Positive outlook

Since 1936 when the first window fan was designed and built by Vent-Axia founder Joe Akester, retrofit ventilation for domestic and smaller commercial buildings has generally featured powered extraction.

Another option exists however, in the form of positive input ventilation, a technique which, in the current climate of saving energy, is finding new favour for condensation control.

The principles behind positive input ventilation are simple. Air is drawn from a source outside the occupied space, usually a loft, where the air is tempered by warmth from the room below and solar heat gains. This air, rather than colder, costlier-to-heat outside air, is then filtered and introduced under pressure into the indoor environment at a central point. Here it mixes with the even warmer air which accumulates beneath the ceiling, thus recycling warmth throughout the building (figure 1).

The cleaner, drier, warmed air continuously displaces stale, moisture-laden air from the living space through natural openings around the building, creating a comfortable, draught-free environment while helping to eliminate the unhealthy and costly effects of condensation and mould growth.

Positive input ventilation is especially attractive to managers of local authority and housing association properties because of its quietness, ultra-low power consumption and minimum impact on heating costs. These are all essential factors in persuading occupiers to accept even the lowest levels of ventilation essential to prevent damage to the fabric and decoration of the building, their furnishings and personal health.

A single, loft-fitted fan makes for a very quick, low-cost installation too – it typically takes less than one hour to complete a neat job.

Corresponding attention must also be directed towards natural ventilation outlets, without which the effectiveness of positive input ventilation could be compromised. Completely up-dating the perforated plate shutters and air-bricks of old, positive input products provide manually or thermostatically-regulated openings for optimum efficiency. For better internal air circulation, door-mounted grilles ensure a continuous flow of replacement air.

For newly constructed buildings that are still drying out, or houses with long-term condensation problems, the positive input ventilation system should be run at full speed for the first 14 days before reverting to the installed settings. This can be easily achieved automatically by means of a selectable link, completing the dry-out of the building in much less time than unassisted natural ventilation, then protecting it indefinitely thereafter.

Field results from local authorities, specifiers, installers and end-users show that a forward-curved, multi-speed centrifugal fan rated at 125-180 m3/h is sufficient for house volumes of up to 400 m3 or floor areas up to 150 m2, while the option of ultra-low speed output of 100m3/h provides a practical minimum volume for night-time running or operation while the premises are empty.

Ultra-low speed can also give extremely quiet performance and diminish power consumption to an almost negligible 4 W – overall running costs can be less than a penny per day.