How does a PIV system work?

PIV (Positive Input Ventilation) units sit in the loft and push filtered fresh air into the house via a ceiling diffuser. The incoming air creates slight positive pressure, forcing stale, moist air out through natural leakage points (gaps around windows, doors, vents). PIV works best in older homes with moderate air leakage.

How much does PIV cost to install?

PIV installation costs £800-£1,500 including unit, diffuser, ductwork, and labour. Running costs are £40-£80/year for electricity (unit consumes 15-30W continuously). Filter replacement costs £20-£40/year.

Is PIV better than MVHR?

PIV is cheaper (£800-£1,500 vs £4,000-£8,000 for MVHR) and simpler to retrofit, but has no heat recovery. MVHR recovers 85-95% of heat, making it more efficient. PIV suits older leaky homes; MVHR suits airtight new builds or deep retrofits.

Positive Input Ventilation (PIV): Pros & Cons

How PIV works, what it costs, and when it's the right choice for your home.

Last reviewed: 20 May 2026 | 4 min read | Verified against 3 sources

PIV (Positive Input Ventilation) systems push filtered fresh air into homes from a loft-mounted unit, creating positive pressure that forces stale air out through natural leakage. Installation costs £800-£1,500. Running costs are £40-£80/year (electricity) plus £20-£40/year (filters). PIV suits older homes with moderate air leakage. It has no heat recovery, so incoming air is cold in winter (raising heating demand). Not suitable for airtight homes (air permeability below 5 m³/h/m² @ 50Pa) where MVHR is better.

How Does PIV Work?

A PIV unit sits in the loft and draws air from the loft space (or outside via a dedicated intake). The air passes through a filter, then the unit pushes it into the house via a ceiling diffuser, usually on the landing or hallway.¹

The incoming air creates slight positive pressure (typically 5-10 Pa above outside pressure). This forces stale, moist air out through gaps around windows, doors, letterboxes, and intentional vents (trickle vents, airbricks). The theory is that dilution and positive pressure prevent condensation by keeping indoor humidity low.

PIV relies on the house having enough natural leakage for stale air to escape. In very airtight homes, PIV has nowhere to push the air, causing over-pressurisation and discomfort.

Pros of PIV

Low cost. £800-£1,500 installed, much cheaper than MVHR (£4,000-£8,000).
Simple retrofit. Requires only a loft-mounted unit and a single ceiling diffuser. No whole-house ductwork.
Effective for condensation in leaky homes. Dilutes indoor humidity and creates positive pressure that prevents damp air settling on cold surfaces.
Low maintenance. Filter changes every 6-12 months (£20-£40). No complex ductwork to clean.
Quiet. Unit is in the loft, so noise in living areas is minimal (typically 20-30 dB).²

Cons of PIV

No heat recovery. Incoming air is cold in winter, raising heating demand. MVHR recovers 85-95% of heat; PIV recovers none.
Relies on loft air or outside air. If drawing from the loft, air can be dusty or cold. Dedicated outside intake adds cost (£200-£400).
Not suitable for airtight homes. Modern homes with air permeability below 5 m³/h/m² @ 50Pa don't have enough leakage for stale air to escape. PIV causes over-pressurisation.
Less control. No zone control (unlike MVHR, which can balance airflows room-by-room). PIV pushes air everywhere equally.
Can cause draughts. If airflow is too high, occupants feel draughts near the diffuser or leakage points.

Costs

Installation

PIV unit (including filter, diffuser, and basic ductwork): £600-£1,000. Installation labour: £200-£500. Total: £800-£1,500.³

Adding a dedicated outside air intake (instead of drawing from loft) costs an extra £200-£400.

Running Costs

PIV units consume 15-30W continuously. Annual electricity cost: £40-£80 (at 34p/kWh, 2026 rates). Filter replacement: £20-£40/year. Total: £60-£120/year.

When Is PIV Suitable?

PIV works best in:

Older homes (pre-2000) with natural air leakage (air permeability above 5 m³/h/m² @ 50Pa).
Homes with persistent condensation where budget doesn't allow MVHR.
Homes with loft space for unit mounting. Not suitable for flats or homes without lofts.

PIV is not suitable for:

Airtight new builds (air permeability below 3 m³/h/m² @ 50Pa). MVHR is the only practical option.
Homes where heat recovery is a priority. MVHR saves more on heating bills.
Flats or homes without loft access. No space for PIV unit.

Installation Cost: £800-£1,500 (inc. unit + labour)
Annual Running Cost: £60-£120 (electricity + filters)
Power Consumption: 15-30W (continuous)
Best For: Older homes, air permeability > 5 m³/h/m²

Sources

Building Research Establishment (BRE), Ventilation Strategies for Existing Homes, BRE Information Paper IP 1/23, 2023. bregroup.com (accessed 20 May 2026)
Energy Saving Trust, Positive Input Ventilation: Performance and Suitability, 2025. energysavingtrust.org.uk (accessed 20 May 2026)
Nuaire, Positive Input Ventilation Product Guide, 2024. nuaire.co.uk (accessed 20 May 2026)

Last reviewed: 20 May 2026 | Word count: 651 | Reading time: 4 minutes