Is positive input ventilation worth fitting?
How a PIV unit actually reduces humidity, the houses where it transforms a condensation problem, the ones where it will disappoint, and what it costs you in heat.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 5 min
- Safety
- warning
Short answer
It works by dilution: a small fan in the loft pushes filtered air continuously down through a ceiling diffuser, displacing the humid indoor air out through the fabric. In a house with persistent whole-house condensation and no other ventilation, it often does fix it. It is not a substitute for extract fans, and it does add a heating cost.
Positive input ventilation is the retrofit that gets recommended when nothing else has worked, and it has a genuinely good record on exactly the problem it is aimed at: a whole house sitting at high humidity through the winter with mould appearing everywhere at once. It is worth understanding what it does before deciding, because the mechanism explains both its successes and its failures.
Safety
What you'll need
- PIV unit sized for the property
- Registered electrician
- Crawl boards, head torch, mask
- Loft or hallway ceiling access
Step by step
- Confirm the problem is whole-house humidity.Mould in several rooms, high hygrometer readings everywhere, windows streaming throughout the house. That is what dilution ventilation is for. Mould in one cold corner of one room is a surface temperature problem and a PIV unit will not fix it.
- Fix the extract fans first.PIV dilutes moisture after it has spread. An extract fan removes it at source before it does. If the bathroom and kitchen fans are absent or not working, do that first — it is cheaper and more effective per pound.
- Check you have somewhere for the air to go.The unit pushes air in, so an equal volume must leave through the fabric — trickle vents, gaps, an open flue, background leakage. In a genuinely airtight house a PIV unit has nothing to push against and will not perform.
- Check the loft is suitable.The unit hangs from the rafters or sits on a frame above the ceiling, needs a power supply, and needs a reasonably clean loft to draw from. A loft full of dust, old insulation debris or bird droppings needs sorting first.
- Choose loft-mounted or wall-mounted.A flat with no loft needs a wall-mounted or through-wall unit drawing from outside. These work on the same principle but do not get the small temperature benefit a loft gives.
- Decide about the heater option.Many units offer a small pre-heater to temper the incoming air on cold days. It removes the cold-draught complaint and it consumes electricity while it runs, so it is a comfort-versus-cost choice.
- Site the diffuser centrally.Usually the landing or the central hallway ceiling, so the air distributes to all the rooms. A diffuser in a corner bedroom ventilates that bedroom and not much else.
- Run it continuously and leave it alone.It is designed to run all the time at a low rate. Switching it off in cold weather is the same mistake as closing trickle vents, and it takes weeks of continuous running before the fabric humidity comes down.
- Measure before and after.Hygrometer readings in two or three rooms for a fortnight before installation and for a couple of months after. That is the only honest way to know whether it worked.
Tips
- Running costs for the fan itself are very low — it is a small continuous fan. The real cost is the heat carried out by the air it displaces, and a pre-heater if fitted.
- In a loft, the incoming air is often a few degrees warmer than outside on a sunny winter day, which is a small bonus rather than a real heat source; do not buy on that claim.
- If the house has a heat-loss problem as well, spending the same money on insulation and fans may do more. Get both quotes before deciding.
Common mistakes
- Fitting PIV instead of fixing a leak or a cold wall — It manages humidity in the air. It does nothing about water entering the fabric and nothing about a wall that is far below dew point, so in those cases it disappoints and the money is wasted.
- Turning it off because the landing feels cool — It only works by running continuously, and the fabric takes weeks to dry down. Intermittent running produces no lasting change in humidity.
Alternatives
- Good extract fans with humidity sensing and overrun: Cheaper, targets the moisture at source, and is the first thing to get right. Often enough on its own.
- A dehumidifier: Removes water without removing heat, and can be moved to the room that needs it. Higher running cost than a PIV fan while it runs, but no heat penalty and no installation.
- Continuous mechanical extract ventilation: A central low-rate extract unit pulling from the wet rooms continuously. Similar retrofit scale to PIV, works by extraction rather than dilution.
- Insulation: If the mould is on specific cold surfaces rather than everywhere, raising those surface temperatures fixes the cause and reduces the bills too.
Questions people ask
Will it make the house cold?
It introduces outside or loft-temperature air continuously, so there is a heating cost, and some people notice cool air at the diffuser on the coldest days. Units with a pre-heater largely remove the sensation at the cost of some electricity.
Does it work in a flat?
Yes, using a wall-mounted or through-wall unit rather than a loft one. Flats are often good candidates because they tend to be airtight, single-aspect and prone to whole-flat humidity.
Do I still need bathroom and kitchen fans?
Yes. Building guidance treats extract at source and background ventilation as complementary, not alternatives, and in practice removing a shower's moisture at the bathroom door is far more efficient than diluting it after it has reached the bedrooms.
Is it noisy?
A properly sized unit at its normal running speed should be barely audible. Noise complaints usually come from an oversized unit, a boost setting left on, or a diffuser fitted too close to a bedroom door.