What is Positive Input Ventilation & How Does it Work?

What is Positive Input Ventilation & How Does it Work?

Condensation on windows, stale indoor air and recurring mould can all be signs that a home isn't getting enough effective ventilation.

Opening windows can help temporarily, but it doesn't always provide consistent airflow throughout the property, particularly during colder weather when windows are more likely to remain closed.

Positive Input Ventilation (PIV) offers a simple whole-home ventilation solution designed to introduce a continuous supply of fresh, filtered air and encourage stale, moisture-laden air out of the property.

But how does a PIV system actually work, where is it installed and how do you know whether it's the right ventilation system for your home?

What is Positive Input Ventilation?

Positive Input Ventilation, usually shortened to PIV, is a type of whole-home ventilation system that gently supplies fresh, filtered air into a property.

Unlike an extractor fan, which actively pulls air out of a particular room, a PIV unit introduces air into the home.

This creates a slight positive pressure within the property. As the fresh air enters and circulates, existing stale and moisture-laden air is encouraged to leave through natural leakage points and purpose-provided background ventilators such as trickle vents.

The result is a more consistent movement of air throughout the home.

You can browse Ventilation Superstore's full range of PIV units, including options from Nuaire and Vent-Axia.

How Does a PIV System Work?

A typical PIV installation operates continuously at a low airflow rate.

The process can be broken down into four main stages.

1. Fresh Air is Drawn Into the Unit

In a typical house installation, the PIV unit is positioned within the loft space.

The unit draws air from the loft before passing it through an internal filter.

Alternative PIV systems are available for properties without a suitable loft, including wall-mounted solutions that draw air directly from outside.

2. The Air is Filtered

Before entering the living space, the incoming air passes through the unit's filter.

This helps reduce the amount of airborne particles entering through the PIV system.

The filter needs to be checked and replaced according to the manufacturer's instructions to maintain the performance of the unit.

3. Fresh Air Enters the Home

The filtered air is normally introduced through a diffuser positioned in a central area such as an upstairs landing or hallway.

Rather than producing a strong draught, a correctly selected and configured PIV system supplies air continuously at a relatively low rate.

The incoming air then begins to circulate through the property.

4. Stale, Moist Air is Displaced

As fresh air is continually introduced, a slight positive pressure is created inside the home.

This helps encourage stale and moisture-laden air towards available exit paths, such as:

• Window trickle vents
• Purpose-provided background ventilators
• Natural gaps within the building fabric

Over time, this process helps dilute humid indoor air and create more consistent whole-home airflow.

What Problems Can PIV Help With?

PIV is commonly considered for existing properties experiencing problems associated with insufficient ventilation.

Condensation

Everyday activities generate moisture inside our homes.

Cooking, showering, drying clothes and even breathing all release water vapour into the indoor air.

When warm, humid air reaches a cold surface, such as a window or external wall, the moisture can condense into water droplets.

Improving ventilation helps remove excess moisture before it has the opportunity to accumulate.

The official GOV.UK Existing Home Ventilation Guide explains that ventilation allows fresh outdoor air to replace stale air and excess moisture within the home.

Mould Associated With Condensation

Persistent condensation can leave surfaces damp enough for mould to develop.

By continually diluting moisture-laden indoor air, a PIV system can help reduce the conditions that contribute to condensation-related mould growth.

However, ventilation isn't a solution for every form of damp.

If moisture is entering a property because of:

• A leaking roof
• Damaged plumbing
• Penetrating damp
• Rising damp
• Defective gutters or drainage
• Another building defect

the underlying source of the moisture needs to be identified and repaired.

PIV can help with ventilation-related condensation and humidity, but it shouldn't be used to disguise an unresolved damp problem.

Can PIV Improve Indoor Air Quality?

PIV is also designed to improve the overall exchange of air within the property.

Poorly ventilated homes can allow stale air, excess moisture and indoor pollutants to accumulate.

By introducing a regular supply of filtered air, PIV helps dilute stale indoor air and encourages greater air movement between rooms.

The extent of filtration depends on the individual PIV unit and filter fitted, so always check the manufacturer's specification if filtration performance is particularly important for your project.

Where is a PIV Unit Installed?

The installation depends largely on the type of property.

Loft-Mounted PIV

For houses with a suitable loft, the most common option is a loft-mounted PIV system.

The main unit is installed in the roof space and connected to a ceiling diffuser, typically positioned around a central landing.

Air is drawn from the loft, filtered and gently introduced into the property below.

This keeps most of the equipment hidden away while allowing the diffuser to distribute air from a central position.

Wall-Mounted PIV

Not every property has access to a loft.

Flats, apartments and some houses may instead use a wall-mounted PIV solution.

These systems draw fresh air directly from outside before filtering and supplying it into the property.

The appropriate type of PIV unit therefore depends on both the ventilation requirements and the construction of the building.

What is a Heated PIV Unit?

Some PIV units include an integral heater.

It's important to understand that this doesn't turn the PIV unit into a home-heating system.

The heater is designed to temper the incoming air when required, helping reduce the sensation of cooler air entering the property during colder conditions.

Depending on the model, the heater may operate automatically according to the temperature of the incoming air.

If you're comparing standard and heated PIV units, check the individual product specification for:

• Heater capacity
• Temperature controls
• Airflow settings
• Energy consumption
• Controls and sensors
• Installation requirements

Is PIV Suitable for New Builds?

PIV is more commonly associated with existing properties and retrofit projects.

Modern new builds are generally constructed with much greater levels of airtightness, and their ventilation strategy should be planned as part of the overall building design.

Depending on the property, systems such as MVHR or another purpose-designed mechanical ventilation solution may be more appropriate.

The correct system should always be chosen according to the building's ventilation requirements rather than assuming PIV is suitable simply because it provides whole-home airflow.

Is PIV Suitable for Flats?

Yes, but a traditional loft-mounted PIV unit won't be suitable where there is no accessible roof space.

Dedicated wall-mounted PIV systems are available for flats and other properties where loft installation isn't possible.

Instead of drawing air from a roof space, these units take fresh air directly from outside through an external wall.

Always check the installation requirements and property suitability of the individual unit before purchasing.

Does a PIV Unit Run All the Time?

PIV systems are generally designed for continuous operation.

Running continuously allows the system to maintain steady airflow and manage moisture levels over time.

Turning the unit off for extended periods can remove the ventilation benefit it has been installed to provide.

Many modern units use low-energy fans and provide several airflow settings so they can be configured according to the property.

The exact operating mode and commissioning requirements depend on the model, so the manufacturer's instructions should always be followed.

Will PIV Make My House Cold?

A correctly selected PIV system is designed to introduce air at a controlled rate rather than create strong draughts.

However, unlike MVHR, a standard PIV system doesn't recover heat from outgoing air.

During colder periods, the incoming air may therefore be cooler than the internal temperature.

Some systems are available with integral heaters that can temper the incoming air for additional comfort.

This is one of the factors worth considering when choosing between different PIV models.

Does PIV Need Maintenance?

Yes, although routine PIV maintenance is generally straightforward.

The filter should be inspected and replaced at the intervals recommended by the manufacturer.

You should also make sure that:

• The ceiling or wall diffuser remains unobstructed
• Airflow isn't blocked by stored items around the unit
• Background ventilators remain available where required
• The unit hasn't been switched off
• The system remains securely installed
• Any unusual noise or change in airflow is investigated

Keeping filters clean is particularly important because a blocked filter can restrict the volume of air passing through the system.

What Are the Main Benefits of PIV?

For a suitable property, Positive Input Ventilation can offer several advantages:

• Continuous whole-home airflow
• Fresh filtered incoming air
• Helps manage excess indoor humidity
• Can reduce condensation
• Can help reduce conditions associated with condensation-related mould
• Relatively straightforward retrofit installation
• No extensive whole-house supply duct network
• Options available for houses and flats
• Low-energy continuous operation on many modern units

Its relatively simple installation is one reason PIV is frequently considered when improving ventilation in an existing home.

When Might PIV Not Be the Right Solution?

PIV isn't automatically the best ventilation system for every property.

Another solution may be more appropriate if:

• You're designing a highly airtight new build
• You require heat recovery
• A full balanced supply-and-extract system is required
• The property's damp problem is caused by a building defect
• There aren't suitable routes for air to move through and leave the property
• The proposed unit isn't suitable for the property's size or layout

Identifying the actual ventilation problem first makes it much easier to choose the appropriate system.

Our guide to choosing the right ventilation system compares PIV with other common options including MVHR, MEV and extractor fans.