Positive Input Ventilation (PIV): Is It the Silver Bullet for London Flats?

The Hype Around “The Robot That Breathes for Your House”

If you’ve been battling damp in London for more than a winter, someone—maybe a surveyor, maybe your neighbour in Hackney—has probably whispered three letters to you: P.I.V.

Positive Input Ventilation units are being marketed aggressively right now. The pitch is seductive: install this white box, and say goodbye to crying windows and black spots forever. But after twelve years in the business, I’ve learned to be skeptical of anything promised as a “miracle cure.”

So, what is it? essentially, a PIV unit is a smart fan. It draws fresh, filtered air from outside (or your loft), pushes it through a filter, and gently pumps it into your hallway. This raises the air pressure inside your flat just a tiny bit.

Think of your flat like a balloon. If you blow air into it, the old, stale, damp air has to rush out through the gaps—trickle vents, under doors, and through keyholes. It forces the moisture out before it can settle on your cold walls.

Why London Flats Are a Nightmare for Airflow

London property stock is… unique. We work in everything from draughty Victorian conversions in Brixton to hermetically sealed new builds in Stratford. Each presents a specific headache that PIV tries to fix.

• The Drying Laundry Issue: It rains here. A lot. You dry clothes on the radiator because you don’t have a tumble dryer or a garden. That moisture has nowhere to go.
• Sealed Shut: In new builds, insulation is so good the property suffocates. In older flats, people seal up chimneys to save on heating bills, inadvertently trapping wet air.
• Security & Noise: If you live on the ground floor in Peckham, you aren’t leaving your bathroom window open while you’re at work. PIV ventilates without unlocking your windows.

I visited a basement flat in Islington last week. The tenant, Sarah, had three dehumidifiers running 24/7. Her electricity bill was astronomical, yet the air still felt heavy. “I feel like I’m living in a Tupperware box,” she told me. This is exactly the scenario where passive ventilation fails.

The Big Question: Does It Actually Work?

Here is the honest answer you won’t get from a salesman: Yes, but with conditions.

We have installed these units and seen condensation disappear within 48 hours. The windows stop streaming. The air smells crisp. It can be impressive. However, calling it a “silver bullet” is dangerous because it ignores the complexity of building physics.

When PIV Fails

A PIV unit manages condensation. It does not fix structural leaks. If your damp is caused by:

• A leaking gutter dripping down the external wall
• Rising damp from a failed damp proof course
• A burst pipe under the floorboards

…then a PIV unit will do absolutely nothing. In fact, you might just be blowing warm air onto a wet wall, which could technically encourage fungal growth in specific conditions. You must fix the source of the water first.

“But I Don’t Have a Loft!”

This is the most common objection we hear in London. Most advice online assumes you have a lovely spacious attic to sit the unit in. If you live in a mid-terrace flat or a basement, that’s not happening.

The Solution: Wall-Mounted Units.

These look a bit like a small air conditioning unit. We install them on an external wall, usually in a hallway or main living area. They pull air directly from outside through a hole we core-drill through the brickwork.

The Nitty Gritty: Costs, Noise, and Cold Air

Let’s talk about the downsides. No technology is perfect, and you need to know what you’re signing up for.

1. “Will it make my flat cold?”

Technically, yes. You are bringing in outside air. In January, that air is cold. However, most modern PIV units come with an integrated heater. This tempers the air before it enters your room. It doesn’t heat the room, but it takes the “chill” off so you don’t feel a draft.

Even without a heater, the theory is that dry air is cheaper to heat than damp air. A damp house takes ages to warm up. So, while you lose a bit of heat bringing fresh air in, you gain efficiency by banishing the humidity.

2. “Is it noisy?”

In a loft? You won’t hear it. Wall-mounted in a small hallway? You might hear a low hum, similar to a fridge running in the next room. Most clients stop noticing it after three days.

3. “What does it cost to run?”

A standard unit (without the heater on) costs pennies a day to run—about the same as an energy-saving lightbulb. If you use the heater version, obviously, that cost goes up, but typically only when the temperature drops below a certain point (usually 10°C).

The Final Verdict

Is PIV a silver bullet? No.

Is it the most effective solution for condensation in London flats where lifestyle changes (opening windows) aren’t working? Absolutely, yes.

If you are a landlord, it protects your investment from tenant lifestyle habits. If you are a homeowner, it improves your air quality drastically. Just remember the order of operations:

Common Client Questions About PIV