Air has a supply contract

Every room needs a continuous supply of fresh air, just like water and power — yet it is the only supply nobody measures. Standards fix it with two criteria at once: air changes on the volume (how many times per hour all the air in the room must be replaced) and airflow per person (the hourly cubic metres each occupant consumes by breathing). And the rule almost everyone ignores is that the stricter of the two criteria commands: a large empty hall is governed by the volume, a small crowded room by the people.

This calculator runs both checks: from the area, the height and the required air changes it derives the required airflow, compares it with the installed one in a coverage index, and checks the hourly cubic metres left to each occupant.

The reference — 80 m² with a 2.7 m ceiling, 0.5 air changes per hour, a 150 m³/h machine, four people — requires 108 m³/h and is covered with an index of 1.39: compliant, with reserve for the peaks.

The formula: one volume, two checks

The required airflow is the room volume multiplied by the required air changes: 80 × 2.7 = 216 m³ of air, which at 0.5 volumes per hour means 108 m³/h. The coverage index is the ratio of installed to required airflow: 150 / 108 = 1.39 — the machine delivers 39% more than the minimum.

The second check divides the same airflow by the people: 150 / 4 = 37.5 m³/h each, above the 30 that comfort standards indicate as the per-person reference. When this second threshold is not met, the verdict flags it even if the volume-based air changes suffice: air is sized twice, and the worse count wins.

Note the physical meaning of 0.5 vol/h: all the air in the home replaced every two hours. It sounds like a lot, and it is the typical residential minimum — because four people, a kitchen and a bathroom produce steam and CO₂ without pause.

How to read the coverage index

Below 0.75 the airflow is severely insufficient: more than a quarter of the required air is missing, and the solution is not a setting but a different machine or duct network. Between 0.75 and 1 the gap is closable without replacing anything: higher speed at critical hours, clean filters, checked ducts.

Between 1 and 1.5 is the right zone: the standard is covered with a healthy margin, without paying in noise and energy for air that is not needed. Above 1.5 the airflow abounds — comfort at the peaks, but every extra cubic metre moved costs fan power and thermal energy, because the expelled air carries away the heat or coolness already paid for.

The index is dimensionless and works in any country with any standard: enter the requirement of your own regulation and the verdict adapts.

The airflow scale

The table keeps the reference room fixed (216 m³, 108 m³/h required) and moves only the machine — from the single fan to the generous centralised MVHR.

The first two rows tell the most common mistake: the small machine bought "so it cannot be heard" that delivers a quarter or half of the changes — and the room that keeps fogged glass and heavy air despite the system. The last two tell its opposite: double or triple the necessary airflow, paid every day in watts and expelled heat.

Where the required air changes come from

The hourly air changes are the parameter that varies with the use of the space, and the range is enormous: 0.3-0.6 vol/h for residential, 1-3 for offices and shops, 3-5 for classrooms and meeting rooms, 5-10 and beyond for commercial kitchens, gyms, windowless bathrooms and laboratories.

The exact value is set by the applicable standard in your country and your contract — national technical norms, building codes, hygiene requirements for premises open to the public. The calculator is parametric precisely for this: you enter the number of your own regulation, not an average decided by others.

The mistake to avoid is copying the value of a different use: the same 150 m³/h machine that covers the home with margin (1.39) drowns in a dense office and vanishes in a classroom — the count in the section on uses shows it row by row.

The people: the second criterion

The volume does not breathe: the people do. Every occupant emits CO₂, moisture and odours at a rate that depends little on the room around them — which is why standards pair the volumetric criterion with a minimum airflow per person, typically around 25-36 m³/h each for sedentary activity, with 30 as a convenient reference.

The per-person criterion becomes dominant as crowding grows: the meeting room with 20 m³ per person is governed by the volume, the classroom with 5 m³ per person by the heads. With the reference machine and eight occupants instead of four, the volume-based air changes do not move at all — but the airflow per head drops to 18.8 m³/h, below the comfort threshold: same room, same machine, different verdict.

That is why the calculator asks for the peak-hour occupants, not the average: air is sized on the worst moment, like lifts.

The open window is not a strategy

The classic objection — "I will open the window" — confuses a random air change with a controlled supply. The winter draught changes too much air in the wrong place, the summer calm too little; and every cubic metre that enters through the window in the heating season brings in air to be heated from scratch, with no heat recovery.

The paradox of modern buildings makes the issue urgent: the more airtight the envelope — new windows, insulation, gaskets — the less the building breathes on its own. The 1970s house ventilated itself by defect, through the cracks; the refurbished one no longer does, and without designed ventilation the moisture that used to leave through the gaps condenses on the thermal bridges.

It is the same account that links this page to the U-value calculator: the efficient envelope and controlled ventilation are not alternatives, they are the two halves of the same project.

Nameplate airflow and real airflow

The number on the data sheet is measured in ideal conditions: free vents, new filters, short ducts. The airflow that actually reaches the rooms is almost always lower, and the causes are ordinary: a clogged filter steals 20-30%, a crushed or overlong duct adds pressure losses, a vent closed "because of the noise" throttles the branch.

The honest check uses the real airflow — measured at the vents with an anemometer or balometer, or at least the nameplate value at the speed actually used, not the maximum. A system that covers the standard only at speed 3, which nobody uses because of the noise, is in practice an undersized system.

Maintenance is part of the sizing: the right machine with the wrong filter is a small machine.

Mistakes that distort the count

The first is the wrong volume: using the gross area instead of the net one, or forgetting that a habitable attic has different average heights — the volume is the base of the whole count. The second is the airflow at maximum speed: the fair comparison uses the speed the system will really run at, day and night.

The third is ignoring the per-person criterion: the studio flat with four flatmates passes the volumetric check and fails the head count. The fourth is copying the air changes of another use: between the bedroom and the commercial kitchen runs a factor of twenty.

The fifth is strategic: sizing on the average instead of the peak. Ventilation is judged on the evening with guests, the full meeting, the class hour — the moment the air runs short is the only one that counts.

The coverage index measures the quantity of air, not its distribution: 150 m³/h all supplied into the living room leave the bedrooms dead even at an index of 1.39 — the design of the network (where air enters, where it is extracted, how it crosses the home) is a craft the ratio does not replace.

Nor does it measure the quality of the supplied air (filtration, intake away from roads and chimneys) or the heat recovery, which decides how much the ventilation costs on the bill but not how many cubic metres it moves.

The count is a parametric estimate for information purposes: it supports preliminary sizing and the check of an existing system, and does not replace the ventilation design nor the regulatory requirements applicable to the specific case.

The scale of uses

The first table keeps the reference machine fixed (150 m³/h on 216 m³) and changes only the use of the space — that is, the air changes the standard requires. It is the fastest way to see how much the requirement commands the verdict.

The exact same machine crosses every branch of the verdict: abundant in the bedroom, compliant in the home, severely insufficient wherever people work. Between the first and the last row runs a factor of twenty — which is why "how much should it ventilate?" without a use is a question with no answer.

The practical reading: first fix the use, then size. The reverse path — buy and hope — produces the offices with heavy air at eleven and the classrooms where nobody can think in the afternoon.

The scale of the people

The second table keeps the same machine and crowds the room: from one to eight occupants, with the comfort criterion of 30 m³/h per person as the comparison line.

The volume-based air changes do not move along the whole table — the room is the same — yet between six and eight people the air per head drops below the reference: it is the second criterion that fails, not the first. The dinner with guests, the full meeting and the whole class live in these rows.

The remedy is not always more machine: variable occupancy is answered by speeds (boost at full hours) and by CO₂-sensor systems, which follow the people instead of ventilating the void.

MVHR and heat recovery: the part the index does not see

The coverage index counts the cubic metres, not what they cost. Two systems with the same index can have very different bills: single-flow ventilation expels air at room temperature and draws in outdoor air to be heated or cooled, while dual-flow with a heat recovery unit transfers 70-90% of the heat from the outgoing air to the incoming one.

The harsher the climate and the higher the airflow, the more the recovery weighs: the hourly cubic metres of this page are also the thermal kilowatt-hours that cross the machine every winter. It is the same reasoning as insulation, applied to the air instead of the wall — and indeed the two twin pages of this cluster do the wall count.

The design rule: first fix the right airflow (this page), then choose the machine that moves it at the lowest running cost. Inverting the order — choosing the efficient machine and hoping it suffices — leaves rooms efficient and badly ventilated.

CO₂ and humidity: the natural testers

The theoretical count has two cheap verifiers every home already owns. The first is a CO₂ meter costing a few tens of euros or dollars: outdoors the air sits just above 400 ppm, and a well-ventilated room typically stays under 800-1000; stable readings above 1200-1500 during occupied hours say the real airflow is below the count, whatever the nameplate declares.

The second is the humidity you can see: fogged glass in the morning, mould halos in the cold corners, lingering odours. They are the symptoms of insufficient air change even before an insulation problem — and they often appear precisely after the refurbishment that made the house airtight.

Used together, the two testers turn the verdict of this page from estimate into diagnosis: if the index says 1.39 but the CO₂ climbs, the problem is not the sizing — it is the real airflow that does not arrive.

Noise: the silent enemy of sizing

Almost no domestic system fails for insufficient nameplate airflow: it fails because the speed that covers the standard is the one you can hear, and after a week the family turns it down. From that moment the system delivers a fraction of the design, and the count of this page must be redone with the airflow of the speed actually tolerated.

The defence is buying size: the machine that covers the demand at speed 2 of 4 works in the quiet part of its curve, the one that covers it only at maximum lives in the noisy part. Moderate oversizing — an index towards 1.5 at the middle speed — is the price of silence, and often the best purchase of the project.

The airflow details matter too: short and wide ducts, gentle bends, silencers on the bedroom branches. The decibel saved is worth real airflow, because it is the airflow nobody will switch off.

Different countries, different standards, same count

The names change from country to country — North American standards reason in minimum airflows per person and per square metre, European ones in indoor air quality categories, national residential ventilation codes in changes and per-room flows — but the structure is the same everywhere: a requirement on the volume, a requirement on the people, and the stricter one commands.

That is why the calculator asks for the vol/h and does not impose them: whoever designs an office in Toronto, a classroom in Madrid or a house in Munich enters the value of their own norm and reads the same dimensionless index. The 30 m³/h per person reference is also a widespread comfort convention, not a law: standards range between about 25 and 36 depending on category and activity.

The one multi-country warning: some regulations also prescribe where to supply and extract (supply in the dry rooms, extraction in kitchen and bathrooms) — a distribution constraint the quantity count does not cover and the local design must respect.

Frequently asked questions

What are air changes per hour (vol/h)?

How many times in one hour all the air in the room is replaced with fresh air. At 0.5 vol/h — the typical residential minimum — all the air in a home changes every two hours; a commercial kitchen at 6 vol/h changes it every ten minutes.

How is the required airflow calculated?

Volume times required changes: floor area × average height × the standard's vol/h. The page reference — 80 m² by 2.7 m at 0.5 vol/h — gives 216 m³ of air and 108 m³/h of required airflow.

What is the coverage index?

The ratio of installed to required airflow: dimensionless, it works with any regulation in any country. Below 1 the standard is not covered; between 1 and 1.5 is the right zone; above 1.5 the airflow abounds and you pay in noise and energy for air you do not need.

Why does the calculator also ask for the occupants?

Because air is sized twice: on the cubic metres and on the people. Each occupant consumes about 30 m³/h of fresh air for comfort — and a small crowded room can pass the volume check and fail the head count: the stricter criterion commands.

How many air changes does a home need?

Residential typically lives between 0.3 and 0.6 vol/h, with 0.5 as the common value. Offices and shops ask 1-3, classrooms and meeting rooms 3-5, commercial kitchens, gyms and windowless bathrooms 5-10 and beyond: the exact value is set by the standard applicable to your case, and it is the calculator's input.

Is opening the windows not enough?

It is a random air change, not a controlled supply: too much with wind and cold, too little in the calm, and every cubic metre entering in winter is air to be heated from scratch with no heat recovery. In modern airtight buildings ventilation must be designed, not entrusted to draughts that no longer exist.

What is an MVHR and when is it worth it?

Mechanical ventilation with heat recovery moves a known, constant airflow, filters the incoming air and recovers heat from the exhaust. It pays off the more airtight the building is: after new windows and insulation it is the second half of the project.

Why is the real airflow lower than the nameplate?

The data sheet measures in ideal conditions. A clogged filter steals 20-30% of the flow, long or crushed ducts add losses, throttled vents cut the branches — and a system run at speed 1 "because of the noise" delivers a fraction of the nameplate value.

How many occupants should I count?

Those of the peak hours, not the average: ventilation is judged at the worst moment, like lifts. With the reference machine (150 m³/h) four people get 37.5 m³/h each; with eight they drop to 18.8, below the comfort reference of 30.

Is a high index always good?

No: above 1.5 the airflow exceeds the standard by half or more, and every excess cubic metre costs fan electricity and thermal energy, because the exhaust carries away heat or coolness already paid for. The right margin sits in the 1-1.5 zone, with the higher speeds in reserve for the peaks.

Does the result hold in every country?

Yes, because the calculator is parametric: the required changes and the per-person threshold vary from norm to norm, but you enter your own standard's value and the coverage index — dimensionless — adapts. No national value is hard-wired into the count.

Does the calculator replace the ventilation design?

No: it measures the quantity of air, not the distribution, the filtration or the heat recovery. It is an informative parametric estimate for preliminary sizing and for checking an existing system — the network design and the regulatory requirements of the specific case remain with the professional.

Updated July 2026