Money goes into high-end equipment. The diffuser, often the cheapest link, still decides whether the energy produced reaches the occupied zone, or leaves before it does.
With a weak last link, part of the energy produced never reaches the occupied zone, and the system makes up for it.With a strong last link, the energy produced is delivered where the people are.
Where the energy goes
Five ways air distribution wastes energy.
A section through a room shown like a CFD result: the contours are the air temperature relative to setpoint. Blue is colder, orange warmer, white at setpoint. Scroll through the five cases and compare the problem with the fix.
01 · Heating stratification·The problem
ColderWarmer1 °F per band · white = setpoint · → airflow
01
Heating stratification
Heating from the ceiling, warm air stays up and runs straight to the ceiling returns. The warmest air leaves the room without heating anyone, part of it is exhausted outside and replaced by outdoor air that must be heated again. The occupied zone stays cold and the thermostat keeps calling for heat.
ACTIF diffusers project warm air downward in heating and horizontally in cooling, with no power and no controls. The heat reaches the occupied zone, and mixed air is what goes back to the return.
Poorly mixed air leaves pockets at different temperatures. The thermostat measures a single spot: sitting in a cool pocket, it is satisfied while the rest of the occupied zone is still too warm. Occupants push the setpoint down, and the whole system works harder than it needs to.
High induction discharges the supply faster and draws in a lot of room air: the supply mixes along the ceiling and the occupied zone is close to a single temperature. Wherever the thermostat sits, it reads what people feel.
In VAV, airflow drops with the load. A low-induction jet loses its Coanda effect, detaches and falls into the occupied zone: cold where it lands, warmer where it doesn’t reach. The system responds to the one spot where the thermostat sits, so parts of the room end up overcooled or overheated.
High-induction diffusers keep the pattern attached to the ceiling, even at minimum airflow, and keep mixing the room air. The occupied zone stays even, and the thermostat reads what people feel.
Take the same low-induction diffuser at minimum airflow, with the thermostat on the far wall. The cold air dumps under the diffuser and the thermostat sits in a hot zone, so the VAV box opens to maximum airflow: far more than the load needs, and the room quickly becomes too cold. The box closes back to minimum, the hot zone returns, and the cycle starts over. Depending on the equipment, these constant swings cost comfort and energy.
A high-induction diffuser stays attached and mixed at minimum airflow. The thermostat reads the room as it is, the box holds a steady airflow that matches the load, and the loop never starts.
Ceiling spaces are tight, so the flexible duct bends hard into the top of the diffuser. The kink adds pressure the fan has to deliver, and it throws the air to the far side: in a diffuser with large openings and an integral collar, most of the air can leave through one side, the rest split between the other three. One side drafts, the other stays warm, and the system answers wherever the thermostat happens to be.
A high-induction face on a pressurized plenum fed from the side: the duct goes straight in, with no kink, and the air builds pressure in the plenum before it leaves through many small openings. Every side gets the same airflow.
ASHRAE 62.1 corrects outdoor airflow by the zone air distribution effectiveness: Voz = Vbz / Ez. Its table gives 0.8 for warm air supplied and returned at the ceiling, a conservative default. The real effectiveness depends on how the air actually distributes in the room, and it can be evaluated with CFD.
Zone air distribution effectiveness
0.8
0.2Table default · 0.8Better mixing · 1.2
01What the table saysA default value for warm air supplied and returned at the ceiling. It knows nothing about your diffuser.
02What CFD saysA simulation shows where the outdoor air actually goes, and what effectiveness the design achieves.
03What the diffuser doesThe better the mixing, the more outdoor air reaches the breathing zone, and the less of it is needed.
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