Positive Input Ventilation (PIV): A Complete Guide

Positive Input Ventilation has become a popular intervention for condensation problems in UK housing. When specified correctly for appropriate properties, PIV systems can be highly effective. But they're not a universal solution, and poor specification leads to wasted investment and disappointed tenants.

How PIV Works

The Basic Principle

A PIV unit is typically installed in the loft space (for houses) or on an internal wall (for flats). It draws air, gently filters it, and introduces it into the property at a controlled rate.

This creates slight positive pressure inside the dwelling, which:

• Dilutes moisture-laden indoor air with drier air
• Encourages stale air to escape through natural leakage points
• Reduces relative humidity throughout the property
• Creates continuous air movement even when windows are closed

Loft-Mounted Units

For houses with accessible loft space:

• Unit draws air from the loft space
• Loft air is typically drier than indoor air
• Some units include heaters for very cold conditions
• Air is introduced through a ceiling diffuser, usually on the landing

Wall-Mounted Units

For flats and properties without lofts:

• Unit mounts on an external wall
• Draws air from outside through a vent
• Filters and (usually) tempers the air before introduction
• More visible than loft units but still relatively discreet

When PIV Is Appropriate

Good Candidates

PIV works well in properties where:

• Condensation is the primary issue: Not penetrating or rising damp
• Ventilation is inadequate: Limited natural ventilation, no MVHR
• Property is reasonably airtight: Too leaky and positive pressure won't build
• Heating is adequate: PIV won't compensate for inadequate heating
• Moisture sources are controllable: Normal occupancy activities

Poor Candidates

PIV is unlikely to resolve issues in properties with:

• Penetrating damp from external defects
• Rising damp from failed DPC
• Severe thermal bridging or cold surfaces
• Excessive draught and air leakage
• Inadequate heating that can't be improved
• Very high moisture production (overcrowding, drying laundry constantly)

Specification Considerations

Unit Sizing

PIV units are rated by air change rate. Typical guidance:

• Small flat (1-2 bed): 0.5-0.75 air changes per hour
• Medium house (2-3 bed): 0.5-1.0 air changes per hour
• Large house (4+ bed): May need larger unit or multiple units

Oversized units can cause draughts and increased heating costs. Undersized units won't achieve adequate air change.

Heating Element

Some units include electric heaters to temper incoming air:

• Prevents cold draughts in winter
• Adds to running costs (typically 20-50W continuously, more if heating active)
• More important for wall-mounted units drawing cold external air
• Some units temper air using loft warmth instead

Controls and Settings

Modern units offer various control options:

• Humidity sensors that increase output when moisture rises
• Temperature sensors that reduce output in very cold conditions
• Boost modes for kitchens and bathrooms
• Manual controls for tenant adjustment

Installation Best Practice

Location

• Central position in the property for even distribution
• Landing or hallway is typical for loft units
• Avoid bedrooms (noise) and bathrooms (moisture source)
• Consider air flow paths through the property

Commissioning

• Verify air flow rate meets specification
• Check no obvious draughts at diffuser
• Confirm controls working correctly
• Set appropriate baseline settings

Tenant Briefing

Crucial for success:

• Explain what the unit does and why it's installed
• Show how any controls work
• Emphasise it should run continuously (not be switched off)
• Explain it complements, not replaces, good ventilation practice

Measuring Effectiveness

What to Monitor

After installation, track:

• Humidity levels: Should see reduction, especially peak levels
• Condensation occurrence: Less frequent, less severe
• Mould growth: Should not recur after proper treatment
• Tenant feedback: Air quality perception, any issues

Expected Improvements

Realistic expectations for a well-specified installation:

• Average relative humidity reduction of 5-15%
• Elimination of condensation on windows (most of the time)
• Prevention of mould recurrence (with initial treatment)
• Improved air quality perception

When It's Not Working

If improvements aren't seen:

• Is the unit actually running? (Check with tenant, verify operation)
• Is the property too leaky for positive pressure?
• Is there a moisture source beyond normal occupancy?
• Was the original diagnosis correct (condensation vs. other damp)?

Costs and ROI

Typical Costs

Return on Investment

Compare against:

• Repeated mould treatment visits
• Redecoration after mould damage
• Complaint handling and officer time
• Potential disrepair claims

For properties with recurring condensation issues, PIV typically pays for itself within 2-3 years.

PIV vs. Alternatives

PIV vs. Extractor Fans

Extractors are better for localised moisture (kitchen, bathroom). PIV provides whole-property ventilation.

PIV vs. MVHR

MVHR (Mechanical Ventilation with Heat Recovery) is more efficient but more expensive and complex to install. PIV is a simpler retrofit solution.

PIV vs. Dehumidifiers

Dehumidifiers remove moisture but don't ventilate. Higher running costs. Better as temporary measure than permanent solution.