Let me confirm the following ASHRAE 62.1-2007 Zone Air Distribution Effectiveness (Ez) issue.

Our project utilizes total heat exchangers for ventilation. We wonder what Ez should we use in the cooling mode most critical situation.

Typical Summer conditions at the project site are indicated below:
Outdoor temperature: 34 degrees C
Indoor temperature: 26 degrees C
Supply air temperature: between 26 and 34 degrees C

These assumptions meet the following situation written in 62.1 Table 6-2:
“Ez=1.0: ceiling supply warm air less than 15 degrees F (8 degrees C) or more above space temperature and ceiling return provided that the 150pfm (0.8m/s) supply air jet reaches to within 4.5ft (1.4m) of floor level. Note: For lower velocity supply air, Ez=0.8."
Note 2: “Warm air" is warmer than space temperature.

We think it’s not acceptable to have air speed at 1.4m above the floor level, because the draft would impair occupants’ comfort. We can find the following statement in ASHRAE 55 section 5.2.4.2:
“5.2.4.2 Draft.
Draft is unwanted local cooling of the body caused by air movement.
...The maximum allowable air speed is specified in Figure 5.2.4.2 as a function of air temperature and turbulence intensity. Alternatively, the following equation may be used for determining the maximum allowable air speed. The predicted percentage of people dissatisfied due to annoyance by draft (DR) is given by ...”

According to the Figure 5.2.4.2, air speed faster than 0.4m/s seems unacceptable. We think that the situation described in Table 6-2 in 62.1 is not compatible with ASHRAE 55 requirement.

If we could interpret the following part of 62.1: “provided that the 150pfm (0.8m/s) supply air jet reaches to within 4.5ft (1.4m) of floor level” as “provided that the 0.8m/s supply air jet at the diffuser, and the air reaches to 1.4m of floor level”, it would be understandable.

We checked the errata and addenda of 62.1 (even though the IEQ p1 asks for compliance only to the errata, and not to the addenda), and found no changes to the Ez coefficients.

Please let us know which Ez value, 1.0 or 0.8 should be used for our project.

Any comments would be greatly appreciated.

Thank you.