Full fresh air means drawing in outdoor air only, treating it and supplying it to the rooms while stale indoor air is exhausted, with no return air mixed in. An air purifier only recirculates indoor air and cannot lower CO₂ levels. Villas usually combine full fresh air with room-by-room supply and exhaust.
With full fresh air, every cubic metre sent into a room is outdoor air, and stale indoor air leaves through a separate exhaust path, with nothing mixed with return air. Systems with return air blend indoor air back into the supply to save energy, but a 50% mixing ratio means a nominal 300m³/h really brings in only 150m³/h of outdoor air, and CO₂ still cannot be exhausted.
Both air conditioning and ventilation carry the word "air", but the air takes a completely different route. A ventilation system with return air draws part of the indoor air back, mixes it with outdoor air and then supplies it to the rooms. The point is to save energy: return air has already been conditioned, so blending it in means a little less heating or cooling.
The cost: the higher the mixing ratio, the less outdoor air actually comes in. At a 50% ratio, a system rated at 300m³/h brings in only 150m³/h of outdoor air. CO₂ still cannot be exhausted, because the return air carries the CO₂ that has already built up indoors.
In a full fresh-air system with no mixing, every cubic metre delivered to a room is outdoor air, and stale indoor air leaves through a separate exhaust path. The cost is higher energy use, offset by heat recovery. That is why we chose full fresh air: health targets cannot be met by watering them down.
Because a purifier does not exchange air. It recirculates air inside a closed box: it can filter particulates and adsorb some formaldehyde, but the CO₂ people exhale is a gas, and a filter cannot stop it. In a 20㎡ bedroom with two adults sleeping overnight and the window closed, CO₂ rises from 420ppm to over 2000ppm, and running the purifier all night does not change that number.
Because a purifier has no "air exchange" step. It draws indoor air in, filters out particulates and blows it back out, circulating inside a closed box the whole time. Particulates (PM2.5) can be filtered, and activated carbon can adsorb some formaldehyde, butthe CO₂ people exhale is a gas, and a filter cannot stop it。
In a 20㎡ bedroom with two adults sleeping overnight and the windows closed, CO₂ rises from 420ppm to over 2000ppm. At that level people feel groggy, sleep more lightly and wake with a dry mouth. Run the purifier all night and the number does not change.
The only solution is to exhaust the stale air and bring fresh air in, which is ventilation. That is why we saythe constant-oxygen metriccan only be met by ventilation.
Both, and take the larger. 30m³/h per person is the national standard minimum, corresponding to roughly 1000ppm indoors; the air-change method calls for at least 2 air changes per hour in quiet zones and 1.5 in active zones, covering emissions from the building and furniture plus infiltration. In sparsely occupied homes the area method gives the larger figure; in crowded homes the per-person method does.
Both, and take the larger.
Per person:30m³ per person per hour is the floor set by the residential standard. The figure derives from the CO₂ a person exhales against an acceptable concentration ceiling, and 30m³/h works out to roughly 1000ppm indoors.
Per floor area:Residential projects are usually calculated by air changes: no less than 2 per hour in quiet zones and 1.5 in active zones. The air-change method covers emissions from the building itself (materials, furniture) and infiltration.
In practice, sparsely occupied homes get the larger figure from the area method and crowded homes from the per-person method. Only the larger value satisfies both. For figures by floor area, seeVilla Fresh-Air Volume Reference Table。
Decisively. Without heat recovery, the equipment supplies all the heat needed to raise cold outdoor air to room temperature in winter; at 75% recovery only 25% has to be supplied. In a climate like Jiangsu, Zhejiang, and Shanghai, humid and hot in summer and damp and cold in winter, heat recovery is not an optional accessory but the key to whether the system runs reliably.
Heat recovery efficiency decides what "air exchange" costs in energy. Put simply, without heat recovery, heating -2℃ outdoor air to 22℃ in winter means the equipment supplies all the heat; at 75% recovery only 25% of that heat has to be supplied.
So in regions with wide seasonal swings (Jiangsu, Zhejiang, and Shanghai are humid and hot in summer and damp and cold in winter), heat recovery is not an optional accessory but the key to stable long-term operation. Judge efficiency by measured performance, not the nominal maximum: many nominal figures are measured under standard conditions and run lower in practice.
Replace on the unit's pressure-drop alarm rather than a fixed number of months; the interval depends on outdoor air quality and running hours. A higher grade is not automatically better: the higher the grade, the greater the resistance, and both energy use and noise rise. The sensible approach is to select for local air quality and ensure sufficient residual pressure.
The interval depends on outdoor air quality and running hours and cannot be generalised. Common practice is to go by the unit's pressure-drop alarm rather than a fixed number of months.
On grades: the pre-filter catches large particles and protects the filters behind it; the medium- or high-efficiency filter handles PM2.5.Higher grade is not always better: the higher the grade, the greater the resistance, and both energy use and noise rise. The sensible approach is to select for local air quality and ensure sufficient residual pressure.
Three sources: mechanical noise from the unit, airflow noise from excessive velocity in the ducts, and throttling noise at the supply and return terminals. The controls: site the unit away from bedrooms with vibration isolation, keep the main duct within 5–6m/s and branches within 3–4m/s, and size terminals to the airflow instead of shrinking them to look tidy. Get all three right and the result is below 30dB(A).
Three sources: mechanical noise from the unit itself, airflow noise from excessive velocity in the ducts, and throttling noise at the supply and return terminals.
Get these three right and bedrooms can reach below 30dB(A) at night, which is where ourconstant-quiet metric comes from。
We recommend running it continuously. The value of ventilation lies in continuous exchange; intermittent operation lets CO₂ and humidity rebound while the system is off. Low-speed running uses little energy and is quiet, and is usually more economical than frequent starting and stopping.
We do not recommend it. A shared duct causes return-air mixing, weakens the fresh-air effect and lets the control logic of the two systems interfere with each other. Keep them separate and coordinate only at the supply terminal locations.
Ventilation lowers indoor formaldehyde by dilution and exchange, not by breaking it down. With a source that keeps emitting, ventilation can only push the concentration to an equilibrium value; it cannot eliminate the source. Controlling pollution at the source still comes first.
It can. That is why ventilation in southern projects usually needs total heat exchange or a separate dehumidification module, not sensible-only recovery. This is a significant difference between northern and southern designs.
They need periodic inspection. Dust accumulating in the ducts becomes a secondary pollution source. Design should include access panels so the ducts can be maintained.
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