PIV Unit Calculator — Positive Input Ventilation Sizing

A Positive Input Ventilation (PIV) unit draws fresh air from the loft space (or outside for wall-mounted models), filters it and gently introduces it into the house through a ceiling-mounted diffuser. This creates slight positive pressure, pushing stale moist air out through natural gaps. It is very effective against condensation.

PIV units are sized based on the total house volume and desired air change rate. A rate of 0.5 air changes per hour is typical. For a 3-bed semi with volume of 200m³, you need 100 m³/hr capacity. Most domestic PIV units offer 60-200 m³/hr and are adjustable.

A built-in heater (commonly a 400W unit, e.g. the Nuaire Drimaster-ECO-Heat) tempers the incoming air in winter to prevent cold draughts from the diffuser. It is recommended for properties in colder regions or where the loft temperature drops below 5°C. The heater is thermostatically controlled, so it only switches on when the incoming air is cold and modulates off once tempered — it adds roughly £30-£60/year to running costs rather than running continuously, but improves comfort significantly.

PIV units use very little electricity — the fan typically draws 6-15 watts, costing about £14-£35 per year at the current Ofgem cap rate. With the thermostatic heater option running in winter, total annual cost is approximately £40-£100. This is much less than a dehumidifier (£150-£300/year) and more effective at preventing condensation.

Loft-mounted PIV is standard for houses with loft access — it draws naturally pre-warmed, filtered loft air. Wall-mounted PIV is for flats and properties without loft access — it draws air from outside through a wall duct. Loft-mounted units are quieter (18-22 dB) than wall-mounted (25-30 dB).