Positive Input Ventilation (PIV)

Positive Input Ventilation (PIV) units are one of the most effective and cost-efficient ways to eliminate condensation, damp, and black mould in UK homes. Rather than extracting air like a standard extractor fan, a PIV system works in reverse — drawing fresh, filtered air from the loft space and continuously distributing it throughout the home. The gentle positive pressure this creates forces stale, humid air out through natural gaps and leakage points in the building fabric, keeping indoor humidity low and preventing the conditions that cause condensation and mould to form.

PIV units are suitable for most UK house types with a loft space. They connect to a single ceiling diffuser — typically in a hallway — so there's no complex ductwork run to every room. Installation is straightforward and most units can be fitted in a day. Running costs are low: most PIV units consume 15–30 watts continuously, which works out to roughly £20–£50 per year at typical UK electricity rates. For colder climates or less well-insulated homes, heated PIV units — such as the Vent-Axia PoziDry Pro with Heater or the Elta PIV ONE PLUS+ — warm the incoming air before it enters the home, making the system effective and comfortable year-round.

Our PIV range includes 7 loft-mounted units from Vent-Axia and Elta, ranging from the compact PoziDry Compact Pro (suitable for smaller properties) to the PoziDry Pro Multi Storey for larger homes. If you're not sure which model is right for your property, read our PIV guide or see our Vent-Axia vs Elta comparison to help narrow it down.

What is a positive input ventilation (PIV) unit?

A PIV unit is a whole-house ventilation system that sits in the loft and continuously supplies filtered fresh air into the home via a ceiling diffuser, usually in the hallway. The gentle positive pressure pushes stale, humid air out through gaps and vents in the building fabric — reducing condensation, mould, and indoor pollutants without requiring a full MVHR installation.

Do I need a PIV unit with a heater?

In most UK homes, yes — a heater is recommended. Loft temperatures can drop significantly in winter, and without a heater the system would introduce cold air into the living space. Units with integral heaters gently warm the supply air to a comfortable temperature before distributing it, making them far more effective year-round. Our best-selling models include the Vent-Axia PoziDry Pro with Heater and the Elta PIV ONE PLUS+.

Is a PIV system easy to install?

PIV units are among the simpler whole-house ventilation systems to install — they don't require ductwork runs to every room. The unit is mounted in the loft, connected to a single ceiling diffuser in the hallway below, and wired to a standard fused spur. Most installations are completed in a day.

What size property does a PIV unit suit?

PIV systems are designed for standard residential properties — typically houses with a loft space. They are not generally suitable for flats or properties without a loft. For properties without a loft, consider a continuous mechanical extract (MEV) or MVHR system instead.

How much does it cost to run a PIV unit?

PIV units are designed to be low-energy, typically consuming 15–30 watts in standard operation — comparable to a low-energy light bulb. Annual running costs are generally in the range of £20–£50 depending on usage and heating requirements.