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.

How do I size a PIV unit?

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.

Do I need the heater option?

A built-in heater (typically 250-500W) 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 adds ~£30-£60/year to running costs but improves comfort significantly.

How much does a PIV unit cost to run?

PIV units use very little electricity — the fan typically draws 7-20 watts, costing about £15-£50 per year. With a 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 or wall mounted — which do I need?

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).

TradeCalculator

PIV Unit Calculator — Positive Input Ventilation Sizing

Size a Positive Input Ventilation unit based on house volume and air changes, with heater option for cold draught prevention and running costs.

House Volume

Total internal volume of house

Number of Floors

Storeys including ground

Mount Type

Heater Option

Tempers cold incoming air

Unit Price (optional)

PIV unit installed cost

How We Calculate This

This calculator sizes a PIV unit based on house volume and target air change rate.

The formula

Required Flow (m³/hr) = House Volume × Air Changes per Hour

Annual Cost = (Watts / 1000) × 24 × 365 × £0.28/kWh

Unit sizes

Small: Up to 60 m³/hr (1-2 bed flat)
Medium: Up to 120 m³/hr (3 bed house)
Large: Up to 200 m³/hr (4+ bed house)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.