What are the drawbacks of positive input ventilation?

The drawbacks are real and worth understanding before committing. Independent guidance warns that forcing warm and moist air into the building fabric can lead to condensation and structural damage². PIV can be cold directly under the fan, may not provide fresh air to the rooms that need it most, and relies on the building fabric and background vents to let air out². It is not a heat recovery system, so the heat in the outgoing air is lost.

This page sets out what PIV does not do, where it falls short of alternatives, and what a household should weigh up. It does not recommend any product or installer.

What positive input ventilation is and how it works

PIV is one of several mechanical ventilation approaches. Independent guidance lists PIV alongside Mechanical Extract Ventilation (MEV), demand-controlled MEV (d-MEV) and Mechanical Ventilation with Heat Recovery (MVHR) as distinct system types⁶. A maker product page describes the principle as introducing filtered air into the dwelling at a continuous rate, encouraging movement of air from inside to outside⁷.

The system depends on somewhere for the air to go. Background ventilation, provided by trickle vents on windows or air bricks in walls, is the usual route, and on its own trickle vents do not provide sufficient ventilation². Door undercuts also matter: they provide airflow between rooms, which helps cross-flow ventilation and reduces the build-up of moist air⁸.

That dependency is the first drawback. PIV does not extract air; it supplies it. If the outlets are blocked, closed or undersized, the air has nowhere to go and the system cannot do its job. Independent guidance notes that trickle vents provide effective background ventilation and allow constant ventilation while a window is closed or locked, with an installation footprint that does not enable intrusion⁹. But they must be open to work.

The SAP calculation reflects this. When PIV is entered, the SAP assessor must enter a minimum of two intermittent extract fans⁴. In other words, the standard methodology does not treat PIV as a complete ventilation strategy on its own. It is a supply system that sits alongside extract provision, not a replacement for it.

Drawbacks of PIV: heat loss and running costs

The central drawback of PIV is that it has no heat recovery. Air is drawn from the loft or from outside and pushed into the home, and the warmth in the air leaving the building is lost. Independent guidance on MVHR makes the contrast explicit: for heat recovery to work efficiently, the MVHR unit and ducting need to be within the insulation layer, not in an uninsulated loft⁵. PIV has no such recovery stage at all.

Running costs depend heavily on whether the unit has a heater. One maker states that non-heated units can operate for less than 1p a day³. That is a maker's figure for its own products, not an independent measurement, and it excludes heated operation. The same maker notes that many units come with optional heaters to temper incoming air during colder months³. A heater changes the running cost picture entirely, and no independent UK figure for heated PIV running costs appears in the evidence reviewed here.

Installation cost is quoted rather than published. One maker states that the price typically includes site surveys, installation and a warranty³, but no figure is given. Prices for PIV are installer-quoted.

The wider context is that ventilation and heating interact. Poor ventilation can trap moisture and everyday pollutants indoors, making conditions like asthma and allergies worse over time¹⁰. But ventilation that loses heat works against the household's energy position. A room-sealed appliance or one with a directly connected ducted external air supply minimises ventilation energy losses from the room and the risk of cold draughts¹¹. PIV does the opposite: it deliberately introduces outside air into the living space.

Where PIV falls short compared with MVHR

MVHR and PIV are not equivalent systems, and the differences matter most in airtight homes. Independent guidance states that whole-house systems like MEV or MVHR offer more consistent results, especially in newer, more airtight homes¹². PIV, by contrast, relies on the building fabric and background vents to release the air it supplies.

MVHR recovers heat; PIV does not. Independent guidance on Passivhaus describes mechanical ventilation with heat recovery as a defining feature of the standard¹³, and a maker source makes the same point about Passivhaus reliance on MVHR¹⁴. The Energy Saving Trust notes that Passivhaus buildings use mechanical ventilation with a heat recovery system attached, and that the technique has higher upfront costs alongside significant cost savings¹³.

MVHR also has stricter installation conditions. Independent guidance states that the MVHR unit and ducting need to be within the insulation layer, not in an uninsulated loft⁵. Official guidance recommends that the MVHR unit should preferably be a Passivhaus Certified Unit⁵. And MVHR suits a particular airtightness range: recommendations vary, but for mechanical ventilation to be appropriate a home will probably want less than half of about 5m3/h.m2@50Pa air permeability⁵.

A maker source notes that a competing mechanical system effectively reduces humidity levels but is more expensive than other mechanical ventilation systems¹⁵. Cost is a genuine differentiator, and PIV is generally the simpler and cheaper intervention. But simplicity is not the same as performance, and in a well-sealed home the absence of heat recovery and the reliance on fabric leakage are real limitations.

Condensation and mould: what PIV does not fix

PIV is often sold as a condensation cure, and the evidence is mixed. Independent guidance warns that forcing warm and moist air into the building fabric can lead to condensation and structural damage². That is the mechanism by which a supply-only system can make things worse rather than better if the moisture has nowhere to escape.

A maker case study reports the opposite. It describes a house refurbishment where the condensation had gone after installing a PIV unit, and returned when the unit was switched off for a week¹⁶. That is a single case study from a manufacturer, and it is not independent evidence.

The two positions are not reconciled in the evidence reviewed here. The outcome appears to depend on the property: how leaky it is, whether background vents are open, and whether the moisture source has been addressed.

What PIV does not fix is the underlying cause. Independent guidance states that problems with damp and mould need to be fixed before other energy saving measures like insulation can be installed¹⁷. Independent guidance on cavity wall insulation makes a similar point: resolve damp issues and fix leaks before installing insulation, as trapping moisture inside can create problems¹⁸. A ventilation system that moves air around a damp building does not repair the damp.

Independent guidance also sets out the health stakes. BEAMA's guidance details the serious health impacts caused by a build-up of condensation and mould in the home, and provides three key ventilation solutions: extractor fans, trickle vents and whole home ventilation systems¹⁹. PIV is not among the three named solutions in that list.

Noise, draughts and loft air quality

PIV units run continuously, and continuous operation brings continuous noise. A maker source describing single flow mechanical ventilation notes that it is permanently active and can therefore generate draughts, noise and heating losses¹⁵. PIV is a single flow system, so the same characteristics apply.

The draught issue is specific: PIV can be cold directly under the fan². That is the point where incoming air enters the room, and in winter it is the coldest spot in the house. Units with optional heaters temper the air, but the maker's own framing is that the heater is an option rather than a standard feature³.

Loft air quality is the other concern. PIV draws air from the loft, and loft conditions vary. Independent guidance notes that loose-fill loft insulation can blow around in draughty lofts¹⁸. A filter is the barrier between that environment and the living space, which is why the maintenance interval matters. One maker states that filters typically need cleaning or replacement every five to seven years for loft mounted systems and every 12 to 18 months for wall mounted systems³.

Noise from outside is a related trade-off. Trickle vents provide ventilation without the need to open windows, reducing noise levels as a consequence, and acoustic options are available in noisier locations⁹. A PIV unit does not offer that acoustic benefit in the same way, because it is a fan running in the loft rather than a passive opening.

Draught-proofing interacts with all of this. Official guidance notes that while draught-stripping is valuable for energy efficiency, it does reduce the free air within the building, and the practitioner should demonstrate how the proposals manage this²⁰. Sealing a home while relying on fabric leakage for PIV exhaust is a contradiction worth resolving before installation.

Is PIV right for your home?

PIV suits some properties and not others. One maker states that PIV systems are suitable for homeowners, landlords and housing providers³, and a maker product page describes PIV as reducing condensation and preventing mould creep in older homes⁷. The same maker notes that older homes with natural gaps may not require trickle vents, while highly sealed modern properties could benefit from additional background ventilators³.

That last point is the crux. PIV works with leakage; airtight homes need a different approach. Independent guidance is clear that whole-house systems like MEV or MVHR offer more consistent results in newer, more airtight homes¹².

For flats and basements without a loft, one maker states that PIV units can be wall-mounted, providing the same airflow benefits as loft installations in houses³. That opens the option to properties that would otherwise be excluded. For loft conversions, the open loft space a loft mounted unit needs is gone, so a wall-mounted unit is the relevant variant.

On approvals, one maker states that PIV systems must be installed by qualified professionals and comply with Building Regulations³. Official guidance notes that internal alterations will probably need building regulations approval²¹. Installing insulation requires an application for approval under the Building Regulations, which can be included as part of an application for the overall project²². Planning permission for internal wall insulation is usually not required, though both planning and listed building consent may be required in some situations²⁰.

The independence question is worth stating plainly. PIV reduces reliance on opening windows and can improve air movement in a leaky home, but it does not reduce reliance on the grid, a supplier or gas. It adds a continuously running fan and, if heated, a continuous electrical load. It has no heat recovery, so it works against the home's thermal efficiency rather than with it. For a household pursuing energy independence, PIV is a comfort and condensation measure, not an energy measure.