressure ventilation)

Description
Electrical fan, usually installed in the roof void, delivers fresh air into the dwelling via the landing. This creates a slight positive pressure within the dwelling which forces moist or stale air out of the dwelling via purpose provided openings or air leakage routes. Typical ventilation rate is 0.5 air changes per hour

Unit cost of installation

Supply and fit is ~£820 including VAT; supply only for a single unit ~£520 including VAT

Bulk cost of installation

Discounts are available from some companies by negotiation.

Maintenance
Periodic cleaning and or replacment of filters.

Lifetime
15 years.

Technical suitability
Fans typically run at low speed continuously, providing low level background ventilation. Fan speed can be boosted either automatically via humidistat or by manual control from occupants.

Some models boost ventilation rates when temperature in the loft is greater than that within the dwelling, effectively collecting solar energy or recycling heat lost from the dwelling. Care needs to be taken to seal around any pipes or cables entering the loft to prevent short circuiting the airflow.

Anecdotal feedback from social housing providers indicates that they can be effective in controlling condensation, however effectiveness will depend on building shape and layout, and whether or not occupants keep internal doors open or closed.

Non-technical constraints
User perceptions of running costs and heat being drawn out of dwelling can lead to units being disabled. Noise can also be a problem depending on how they are installed; some units can be suspended from rafters to cut down on vibration noise.

Potential concerns with fuel poor
No specific issues.

Ancillary benefits
Delivered air can be filtered so improving indoor air quality - some units have filters fine enough to remove pollen for example.

Trigger points for work
Minor or major refurbishment. Condensation which may be more related to occupant lifestyle, rather than the condition of fabric or heating system.

Performance Limitations

Technical Guidance Available
» CE124/GPG 268 'Energy-efficient ventilation in housing. A guide for specifiers on the requirements and options for ventilation'