Positive Pressure Ventilation, Explained — With a Bicycle Tyre
A positive-pressure ventilation system (PPV) keeps a home cleaner than the outdoor air around it by pushing filtered outdoor air in, slightly raising the internal pressure. Because the inside of the home is at higher pressure than the outside, dirty air physically cannot enter through gaps and cracks. It is the same principle that keeps a bicycle tyre inflated: as long as the pump is faster than the leak, the inside stays pressurised. This page explains why that physics matters for indoor air quality in India.
Key idea in one paragraph
Most ventilation systems try to push dirty air out. Positive pressure does the opposite: it pushes clean air in. The result is that every leak in the building envelope leaks outward, not inward. Outdoor dust, vehicle exhaust and PM2.5 are blocked at the gap they would normally enter through.
Key numbers
- 10–25 Pa — typical positive pressure inside a PPV-equipped home (about the pressure under 1 mm of water)
- 500 m³/h — typical effective fresh-air supply for a 2–4 BHK home
- ~99.95% — particle capture by an H13 HEPA filter (EN 1822)
- Under 10 µg/m³ — indoor PM2.5 we measure in PPV-equipped homes when outdoor air is 200+ µg/m³
The bicycle tyre analogy
A bicycle tyre is sealed but never perfect. There is always a slow leak around the valve or through the rubber. The tyre stays hard because the air pressure inside is much higher than the air pressure outside. The leak goes one way: out.
Now reverse it. A flat tyre has the same pressure inside and outside. Pump in some air and the pressure rises until it pushes back against the rim and the road.
A house works the same way. A perfectly sealed house does not exist. Every door has a gap of a few millimetres. Every window has a seal that has aged. Every electrical conduit and AC pipe punches a small hole through a wall.
- A normal house at equal pressure with the outside: air moves both directions through every gap, driven by wind, temperature, and convection. Pollution leaks in.
- A PPV house: a small fan continuously pushes filtered air in. The pressure inside sits a few pascals higher than outside. Now every gap leaks out. Pollution cannot enter, because air is always flowing the wrong way for it.
This is the same logic used to keep operating theatres free of bacteria, semiconductor cleanrooms free of dust, and aircraft cabins free of outside pressure changes.
Why this matters in India
Three reasons:
1. Outdoor air is bad year-round, not just in winter. Delhi NCR’s PM2.5 sits above the WHO 24-hour limit on roughly 320 days a year. Indoor concentration tracks outdoor concentration closely in any home that exchanges air naturally.
2. Indian homes are no longer “leaky” in the helpful sense. Aluminium and uPVC windows, gasketed doors, and modern construction have reduced natural air-change rates by ~80% versus 1990s homes. The home leaks less heat — and less fresh air.
3. Air purifiers don’t solve the problem. A purifier removes PM2.5 from the room it sits in. It does not remove CO₂, it does not address other rooms, and it cannot stop fresh PM2.5 from entering the moment a door opens.
PPV addresses all three by treating the home as a single pressurised volume, not a collection of rooms.
How a residential PPV system is built
Every PPV system has four parts:
- An intake on an outside wall. A core-cut or glass-cut hole, typically 10 inches in diameter, with a louvered grille to keep out leaves and birds.
- A filtration unit mounted just inside, containing a washable pre-filter and an H13 HEPA filter. This is the “fresh air system” the customer sees.
- A fan — an EC (electronically commutated) motor running at low speed for whisper-quiet 24/7 operation. Variable speed via a potentiometer.
- A duct distributing filtered air into the home, typically opening into a central corridor so air flows passively into all rooms.
The fan runs at very low speed continuously, pulling 30–40 watts. Electricity cost for 24/7 operation in Delhi is under ₹170/month.
What it doesn’t do (and what fixes that)
PPV pressurises the whole home and pushes air in. For the system to work, dirty air has to have somewhere to leak out. In a Delhi home, the natural exit points are:
- The kitchen exhaust — the chimney/exhaust fan is the planned escape route for cooking pollutants. Keep it on whenever you cook.
- The bathroom exhaust — handles humidity and odours.
- Door and window gaps — quiet, continuous outflow
This is also why kitchen pollutants do not spread into the rest of a PPV home: pressure differential plus the kitchen exhaust means air can only move in one direction.
How PPV compares to ERV, HRV and exhaust-only ventilation
- Exhaust-only (EOV): pulls air out, creating negative pressure. Cheap, but it sucks in dirty unfiltered air through every crack — exactly the wrong direction for India.
- HRV (heat recovery ventilator): two-way ventilation that recovers heat between incoming and outgoing streams. Designed for cold European/North American climates. Filtration is secondary.
- ERV (energy recovery ventilator): like HRV but also transfers humidity. Useful in humid climates with HVAC, but expensive and ducted heavily.
- PPV: one-way fresh-air supply with heavy filtration. Built for outdoor pollution rather than energy efficiency. The right fit for Delhi NCR.
A longer comparison lives at ERV vs HRV vs PPV.
What positive pressure cannot do
PPV does three things very well: lowers PM2.5, lowers CO₂, and stops infiltration. It does not:
- Cool or heat the home (use AC and a heater for that — the heater add-on for aqi0 ships from October 2026)
- Replace a kitchen exhaust during cooking
- Replace a bathroom exhaust during a shower
- Clean carpets, mattresses, or upholstery already loaded with dust and VOCs
It is a continuous-supply system, not a magic box.
FAQ
How is positive pressure different from an air purifier? A purifier filters air already inside the room. PPV brings in new outdoor air that has been filtered. PPV also lowers CO₂; purifiers don’t.
How much pressure are we talking about? About 10–25 Pa. That is roughly the pressure exerted by one millimetre of water. You cannot feel it, but every leak in the home now flows outward.
Does it work with the AC on? Yes. The AC recirculates room air for cooling; PPV brings in new air for breathing. They run in parallel.
Will my kitchen smells spread to the rest of the house? No. Positive pressure plus an active kitchen exhaust means cooking air can only move toward the exhaust, not toward the living room.
What if I open a window? The pressure equalises while the window is open and PPV’s protection is reduced. Within 30–40 minutes of closing the window, indoor air returns to a clean baseline.
Is the noise an issue? A correctly sized aqi0 system at normal speed (4–5 V supply) is comparable to a ceiling fan on speed 2 — audible if you listen for it, not noticeable once you stop.