There is a quiet assumption built into most ventilation design: that air goes where it is sent. Put a supply vent in a room, connect it to a fan rated for the right airflow, and the room is ventilated. The number on the equipment becomes the number in the room.

It is a reasonable assumption. It is also, in a real building, frequently wrong.

Air is not obedient. It does not follow drawings. It follows pressure, and it takes the path of least resistance — and in a real building, the path of least resistance is rarely the one the designer intended. Before a ventilation system can claim to ventilate a space, the air it moves has to actually arrive there. That journey, from where air enters a building to where it is needed, is where a great deal goes wrong.

Three forces move air through a building: the stack effect, wind, and mechanical systems. Only the last one is under the designer's control.

The stack effect is buoyancy at building scale. Warm indoor air is lighter than cold outdoor air, so in winter it rises, drawing air in at the bottom of a building and pushing it out at the top. The taller the building and the colder the climate, the stronger the effect. Wind adds its own pressure — pushing on one face of the building, pulling on others, shifting constantly. And a ventilation fan adds pressure of its own, trying to direct air where the design wants it.

The mistake is to assume the fan is in charge. It is one pressure source among three, and the other two do not care about the design. As the building-science literature puts it plainly, the pressure differences created by stack effect and wind are often similar to or greater than the ones created by the mechanical system. The fan does not get to overrule the building. It competes with it.

This is most visible in multi-unit residential buildings, where air supplied at a central point has to travel — up shafts, along corridors, through gaps around suite doors — to reach the units it is meant to serve. Every step of that journey is a chance for the air to go somewhere else.

The most thorough field measurement of this comes from RDH Building Science, a Canadian building-science firm. In a study of a thirteen-storey Vancouver building ventilated by a rooftop unit — a building typical of much of North America's residential stock built from the 1970s through the 1990s (and arguably later) — RDH measured what the system actually delivered. The rooftop fan was working correctly, moving its full design airflow. But by the time that air had travelled down the shafts and along the corridors, only a fraction of it reached the suites. Lower-floor suites received roughly an order of magnitude less ventilation than upper-floor suites, driven by the stack effect pulling air upward through the building. Some suites were starved; others were over-served. The system moved the right amount of air. It simply did not arrive where it was designed to.

This isn't a story about one unusual building — RDH is clear the problem isn't unique to the tower they studied. Buildings of that generation, built the same way, tend to move air the same way, and the failure they measured shows up across the type — not as a defect of one building, but as a property of how these buildings work.

A separate BC Housing framework study by the British Columbia Institute of Technology modelled the same problem and reached the same conclusion — and explicitly cited RDH's field measurements as confirmation. In its simulations of a corridor-ventilated building, suite ventilation rose floor by floor with the stack effect, over-ventilating the top and starving the bottom. The report's word for this uneven distribution in existing buildings was inevitable. The field measurement and the model agree: in a real building, air moved on a long journey does not distribute the way the design assumes.

How far the gap opens depends on the building — its height, its airtightness, its exposure, its climate. That variability is itself the point. You cannot reliably predict where a given building will send its air, which means you cannot reliably design around it. This is the heart of why ventilation doesn't reach every room it was designed to serve: the air is moving the whole time, just not to the places the drawing promised.

The same problem appears at smaller scale, in two distinct ways: within a single room, and between suites. Within a single room, ventilation air can enter and leave again before it ever mixes with the air people are breathing — a short-circuit that lets a system deliver its rated airflow while leaving the breathing zone stale. This is a well-described phenomenon in the ventilation-efficiency literature: air can move through a room without doing its work in it. Residential ventilation is low-flow by nature, which makes this easy to do and hard to notice.

Between suites, the mechanism is different: the building's pressure differences push air through whatever paths exist — gaps around doors, penetrations for plumbing and wiring, shared exhaust stacks. The BCIT study examined a low-rise multi-unit residential building from the same era identified in the RDH study, where residents complained of second-hand smoke and cooking smells appearing in suites that were not their source. The mechanism was exactly this: air, and the pollutants it carried, migrating between units along the building's hidden pathways rather than staying where it belonged. The system was, in effect, ventilating the wrong suites with the wrong air.

All of this points to a distinction that matters more than it first appears. There is a difference between the airflow a system is rated to move and the airflow it actually delivers to the people who need it.

The BC Housing framework names two requirements for a ventilation system that genuinely works. The first is responsiveness: providing the right amount of air where it is needed, when it is needed. The second is effectiveness: ensuring that supplied air actually reaches the breathing zone and dilutes the pollutants there, rather than short-circuiting past it. Neither is the same as the number on the fan. A centralized system can meet its rated airflow on paper and still fail both — because most of that air never completes the journey to the room, and what does arrive may never reach the people in it.

Meeting the airflow requirement and delivering the airflow are two different achievements. The journey is the difference between them.

This is where LUNOS decentralized ventilation changes the problem rather than fighting it.

A through-wall system like LUNOS does not move air across the building. It supplies and exhausts at the wall of the room it serves — a short, direct path, no shaft, no corridor, no shared network. It meets the same code-required airflow a centralized system is rated for; the difference is that with no journey to survive, the rated airflow and the delivered airflow are the same number. The air enters the room that needs it and leaves from the room that needs it. There is no long path for the building's forces to redirect.

That is the heart of it: LUNOS works because it doesn't fight building forces over long distances. It does not try to overpower the stack effect or win a pressure contest with the building. It simply removes the journey on which those forces act.

This is also why decentralized ventilation is the right tool for some buildings and not others. The same physics that defeats long-journey delivery sets a limit: in very tall buildings, the stack-driven pressures grow strong enough to interfere with any ventilation strategy, decentralized included. The building-science literature is clear that as a building rises, natural forces become progressively harder for any mechanical system to control. Decentralized ventilation is exceptionally well suited to the low- and mid-rise buildings that make up the vast majority of existing residential stock — and it is not a universal claim for every building of every height. It is the right answer for the right conditions.

The honest counterpoint belongs here too: a centralized system, with its single point of control, can be the better choice when the goal is keeping outdoor pollutants out — sealing the building and filtering everything that enters during a wildfire-smoke event, for instance. Different problems favour different shapes. But for the everyday job of delivering fresh air to the rooms where people actually live, the long journey is a liability, not a feature.

A vent in a room does not mean the room is ventilated. The air still has to get there, and in a real building, getting there is the hard part.

So the useful question about any ventilation system is not how much air its equipment is rated to move. It is whether that air actually reaches the people it is meant to serve — reliably, room by room, in the building as it really behaves rather than as the drawing imagines it. The health of the building and the health of the people inside it both depend on that arrival. Everything else is just air moving somewhere.

At Lunos Canada, this is the problem we work on: ventilation that reaches the room it's meant to, without depending on a journey through the building to get there. If you're weighing ventilation for a retrofit or a new build — or just want to talk through how decentralized ventilation would fit your project — we're always glad to hear from you. Reach out any time.