Key Takeaways
- The unit is fixed to the loft floor or ceiling joists in a central position above the hallway
- A short length of flexible insulated duct connects the unit to a ceiling spigot fitted in the hallway ceiling below
- A ceiling diffuser grille covers the spigot — typically a plain round or square white grille that sits flush with the ceiling
- The unit is wired to a standard 3A or 5A fused spur — this requires a qualified electrician
There are no ductwork runs to individual rooms, no ceiling voids to chase, and no visible pipework. The diffuser in the hallway is usually the only visible sign of the system from inside the property. Physical installation takes most experienced installers two to three hours; the electrical connection adds a further 30–60 minutes. Many homeowners with basic DIY skills handle the physical installation themselves and have an electrician connect the power supply.
For properties without a loft — flats, ground-floor apartments, bungalows with no loft access — wall-mounted PIV units are available that draw air directly from outside through an external wall core drill. These are less common but cover the same ventilation principle.
Do I Need a Heated or Unheated PIV Unit?
This is the most important specification decision when choosing a PIV unit, and for most UK homes the answer is a heated model.
Loft temperatures track outdoor temperatures closely — on a cold January night, the loft may be at 2–5°C. A standard unheated PIV unit would be drawing that air into the living space continuously, which is noticeable and uncomfortable, and can make the system less effective because occupants switch it off or reduce the airflow setting to compensate.
Heated PIV units — such as the Vent-Axia PoziDry Pro with Heater or the Elta PIV ONE PLUS+ — include an integral heating element that activates when incoming air temperature falls below a set threshold, warming the supply air by a few degrees before it enters the home. The heater doesn't run constantly — only when needed — so the additional running cost is modest.
Unheated models are best suited to well-insulated properties where the loft is partially conditioned, or warmer southern UK climates where winter loft temperatures don't drop as severely. If in doubt, go for a heated model.
How Much Does a PIV Unit Cost to Run?
PIV units are extremely cheap to run compared to the alternatives. A typical unit consumes 15–30 watts on its standard continuous ventilation setting — similar to a low-energy light bulb. At current UK electricity rates, this works out to roughly £20–£50 per year for the ventilation function.
When the integral heater is active (on heated models), power consumption rises — typically to 500–1,000 watts while heating. But because the heater only activates when incoming air is cold and turns off once the air reaches temperature, average annual heating cost is typically modest — adding £30–£80 per year in a typical UK winter, depending on the property and climate.
By comparison, a dehumidifier uses 300–500 watts continuously, treats only one room, needs its water reservoir emptied regularly, and doesn't address the root cause of the problem. For a whole-home condensation solution, PIV is the lower-cost option in both purchase price and running costs.
How Long Does a PIV Unit Take to Work?
Most homeowners notice a visible improvement within two to four weeks. Condensation on windows and cold surfaces typically reduces within the first week or two as indoor humidity starts to drop. Surface mould that has formed in corners or on window frames will dry out within the same timeframe and become easier to clean and treat.
For properties with significant existing mould, it's worth cleaning surface mould at the point of installation — PIV will stop it coming back, but it won't remove mould that's already established. Structural dampness (rising damp, penetrating damp through defective masonry or roofing) requires separate remediation; PIV addresses condensation moisture, not water ingress.
PIV vs Dehumidifier: Which is Better?
Both reduce indoor humidity, but they work very differently and suit different problems.
A dehumidifier draws humid room air over a cooled surface, condenses the moisture out, and collects it in a reservoir. It's portable, cheap to buy, and effective in a single room — but it needs regular emptying, uses significant electricity, and doesn't address the source of the moisture problem across the whole property.
A PIV system continuously replaces indoor air with fresh outdoor air, reducing whole-home humidity at source. It covers the entire property, requires minimal maintenance (filter changes every few years), costs less to run, and works automatically without any intervention.
If condensation or mould is limited to one specific room and you don't want a permanent installation, a dehumidifier is a reasonable short-term fix. For whole-home condensation problems — which is the majority of UK cases — PIV is the more effective and cost-efficient long-term solution.
Frequently Asked Questions
What is positive input ventilation and how does it work?
Positive input ventilation is a whole-house ventilation system that introduces a continuous supply of fresh, filtered air into a property from the loft, creating a gentle positive pressure that forces stale, humid air out through natural gaps in the building fabric. It works by running a low-power fan in the loft 24 hours a day, connected to a single ceiling diffuser in the hallway. The result is a sustained reduction in indoor humidity that prevents condensation and mould from forming throughout the whole property.
How much does PIV installation cost?
The unit itself typically costs between £250 and £500 depending on the model and whether it includes an integral heater. Installation cost varies by region and installer, but a typical loft PIV installation — supply and fit, including electrical connection by a qualified electrician — is usually £300–£600 in total. Some installers offer supply-and-fit packages. The unit itself is the main cost; the installation is relatively simple compared to systems like MVHR that require full ductwork.
Is positive input ventilation suitable for flats?
Standard loft-mounted PIV units require a loft space and are not suitable for flats or apartments. Wall-mounted PIV units, which draw fresh air from outside through an external wall, are available for flats and properties without loft access. For flats with multiple wet rooms and higher ventilation requirements, a continuous mechanical extract (MEV) system or MVHR may be more appropriate — contact us at hello@efans.co.uk and we can advise on the right solution.
How often do PIV filters need changing?
Most PIV unit filters should be inspected annually and replaced every two to five years depending on the model and local air quality. Units installed in areas with high pollen counts, dust, or near roads may need more frequent filter changes. The filter is typically located in the loft unit and is accessible without removing the unit from the installation — filter replacement is a straightforward task most homeowners can do themselves. Vent-Axia PoziDry filters and Elta PIV ONE filters are stocked at eFans.
Can a PIV unit run silently?
PIV units are designed to run continuously at low speed, and at their standard operating setting most models are very quiet — typically generating less noise than a domestic fridge. The unit is in the loft with the diffuser in the hallway, so the sound path into the living space is minimal. At boost settings the airflow noise through the diffuser becomes slightly more noticeable, but units like the Vet-Axia PoziDry Pro are specifically designed with anti-vibration legs and acoustic foam to minimise loft noise transmission.
Is PIV the same as MVHR?
No — they are different systems. PIV (Positive Input Ventilation) is a simple supply-only system: one loft unit, one diffuser, introducing fresh air to create positive pressure. It's designed for retrofit in existing homes and is relatively simple and affordable. MVHR (Mechanical Ventilation with Heat Recovery) is a supply and extract system that requires ductwork to every room — extracting stale air from wet rooms and supplying fresh air to habitable rooms, recovering heat in the process. MVHR is more efficient and provides better air quality but is considerably more expensive and disruptive to install. For most existing UK homes with condensation problems, PIV is the right solution. Browse our MVHR range if you're specifying for a new build or major renovation.
Which PIV unit should I buy — Vent-Axia or Elta?
Both are excellent. The Vent-Axia PoziDry Pro range is the most widely installed PIV system in the UK, with a strong service network, well-established filter supply chain, and a 5–7 year warranty depending on model. The Elta PIV ONE PLUS+ uses heavy-duty galvanised steel construction and includes built-in sound attenuation — it's a more robust unit that suits installers and landlords who want something built to last. For most homeowners, the PoziDry Pro with Heater is the go-to choice on value and availability. Read our full Vent-Axia vs Elta PIV comparison for a detailed head-to-head.