Positive input ventilation (PIV) is a quiet fan unit, usually fitted in the loft, that draws in filtered air and gently pushes it into the home through a ceiling diffuser, often on the landing. This slowly replaces moist indoor air and dilutes humidity, which often helps with condensation and mould. It does not fix rising damp, penetrating damp or leaks, so we check the cause before recommending one.
How does a PIV unit work?
A PIV unit is a small, low-energy fan in a housing with a filter. In a house with a loft, it sits in the roof space and takes air from there, which is usually a little warmer and drier than outside air. The air passes through the filter and is fed through a short duct to a diffuser in the ceiling, usually on the landing at the centre of the house.
Because the unit is adding a steady supply of air, the home sits under very slight positive pressure. Moist, stale air is gradually pushed out through the natural gaps in the building — trickle vents, door gaps, extraction routes — and replaced with fresher air. Over time that reduces humidity across the whole house, rather than just in the room with a fan.
Many units have an adjustable fan speed and a heater option that tempers incoming air on very cold days. In flats or homes without a loft, wall-mounted units are available that take air directly from outside.
Is PIV right for my home?
PIV often works well where the main problem is condensation spread across several rooms — bedrooms with mould in the corners, streaming windows throughout the house, a musty smell on the landing. It is less useful when the damp has a different cause.
| Situation | Is PIV likely to help? |
|---|---|
| Condensation and mould in several rooms, worse in winter | Often, yes — this is what PIV is designed for |
| Steam and mould only in the bathroom or kitchen | Better tackled first with a good extractor fan at the source |
| Damp patches that grow after rain | No — the water entry needs repairing |
| Tide marks and salts low on walls | No — this needs diagnosis and possibly rising damp treatment |
| Damp from a plumbing leak | No — the leak needs fixing |
How well PIV works depends on the home: its size and layout, how airtight it is, where the moisture is produced, and whether kitchens and bathrooms still have working extraction. PIV works best alongside effective extractor fans, not instead of them. That is why, as with all our mould and condensation work in Liverpool, we start with a proper look at the property and, where it helps, a short period of humidity monitoring.
What happens during a PIV installation?
- Assessment. We confirm the damp is condensation-led, check the loft space and access, and choose the best diffuser position, usually a central landing or hallway ceiling.
- Quote. You get a written quote that sets out the unit, the position and the electrical work needed.
- Fitting the unit. The unit is mounted in the loft clear of insulation, with a short duct to the ceiling diffuser.
- Electrical connection. Wiring is carried out by a qualified electrician, with the work notified under Part P where the regulations require it and the correct paperwork provided.
- Setting up. We set the fan speed to suit the size of the home and explain the controls, including any heater setting.
- Follow-up. We explain what improvement to expect and how long it can take for walls and furnishings to dry out.
Most PIV installations in a standard house take a few hours. Flats and wall-mounted units can take longer depending on the wall construction.
Please do not wire a PIV unit yourself. Connecting fixed equipment to the mains can be notifiable under Part P and carries a real risk of electric shock or fire. We use qualified electricians for all electrical work.
Looking after a PIV unit
- The filter needs checking and changing from time to time, following the manufacturer’s instructions.
- Leave the unit running — it is designed to run continuously at a low speed.
- Keep the diffuser clear and do not cover it when decorating.
- If you notice the landing feels draughty, the speed or heater setting can usually be adjusted.
PIV in Liverpool homes
Many Liverpool terraces have a straightforward loft and a central landing, which suits a loft-mounted PIV unit. We fit them in older solid-wall terraces in areas like Everton and West Derby, where condensation in back bedrooms is common. Inter-war semis on estates such as Norris Green often have similar layouts. For flats and maisonettes without their own loft, we look at whether a wall-mounted unit is practical.
For landlords, PIV can be one part of a clear, recorded plan to tackle condensation in a rented home, alongside fans and mould treatment. We can provide written notes and photos of the installation for your records.
What affects the cost of PIV installation?
The main factors are the type of unit (loft or wall-mounted, with or without heater), access to the loft, the length of the duct run, the electrical connection needed, and whether mould treatment or other ventilation work is done at the same time. National guide figures are summarised in our damp and mould cost guide; we quote each home individually.
Wondering whether PIV would help your home?
Call 07777 659786 or use the form. Tell us which rooms have condensation or mould and whether you have a loft, and we will call back to talk it through. PIV is one of several options we use for professional mould removal and prevention across Merseyside.
Frequently asked questions
Does a PIV unit make the house cold?
A PIV unit takes air from the loft, which is usually a little warmer than outside, and many units have a heater option that tempers the air on very cold days. Some people notice a slight draught near the diffuser at first; adjusting the fan speed or heater setting usually sorts that. Drier air also tends to feel warmer and is easier to heat.
How much electricity does a PIV unit use?
PIV units are designed to run continuously at low power, so the fan itself uses relatively little electricity. Running costs rise if the heater option is used a lot in cold weather. The manufacturer’s figures for the specific unit give the best guide, and we can talk you through them when we recommend a model.
Can PIV be fitted in a flat?
Yes, in many cases. Where there is no loft, a wall-mounted PIV unit can take air directly from outside through an external wall. Whether it is practical depends on the wall construction, the layout of the flat and, in a block, any permission needed from the freeholder or management company.
Will a PIV unit get rid of mould?
PIV helps prevent mould by lowering humidity, but it does not remove mould that is already there. Existing mould should be cleaned and treated, and materials it has grown into replaced where needed. We usually combine PIV with mould removal and, where necessary, better extraction in the kitchen and bathroom.
How long before I notice a difference with PIV?
Many households notice fewer wet windows within a few weeks, although it can take longer for walls, furnishings and stored items to dry out fully, especially in winter. Results depend on the home and how much moisture is produced. Simple humidity monitoring before and after is a good way to see the change.