How to install a positive input ventilation unit

A PIV unit sits in your loft and gently pushes filtered air down into the hall, raising the pressure across the whole house just enough to push damp, stale air out. It is one of the most effective ways to tackle condensation damp in UK homes, and the installation is within reach for a methodical DIYer. The electrical connection is simple, but position and airflow direction matter a great deal.

Helpful video reference. The video above is from the EASY DIY channel: "Positive Input Ventilation Unit PIV Install". It follows a Vent Axia PureAir Home with Heater installation, which is a common UK unit, and covers the key steps clearly. Watch it end-to-end before you start so you understand how the duct and diffuser fit together.

Before you start. Turn off the circuit you plan to use at the consumer unit and confirm dead with a voltage tester before you make any electrical connections. Check the loft has enough clearance to work safely and that you will be stepping on the joists, not the plasterboard between them. A PIV unit needs a power supply: if there is no socket or fused spur nearby and you are not confident running a new cable, call an electrician for that part.

1. Confirm PIV is the right solution

PIV works by pressurising the house from above, so the loft needs to be reasonably sealed from the outside world and insulated to at least current standards. If warm, moist air is getting into the loft directly from a broken soffit or a missing tile, sort that first or the unit will just move damp air around rather than diluting it.

The ideal candidate is a house with persistent condensation on windows or black mould spots in corners, caused by inadequate ventilation rather than a structural water ingress problem. If water is coming in through the roof or walls, a PIV unit will not fix it.

2. Choose the right unit

UK brands include Vent Axia, Nuaire, EnviroVent and BPC Ventilation. Most loft-mounted domestic units push 20 to 50 litres per second and have an integral spigot heater to take the edge off in winter. Check the unit is rated for your loft temperature range and that the heater wattage is within the rating of the circuit you plan to use.

For a standard three-bedroom semi, a 25 to 35 l/s unit is sufficient. Larger units are not always better: too much positive pressure can cause draughts through trickle vents and make a house feel cold.

3. Position the unit over the hallway

The diffuser should sit in the ceiling of the hallway or landing, as centrally as possible over the stairwell. This placement lets the pressurised air reach every room through the gaps around closed doors. The unit itself goes directly above, secured to a joist or a cross-bearer so it cannot vibrate loose.

Measure and mark the ceiling position from below using a plumb bob or a long, straight rod pushed up through a small pilot hole to find the exact spot in the loft. The diffuser collar is typically 125 mm in diameter.

4. Cut the diffuser aperture

Most PIV units use a 127 mm hole saw. Drill a pilot hole from below first, then cut from below to avoid tearing the ceiling finish. Have someone in the loft to catch any falling insulation or debris. If there is a joist directly where you need the hole, you will need to move the position slightly rather than cut into the joist.

Feed the flexible insulated duct up through the aperture and leave enough slack to connect comfortably to the unit spigot.

5. Secure the unit in the loft

Most PIV units come with a hanging bracket or a base plate. Mount the unit so it is level: the internal condensate tray needs to drain correctly. Keep the unit clear of deep mineral wool insulation so that the air intake can draw properly from the loft space. The manual will specify a minimum clearance, typically 300 mm.

Connect the flexible duct to the unit spigot and tighten the collar clip. The duct should run downward with no kinks or sharp bends.

6. Wire the unit to a fused connection unit

Most loft PIV units run from a standard 13 A ring circuit via a 3 A fused connection unit (FCU). The FCU gives the unit its own isolation point, which is useful for filter changes. Locate the nearest accessible circuit in the loft, be that a lighting cable or a socket feed routed through the ceiling, and connect the FCU into it correctly.

If you are picking up from a lighting circuit, the FCU must be the switched type so the PIV unit has its own switch independent of the lights. Terminate the PIV unit's flex at the FCU's load terminals in line with the manufacturer's wiring diagram.

Confirm the earth is continuous from the FCU through to the unit casing. PIV units are Class 1 appliances.

7. Fit the ceiling diffuser

From below, push the diffuser plate up to the ceiling and connect the duct collar. Most UK units use a push-and-twist or clip-ring fitting. Adjust the directional fins so the airflow points straight down into the hallway rather than across the ceiling. Tighten any retaining screws.

8. Restore power and test

Switch the circuit back on. The unit should start quietly on its lowest speed. After a few minutes, hold a piece of tissue below the diffuser: you should see gentle but definite movement caused by the airflow. Most modern PIV units have an LED indicator that shows operating mode.

Set the boost speed and heater controls according to the manual. In a typical UK home, the low continuous speed is used all year and the heater only activates when the loft temperature drops below around 10°C.

Stop and call an electrician if: you cannot identify a safe circuit to pick up from, the loft has no earthed circuit anywhere nearby, the unit's heater wattage exceeds the circuit's spare capacity, the ceiling has a metal mesh or unusual construction that makes cutting the hole dangerous, or you find any scorched cable or damaged accessories when you access the loft space.

When to call us

The mechanical side of a PIV installation, choosing position and cutting the hole, is straightforward. The electrical part, particularly if it involves running a new cable or adding a spur in the loft, can become Part P notifiable work. If the job needs more than connecting an FCU to an existing circuit, give Richard a call.

Need help with ventilation wiring in Sandwich?

Richard can wire a fused connection unit for a PIV or MVHR unit, or run a dedicated circuit if needed, on the small-jobs rate of £10 per 10 minutes.

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