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Understanding PIV Systems

Approx. 6 min read

Introduction

Positive Input Ventilation (PIV) systems are an effective, low-energy solution for improving indoor air quality and controlling condensation in residential properties. By continuously introducing filtered air from a central unit, PIV systems create a gentle, positive pressure that drives out stale, moisture-laden air through natural leakage points and trickle vents.

Originally developed for social housing, PIV technology is now widely adopted in both retrofit and new build applications due to its simplicity, reliability, and energy efficiency.

How PIV Works

A PIV unit is typically installed in the loft (for houses) or within a central hallway (for apartments). The system draws in external air — either directly from outside or from the loft space — passing it through a high-grade filter and, if required, an integral heater to temper the air during colder months.

This clean, tempered air is then gently diffused throughout the dwelling via a ceiling-mounted diffuser, creating a slight positive pressure. As this air moves through the property, it displaces stale air and moisture, expelling them through natural leakage paths and purpose-designed vents.

System Components

Key elements of a PIV system include:

  • Central fan unit – energy-efficient motor with variable speed control.
  • Filter – G3 to G4 (standard) or higher-grade filters for allergen reduction.
  • Heater element – optional preheater to maintain supply air temperature.
  • Ceiling diffuser – distributes fresh air evenly across living areas.
  • Sensors and controls – humidity, temperature, and air quality monitoring options.

Benefits of PIV Systems

  • Condensation and mould prevention – maintains relative humidity at safe levels.
  • Improved air quality – reduces pollutants, allergens, and volatile organic compounds (VOCs).
  • Low running costs – typically <10W power consumption; minimal maintenance.
  • Ease of installation – non-invasive retrofit solution requiring no ducting network.
  • Thermal comfort – optional heaters prevent cold draughts during winter.

Design and Installation Considerations

When specifying a PIV system, consider:

  • Property airtightness – PIV works best in homes with moderate leakage; excessive airtightness may restrict exfiltration paths.
  • Air source – systems drawing from lofts must ensure clean, ventilated loft air or use ducted external intake models.
  • Diffuser placement – should be central to the dwelling for balanced air distribution.
  • Heating element control – thermostatic control prevents unnecessary energy use.
  • Maintenance – filters should be inspected and replaced every 6–12 months.

Compliance and Standards

While PIV is not explicitly covered in Building Regulations Part F, it can be used as an alternative means of compliance when designed and commissioned appropriately. It should meet the general performance requirements for adequate ventilation and indoor air quality as outlined in:

  • Approved Document F (2021 edition)
  • BS 5250:2021 – Management of Moisture in Buildings
  • CIBSE TM59 – Overheating and Indoor Air Quality

Documentation of design intent and performance is recommended for Building Control submission.

Applications

PIV is particularly effective for:

  • Existing homes suffering from condensation or mould.
  • Loosely sealed dwellings with limited mechanical ventilation.
  • Apartments (via wall-mounted PIV units).
  • Retrofit projects requiring minimal disruption.

Flexivent Expertise

Flexivent supplies and installs PIV systems tailored to your property type and occupancy requirements. Our team ensures correct specification, balanced airflow, and compliance with current Building Regulations. Contact Flexivent to discuss how PIV can improve air quality and eliminate condensation issues in your home or project.