Terminology is important !

Draught

draught is unwanted cooling of the body caused by air movement, which depends on the air speed, air temperature, turbulence intensity, activity and clothing of the building occupants.

The predicted percentage of people dissatisfied due to annoyance by draught (DR) is limited to:-

10% for Class A and 20% for Class B and C environments,

All of which prescribe a mean air velocity within 0.13 to 0.22m/sfor a local air temperature of 24oC

Don’t nod off just yet notes 

• “Sister strategies” – focused air delivery & mixing air delivery.  

• Cooling & energy performance is intrinsically linked to air diffusion because ultimately air diffusion influences suction pressures.

• Air rarely moves predictably• “Air diffusion first” can often reveal opportunities for saving energy

Natspec is OK, right?

Specification elements for Indoor Air Quality & Comfort

• VAV System – Mixed flow ADPI– Zone control critical

• Displacement System– Mixed flow for low ceilings– Individual occupant control– Pollutants rise to extract

• Chilled Ceiling– 100% outdoor air @ 1.5/L/s per m2

– Zone control

Potential Diffusion features: • Air Diffuser Performance Index

(ADPI) • Temperature • CO2 concentration• Furniture off gassing• VOCs• Sick Building Syndrome

Why is this important ?

Fundamentals are affected

• AS1668.2 (Fresh air) does not standardize how well the air is diffused.

• ISO7730 (PMV) does not specify velocities.

• The use of PMV in ASHRAE 55 is limited to velocity< 0.2m/s.

• Hot Air rises.

• ACE is strongly driven by air patterns, OA, TD 

• The higher the ADPI, the higher the % of occupants satisfied w/ thermal conditions. 

• Design is for heating &  cooling extremes & everywhere inbetween

The “New Normal”

“The pace of change in the workplace 

is such that the mindset and 

disposition of the worker is the 

Number 1 CEO concern…” THE FUTURE OF WORK /ASPEN INSTITUTE

New systems with user

interaction

Comfort

...affects our 

performance

Comfort

...differs 

between gender

Air movement

...is 

fundamental

This is normal

Boss sits here!

Comfort & Well‐being

Complaints incl: Choice

Productivity

“Do you think having control of air movement will enhance or hinder productivity?”

Control

Air Diffusion Performance Index

ADPI is the % of measurements within an occupied volume where the (Eff Draft Temperature) is between -1.5 to +1C of setpoint with air velocity <0.35m/s.

Strategies

• Focused air delivery • Mixing air delivery

Focussed Delivery – workstation air

Focussed Delivery – Workstation air

3500

Focussed Delivery‐ UFAD / Displacement

Focussed Delivery‐ Displacement

Mixing Delivery ‐ Flexibility

Mixing Delivery – Wide Temperatures

Types of Variable Air Volume Systems• VAV System

– RSH < 200 W/m2

– Supply air > 12°C– S/R air < 10 K– Standard Induction diffusers– < 500 L/s per diffuser

Low Temp VAV System– RSH < 200 W/m2

– Supply air > 8°C– S/R air < 16 K– High induction air diffusers– < 500 L/s per diffuser

Variable Airflow & Coil Control

Cool KW = 55 Sens & 47KW LatentCOP = 3.1Comp Input = 32.9 KWAirflow = 5050L/s Fan Energy 7.7KW absorbed

Cool KW = 79.5 Sens & 19.9KW LatentCOP = 4.2Comp Input = 23.8 KWAirflow = 5050L/s Fan Energy 7.7KW absorbed

What are we playing for?

Combining entrainment and swirl

Is combining focus and mixing

Sinusoidal air movement is real

Below brief, above brief and compliant

3 minutes later

Equilibrium 

Airflow variation = arm wrestling between AHUs

Pressure sensors

Common Atrium/ Open space

Try “Air Diffusion First”

If I had my time again...

ScenarioFocussed or 

Mixed Advantages

Large atrium or mall Mixed Little duct required

Commercial office – high churn expectations 3.5 flr to flr

Focussed  UFAD / Task Air

Mixed LT VAV

BothHigh turndown 

capability

Retail tenancies MixedFlexibility of diffuser 

placement

Try “Focussed” strategies first – they are intrinsically lower energy and duty.

Where it is impractical (large retail or warehouse spaces), try mixing strategies.

Where VAV turndown is important, try mixing and possibly motorised diffusers.

Summary• Air Diffusion is essential to thermal comfort and air quality delivering air where people are.

• Style of air diffusion is related to coil and airflow decisions & thus energy

• Air rarely moves predictably.• Try looking “Air diffusion first” for a change.• Air diffusion needs to be deliberate in design.• Our clients are searching for higher levels of comfort & IAQ without energy penalty.