PIV units, and the question that decides whether yours is worth buying
A positive input ventilation unit works on moisture in your air, which makes it a real answer to condensation and no answer at all to the other two kinds of damp. The manufacturers' own wattages costed against the current capped rate, the loft inspection they ask for first, and why every published running cost in this market is about the heater.
By The Damp GuyLast updated
The short version
A PIV unit gently pressurises your house with filtered air from the loft, pushing moisture-laden air out through the gaps the building already has. So it acts on water that is in your air. That makes it a genuine fix for condensation and the mould that follows it, and it does nothing whatever for rising damp or penetrating damp. The fan is cheap: 6.0W on a three-bedroom setting, which is £13.83 a year at the capped 26.32p per kWh. The heater element is 400W or 500W, roughly 83 times the fan, and that is where the whole cost argument actually lives.
A PIV unit reaches the air in your house and nothing else
That one sentence settles most of the buying decision, and it is the sentence almost nobody selling one writes down.
| Kind of damp | Does a PIV unit help? | Why |
|---|---|---|
| Condensation | Yes | The water is suspended in your air, which is the one place the unit can reach. It dilutes and displaces that air continuously, and unlike opening a window it does not throw your heat out with it. |
| Penetrating | No | Water is crossing the wall from outside at a specific defect. Moving air around the landing does not reach the outside face of a wall, and the wall refills as fast as the room dries. |
| Rising | No | Moisture moving up through the structure is not in the air until it has already come through. Pressurising the hallway changes nothing about what is arriving at skirting height. |
That is not a controversial claim and it is not mine. It is in the ranking set, once. Out of the ten UK pages that come up for this search, the one that states it plainly is the one published by a firm that sells portable power rather than ventilation: "If you have a "rising damp" issue or a leaking pipe behind a wall, no amount of airflow will fix it. PIV deals with the air, not structural water ingress." The same page adds that structural issues have to be repaired first.
Everybody else in that top ten either makes PIV units, fits PIV units, or sells the damp treatment PIV competes against. None of them is lying about the mechanism. They simply have no reason to open with the sentence that disqualifies two thirds of their enquiries, and a homeowner reading nine pages in a row can finish all nine without meeting it.
So the test comes before the purchase, and it is free. Condensation tracks the cold and gets worse on the coldest mornings. Penetrating damp tracks the rain. Rising damp tracks neither and sits at a fixed height regardless of the weather. If yours is one of the last two, a four-figure quote for positive input ventilation is money spent on the wrong variable, however well the unit works.
The loft inspection the maker asks for before anybody fits one
It is printed in Nuaire's own installation leaflet, it costs a torch and ten minutes, and no consumer page in the country reproduces it.
A PIV unit is not a machine you drop into a loft. It is a machine that assumes seven things about the loft it is going into, and the installation instructions list them before they say a word about mounting anything. Here they are, in the manufacturer's wording rather than mine, because a paraphrase of an installation instruction is an opinion.
- Check to ensure that the loft has adequate ventilation. There may be occasions where a loft is so well sealed that additional ventilation may have to be provided by the owner/occupier.
- Ensure that all water tanks are covered and sealed.
- Check that all water pipes are lagged.
- Ensure that any extract fans are discharging to outside and not into the loft.
- Check that the loft hatch is tightly sealed.
- Ensure that all holes in the ceilings are sealed i.e. ceiling light fittings etc.
- A visual inspection of any flues or chimneys for leakage in the loft should be carried out by the installer.
Read the fourth one twice. An extractor fan that discharges into the loft instead of outside is a genuinely common defect that passes every casual check, and it is putting a bathroom's worth of water vapour into the space a PIV unit is about to draw from. Fit the unit without finding it and you have built a circuit: the shower steam goes up into the loft, and the unit pushes it back down onto your landing, filtered.
The sixth one is the same shape of problem. Unsealed ceiling penetrations, downlighters especially, are the route warm wet air takes into a cold roof void, and they are the usual reason for condensation and blackening on the underside of a roof. Sealing them is on this list because the unit needs a dry, ventilated loft to draw from. Which is worth noticing. Preparing a house properly for a PIV unit means fixing the two defects that most often cause the damp in the first place.
And the seventh is a safety item rather than a performance one. Pressurising a dwelling changes the pressure a chimney or an open flue is working against, so the same document carries a flat instruction: "Precautions must be taken to avoid the back-flow of gases into the room from the open flue of gas or other fuel-burning appliances." If you have an open fire, a back boiler or an open-flued gas appliance, that is a conversation to have with the installer before the unit goes in, not after.
You can run most of this yourself in an evening. If a quote arrives without any of it having been looked at, you now know what to ask.
The cold air question has an answer, and it is a setting rather than a reassurance
Air from your loft in January is cold air. Every maker builds a temperature response into the unit, and they publish the thresholds.
This is the objection that comes up more than any other, and the market answers it with adjectives. The sellers say the effect should not be severe. The operators are more useful: a housing association tells its own tenants how to check the unit is running, and the instruction is to hold a hand in front of the grille and feel for "gentle, cool air". That is the honest description. The air is cool, because it came out of your loft.
What it does about that is published, and it is worth reading because the cautious setting is not the one the unit arrives on.
- Nuaire, factory default (option 1). Below 19C in the loft it runs normally. Above 18C and below 24C it boosts to recover heat. Above 23C it goes to standby.
- Nuaire, option 5. Above 27C or below 5C in the loft, the unit switches itself to standby. This is the only setting that stops it in the cold, and it is not the default.
- Vent-Axia. A loft sensor adjusts air volume automatically. Above 27C in the loft, adjustable, the unit enters standby.
- EnviroVent. Above 25C in the property, most units go into sleep mode and switch back on when it drops.
The one to notice is Nuaire's option 5, which parks the unit below 5C in the loft. That is the manufacturer's own answer to somebody who cannot bear the cold draught in deep winter, and it is not the factory default. On the default setting, option 1, the unit carries on supplying loft air at whatever temperature the loft happens to be. If your house feels colder since one went in, the setting is the first thing to ask about.
Notice also what those thresholds tell you about the rest of the year. Above 23C, 25C or 27C, depending on the maker, the unit stops. Between the two ends it boosts instead. A loft warmed by the sun is free heat, and pushing it down into the house recovers warmth that would otherwise go out through the roof. Nobody selling a PIV unit is pretending it is a way of heating the house, and EnviroVent says so in terms about its own heater: "Although a Positive Input Ventilation unit comes with a pulse heater, this is used to temper the air should the temperature outside drop, but it's not a form of heating to be used within a property."
What it costs to run, with the arithmetic shown
Published wattages from two manufacturers' own datasheets, multiplied by hours, multiplied by the capped 26.32p per kWh. That is the entire method.
| What is running | Published draw | An hour | Over a year |
|---|---|---|---|
| Nuaire Drimaster-Eco LC at its stated minimum, running always | 1.6 W | 0.04p | £3.69 |
| Vent-Axia PureAir Home, one-bedroom trickle, running always | 3.1 W | 0.08p | £7.15 |
| Vent-Axia PureAir Home, three-bedroom trickle, running always | 6.0 W | 0.16p | £13.83 |
| Vent-Axia PureAir Home, three-bedroom energy recovery, running always | 12.0 W | 0.32p | £27.67 |
| Nuaire Drimaster-Eco LC at its stated maximum, running always | 17 W | 0.45p | £39.20 |
| Nuaire's 400W heater element, one hour a day | 400 W | 10.5p | £38.43 |
| Vent-Axia's 500W heater element, one hour a day | 500 W | 13.2p | £48.03 |
Look at the gap that opens between the fan rows and the heater rows. The fan on a three-bedroom setting draws 6.0W. The heater element draws 500W. That is about 83 times as much, and it reframes the question people actually ask. Running a PIV unit costs a few pounds a year. Running a heated PIV unit costs whatever its heater costs, and everything else is noise.
The sharpest way to see it: switch that 500W element on for one hour a day and it comes to £48.03 across the year. The fan, running every hour of the same year, comes to £13.83. One hour a day of heat costs 3.5 times what the ventilation costs altogether.
Those are my sums, not a manufacturer's claim, and they are deliberately simple so you can redo them. Wattage divided by 1,000 to get kilowatts, multiplied by hours, multiplied by your own unit rate. Nothing here accounts for the unit dropping into standby in summer or boosting on a warm afternoon, both of which move the real figure. The point is the order of magnitude, and the order of magnitude is not in dispute.
One comparison worth holding on to, since it is the machine most people weigh this against. A dehumidifier draws 150 to 260 watts as a compressor unit and around 330 watts as a desiccant, for one room with the door shut. The PIV fan draws single figures for the whole house. That is a real argument in the unit's favour, and it is an argument about condensation only.
Five published running costs, checked against one dated rate
Every figure in this market is asserted rather than worked out, and two of them rest on an electricity price that no longer exists.
| Who published it | What they say | The rate behind it | Checked at 26.32p |
|---|---|---|---|
| Homes Plus | around £30 a year to run | 15p per kWh, stated | £30 at 15p is 200 kWh a year, which is a continuous draw of about 22.8W. The same 200 kWh at the capped rate is £52.64. |
| PIV UK | about £7 a year, or under 2p a day, for a fan motor drawing about 5W | not stated; the two figures imply about 16p per kWh | 5W running always is 43.8 kWh a year, which at the capped rate is £11.53. |
| Permagard | as little as 1p to 2p per day for an unheated unit | not stated | At the capped rate that is a draw of 1.6W to 3.2W, which is the bottom of Nuaire's own published range rather than a mid setting. |
| Permagard | up to £100 a year for a heated unit in a large house | not stated | £100 at the capped rate is 380 kWh, which is about two hours a day of a 500W heater across the year. |
| Evergreen Power | as much as £1 a day with the heater running, on a 500W element | not stated | £1 a day at the capped rate is 3.8 kWh, which is about seven and a half hours a day of a 500W heater. |
The first row is the useful one, and it is the only publisher in the set that showed its working. A housing association fitting these units across its own stock tells tenants they cost around £30 a year, "based on an average electricity unit price of 15p". That is an honest figure and a stale one. The same consumption at the capped 26.32p is £52.64. Nothing about the unit changed. The price of electricity did.
The second row is the same problem without the disclosure. An installer quotes about £7 a year for a 5W motor, which only works if electricity costs about 16p. At today's capped rate the same 5W comes to £11.53. Neither figure is dishonest and both will be repeated in a sales conversation next winter, which is the whole reason for printing the sum rather than the answer.
Two of the rows are about the heater, and notice how far apart they are. One seller puts a heated unit at up to £100 a year, which works out at roughly two hours of heating a day. Another puts it at as much as £1 a day, which is seven and a half. Both are describing the same 500W element. What separates them is how cold the loft is and how the thermostat is set, and neither publisher says so.
What the box costs, and what four publishers say the job costs
The hardware is well documented and the fitted price is not. The published ranges for the same job differ by a factor of ten.
Start with the part that is easy to check. These are the listed prices at one UK retailer on 11 September 2026, including VAT. One merchant, one day, and they will move, so treat them as a shape rather than a quote.
- £361.80 Nuaire Drimaster Eco LC with Loft Control, for a property with a loft.
- £344.70 Nuaire Flatmaster, for a property with no loft.
- £429.60 Nuaire Drimaster Eco with Hall Control, for a property with a loft.
- £469.50 Nuaire Flatmaster 2000 Heat, for a property with no loft, heated.
- £508.80 Nuaire Drimaster Eco Link HC, for a property with a loft.
- £678.30 Nuaire Drimaster Eco Heat HC, for a property with a loft, heated.
The heater is the biggest single decision on that list. Going from the cheapest loft unit to the heated one in the same range costs £316.50 more to buy, and then it is the component that carries almost all the running cost. Both of those figures point the same way, which is at least consistent.
Now the fitted price, which is where the published answers come apart.
| Who published it | What the figure covers | The figure |
|---|---|---|
| Permagard | the unit alone, before fitting | £300 to £1,000 |
| EcoFlow | the unit and professional installation | £400 to £1,000 |
| PIV UK | supply and install, fixed price including VAT | £995 |
| Evergreen Power | the system including installation | £2,500 to £4,000 |
Those are four publishers describing one job, and the top of the highest range is ten times the bottom of the lowest. They are not averaged here and they should not be averaged anywhere: collapsing them into a midpoint would invent a precision none of them supports, and would throw away the only thing the rows are useful for.
What they are useful for is the question you take into the conversation. The hardware is listed publicly at under £700 including VAT even for the heated loft unit. The retailer selling it says the installation "should take under an hour" once an electrician makes the mains connection. So if a quote lands at four figures, the honest question is not whether it is too much. It is what the difference is buying: an assessment, a longer job than an hour, work on the loft itself, a guarantee, or margin. Any of those can be a fair answer. Not being told which is the problem.
Worth checking either way: three-storey properties need more than a standard install. Nuaire makes a smoke or carbon monoxide alarm interlock mandatory there, and EnviroVent describes fire-stopping requirements above 4.5 metres plus a specialist metal diffuser. That is a real cost difference with a stated reason behind it, and it is the kind of line a quote should name.
Against the alternatives, and what each one needs to already be true
Every remedy here pulls the same lever: moisture in the air. What separates them is reach, and what each one silently assumes about your house.
| The option | Where it acts | What it needs to already be true | What it will not reach |
|---|---|---|---|
| PIV unit | The whole house, from one ceiling diffuser at its centre | A loft that is already well ventilated, ceiling penetrations sealed, and leakage paths for the air to get back out | A wet room that is not off the same central space, per its own makers |
| Extract fan at the wet room | At the point the moisture is made, before it reaches anywhere else | A duct that genuinely discharges outside rather than into the loft | Any room it does not serve |
| Dehumidifier | The air in one room, door shut | A room warm enough for the type you bought, and nothing resupplying it | Water arriving through the structure, at any rate at all |
The middle column is the one to sit with, because it is where the sales conversation usually skips. A PIV unit is sold as a whole-house answer and it is one, on conditions. Two makers say the same thing about the limit in their own documents. Vent-Axia: "En-suites and utility areas should be serviced by continuous mechanical extract ventilation." EnviroVent says a wet room not connected to the hallway needs its own continuous ventilation. So a bathroom behind a bedroom door is outside the unit's reach and still needs extraction at the published rate for that room.
There is a condition at the other end too, and it catches newer houses. EnviroVent: "This wouldn't be a suitable solution for extremely airtight properties, as there's a need for airflow between rooms and atmosphere." The unit works by pushing air in and letting the house leak it out, so a very well sealed property has nowhere to put it. That is the one case where a draughty old house is the easier customer.
None of these three is a repair. They manage moisture in the air, and government guidance on damp and mould is blunt about the difference between clearing a symptom and dealing with a cause. If the air is already dry and the wall is still wet, none of the three was ever going to be the answer, and an inspection by somebody with no interest in selling you the treatment is the money better spent.
About this resource
The wattages and airflow figures are the manufacturers' own published specifications, read on 11 September 2026: Vent-Axia's Lo-Carbon PureAir Home datasheet for the bedroom-by- bedroom performance guide and the 500W heater, and Nuaire's DRI-ECO-LC datasheet and installation leaflet 671754 for the 1.6W to 17W range, the speed settings, the temperature control options and the loft inspection. Nothing in those figures is measured by me. The unit rate is Ofgem's published cap of 26.32p per kWh with a 54.83p daily standing charge, for 1 October to 31 December 2026, Direct Debit, England, Scotland and Wales. Every running cost on this page is my multiplication of those two sourced numbers, shown as arithmetic so you can check it against your own tariff.
Two sourcing limits, stated rather than hidden. The two Nuaire PDFs were read from the copies hosted by a retailer of the unit, because Nuaire publishes no direct link to them from its own product pages. And the one page in the ranking set that sells damp treatment returns an error to automated reading, as does the only non-commercial result on that page, so neither contributes anything here. Nothing about what owners report appears on this page for that reason.
No independent evaluation of PIV could be found. Nuaire's filter page says its filters are "BRE tested and proven to improve air quality" without a report reference, and I could locate no consumer body, government or research assessment of the technology to read. That absence is reported rather than filled in with a seller's claim, and it is the same situation this site found the last time it looked at PIV. The prices are one retailer on one day for the hardware, and four separate publishers for the fitted job, kept apart and attributed because they disagree.
No unit is recommended and no installer is named, because nothing on this site earns a commission and no testing has been done. There is no noise figure either: no datasheet I read publishes one in decibels, and "almost silent" is an adjective, not a measurement. This site does not survey, treat or price work, and it cannot tell you what is happening in your building. If your air is dry and the wall is still wet, that is the point to get an independent inspection.
Common questions
Do PIV units actually work?
On condensation, yes, and that is the whole of it. The unit pushes filtered air in continuously so moisture-laden air is forced out through the gaps a house already has, which acts on water that is sitting in your air. It does nothing about water arriving through a wall or up through a floor. The bluntest statement of that in the whole market comes from the one publisher in the top ten with no PIV to sell: "PIV deals with the air, not structural water ingress." Work out which kind you have before you spend anything, because two of the three are not air problems.
What are the disadvantages of a PIV unit?
Four, and they are mostly conditions on your building rather than faults in the machine. The air comes from your loft, so a cold loft in January delivers cold air, and the fix for that is a heater element drawing 400W or 500W against a fan drawing single figures. It needs a loft that is already well ventilated and a ceiling that is already sealed, which is a condition, not a feature. Both Vent-Axia and EnviroVent state that an en-suite or a kitchen off the central space still needs its own continuous extract, so it is not a whole answer. And EnviroVent says plainly it is unsuitable for very airtight properties, because the air has to get back out again.
How much does it cost to install a PIV system in the UK?
Four publishers give four different answers and the spread is close to tenfold. EcoFlow says £400 to £1,000 for the unit and professional installation. PIV UK sells a fixed-price national supply and install at £995 including VAT. Evergreen Power says £2,500 to £4,000 including installation. Against that, a retailer lists the Nuaire loft units themselves at £361.80 to £678.30 including VAT. Nobody is wrong, exactly, but they are not describing the same job, and that is the thing to establish before you accept a number.
Where should a PIV unit be installed?
In the ceiling at the centre of the house, not in a room. Nuaire specifies the central hallway in a single-storey property, or the ceiling of the top-floor landing in anything with two or more storeys. Vent-Axia says over the stairwell in a conventional house, or the main hall in a bungalow. The unit itself sits in the loft above it. The point of the position is that air has to be able to reach every room from one diffuser, which is also why a room you have to walk through another room to get to is the one it will not serve.
Is a PIV unit better than a dehumidifier?
They solve the same problem at different scales and different costs. A dehumidifier draws 150 to 260 watts as a compressor unit, or around 330 watts as a desiccant, and treats one room with the door shut. A PIV fan draws single-figure watts and treats the whole house from one diffuser, but it needs a ventilated loft, sealed ceiling penetrations and leakage paths out. Neither touches water arriving through the structure. If the problem is one cold bedroom, the machine you can move is the cheaper experiment. If it is the whole house, the one that runs on 6 watts is the cheaper habit.
Can an electrician install a PIV unit?
An electrician is required for part of it, and for a straightforward loft installation that is usually the whole job. Nuaire states that the connection to the mains "must be carried out by a qualified electrician". EnviroVent says the unit "needs to be installed by a person competent to do so" and that the work involves an electrical connection plus setting the unit up for the property. Two cases need more than that: a three-storey property, where Nuaire makes a smoke or CO alarm shutdown interlock mandatory, and anywhere with an open-flued gas or solid-fuel appliance, where the instructions require precautions against back-flow of flue gases.
Do PIV units use a lot of electricity?
The fan does not and the heater does. Vent-Axia publishes 6.0W for a three-bedroom house on trickle, which is 0.16p an hour and £13.83 across a whole year at the capped 26.32p per kWh. Nuaire publishes 1.6W to 17W for its unit, so £3.69 to £39.20 a year on the same sum. The heater element is 400W or 500W. Running the 500W element for one hour a day comes to £48.03 over a year, which is three and a half times what the fan costs running every hour of that same year. So the honest answer is that a PIV unit is cheap to run and a heated PIV unit is a different question.
Is my PIV unit making my house cold?
It is introducing loft air into your landing continuously, so in January it is introducing cold air, and the operators know it. A housing association tells its own tenants to check the unit is working by holding a hand to the grille and feeling for "gentle, cool air". What to do about it is a setting, not an opinion. Nuaire publishes five temperature control options and the factory default is not the cautious one: option 5 puts the unit into standby below 5C in the loft, and the default, option 1, does not. Ask which option yours is set to. The other answer is a heater element, which costs money to buy and more to run.
Will a PIV unit stop loft condensation?
Not directly, and the relationship runs the other way round. The unit needs the loft to be well ventilated before it goes in, not after. Nuaire's own installation instructions open with checking that the loft has adequate ventilation and note that a loft can be so well sealed that the owner has to add more. The same checklist then asks for every ceiling penetration to be sealed and for extract fans to discharge outside rather than into the loft, which are the two things that cause loft condensation in the first place. So a correctly prepared PIV installation fixes the causes on the way past. The unit itself is downstream of them.
Is it okay to use a PIV unit in the summer?
Yes, and it largely decides for itself. All three major makers build in a shutdown, and they each pick a different number, which is worth knowing rather than averaging. Nuaire's factory default drops the unit into standby once the loft goes above 23C. Vent-Axia uses 27C, adjustable. EnviroVent says most of its units sleep above 25C in the property and wake up when it drops. Below those thresholds the units run heat recovery mode instead, boosting airflow to pull warmth out of the loft before it escapes through the roof. Nothing here needs switching off by hand, and Nuaire specifically says the unit must not be wired through a timer.