Containment, Capture and Process Airflow

Containment controls a pressure difference; the flow is a consequence

An enclosure built to contain a hazard is held at NEGATIVE pressure relative to everything around it, so that any leak in its envelope flows inward. The controlled quantity is the pressure differential, and the airflow is whatever it takes to maintain it.

That inverts the usual relationship between quality and quantity. A well-sealed enclosure holds its differential on a modest flow; a leaky one needs far more air to hold the same differential, because the extraction has to remove everything leaking in as well as the design air change. Sealing the enclosure REDUCES the plant required, which is the opposite of how ventilation is usually sized.

The air change rate quoted alongside it does a different job: it dilutes and removes airborne fibre or dust within the enclosure so that the concentration falls between disturbances. Both criteria have to be met, and the flow that satisfies one does not automatically satisfy the other.

The practical consequences are about continuity rather than arithmetic. The differential has to be monitored and recorded for the whole period the enclosure exists, including overnight; an airlock is what lets people pass without losing it; and a unit sized exactly at the calculated flow has no margin for the filter loading that will occur, which is why the filters' dirty-condition resistance belongs in the selection.

A cleanroom is the same idea with both signs reversed

A cleanroom holds POSITIVE pressure, so that leakage flows outward and contamination cannot enter through the envelope. Same mechanism, opposite direction, and a cascade of differentials between rooms of different cleanliness rather than a single boundary.

The airflow, though, is driven by something else entirely: particle removal. Air is recirculated continuously through high-efficiency filters, and the rate required rises steeply with the class — each step toward a cleaner classification costs a multiple of the air change rate rather than an increment, which is why the cleanest rooms run unidirectional flow over the whole ceiling rather than a change rate at all.

Classification is by measured particle count at a stated size, in a stated occupancy state, which is a specification the calculation cannot deliver. An airflow figure places a design in the right region; the room's class is established by measurement, and it is established three times — as built, at rest and operational — because people and process are the dominant particle sources.

Filter pressure drop matters more here than anywhere else in the family, because the whole air volume passes through high-resistance filters continuously and their resistance rises as they load. A fan selected at clean-filter resistance falls off its curve as the filters age, and the air change rate — the thing the classification depends on — falls with it.

Extraction over a source is a capture problem, not a room problem

A cooker hood's duty does not come from the size of the kitchen. It comes from the thermal plume the appliances beneath it produce and from whether the hood's geometry can capture that plume before it escapes.

Two geometric facts govern it. OVERHANG — how far the hood extends beyond the appliance on each open side — determines whether a plume drifting sideways is still under the canopy. And the number of open sides determines how easily room air currents can peel the plume away.

That is why an ISLAND hood needs substantially more extraction than a wall-mounted one over identical appliances: a wall hood has a wall helping it on one side and a plume that rises against it, while an island hood is open on four sides and has nothing but its own capture velocity. Sizing an island hood from a wall hood's figure is a common and expensive error.

The other half is MAKEUP AIR. Extracting a large volume from a building removes it from somewhere, and in a reasonably tight building the only paths are the ones that exist — including, if there is one, an open-flued appliance. Above a modest extraction rate, codes require makeup air to be supplied deliberately rather than left to find its own way in, and that supply is itself a heating and cooling load that has to be conditioned.

Worst-case depressurisation is a safety test, not a calculation

Everything in the previous section converges on one question: with all the extraction running, does the building go negative enough to pull combustion products back down a flue?

The test that answers it is procedural rather than arithmetic. Every exhaust appliance is switched on — kitchen hood, bathroom fans, dryer, any ducted vacuum — interior doors are set to the configuration that produces the worst pressure in the room containing the appliance, and the flue is then checked for spillage under those conditions while the appliance fires.

The reason it has to be a test is that the answer depends on the building's leakage distribution, which is not knowable from a drawing. A house that passes with one fan running can fail with two, and a house that passed at handover can fail after it is draught-proofed, after a new extractor is fitted, or after a door is replaced with one that seals.

So a calculated depressurisation figure is a screening tool with a specific use: deciding whether a test is needed and what makeup air might be required. Where an open-flued appliance exists, the test governs — and the reliable design answer is to remove the question by using sealed-combustion appliances, which take their air from outside and are indifferent to the pressure in the room.

Humidification is mostly conditioning the air you brought in

The moisture a humidifier must add is the mass flow of dry air multiplied by the difference between the humidity ratio it has and the one it needs. It is a latent load, and it is entirely outside the sensible-heat arithmetic that sizes the heating.

The counter-intuitive part is where the load comes from. In cold weather the outdoor air is very dry in absolute terms even at high relative humidity, so almost all of the humidification duty is conditioning the OUTDOOR air being brought in — ventilation and infiltration — rather than replacing what occupants use.

That makes humidification demand a function of the building's airtightness. A leaky building needs a large humidifier and will still struggle; the same building sealed needs a fraction of the capacity, because the load is the air exchange itself. Improving the envelope is a humidification measure, which is not how it is usually presented.

It also sets a ceiling that has nothing to do with capacity. Raising indoor humidity raises the dew point, and any surface colder than that dew point will condense — so in a building with poorly insulated windows or thermal bridges, the humidity that can be maintained is limited by the coldest surface rather than by the plant. A humidifier sized to a target the envelope cannot support will produce condensation and mould rather than comfort.

The attic fan: the arithmetic is right and the idea is usually wrong

A powered attic ventilator moves the airflow it is rated for, and the calculation that sizes it is correct. The problem is where the air comes from.

An attic fan creates a negative pressure in the attic, and it draws replacement air through whichever paths offer least resistance. If the intake venting is inadequate — which it commonly is, as the ventilation paper describes — a substantial share arrives through the CEILING PLANE, from the conditioned space below: through light fittings, loft hatches, service penetrations and the top plates of partitions.

So the fan is extracting air that has already been paid for, and replacing it with outdoor air drawn into the house. Measured studies of these units repeatedly find the cooling energy lost this way exceeding the cooling saved by the cooler attic, which is why several energy programmes recommend against them and why the recommendation surprises people.

Depressurising the house has the second consequence too, and it is the serious one: the same negative pressure can backdraft an open-flued appliance, which is the scenario the previous section tests for.

The honest framing on the page is therefore that the calculator sizes a fan correctly and that the fan is frequently the wrong answer. Sealing the ceiling plane and providing adequate passive intake ventilation addresses the same attic temperature without either penalty — and if a powered ventilator is still wanted, the intake area has to be provided first, because the fan will otherwise take its air from the house.