Managing Under Less Than Ideal Conditions
The above charts are based on relatively ideal conditions—conditions that do not always exist. The Standard goes into considerable detail about the limits for non-ideal conditions and we will briefly introduce them here.
Elevated Air Speed
Increasing the air speed over the body causes increased cooling. Elevated air speed can be used to advantage to offset excessive space temperatures. The temperature limits specified are increased by up to 5°F, as long as the air speed is within the occupant’s control and limited to below 160 ft/min.
The personal desk fan provides a simple example of placing air speed under individual control. For example, in the case of a naturally ventilated space where the acceptable temperature range was 71°F to 80°F, the acceptable temperature range would be increased to a higher range of 71°F to 85°F with the addition of a fan that was controlled by the occupant.
Draft
Draft discomfort depends on air temperature, velocity and turbulence. In general the steadier the draft the less the discomfort—it does not draw attention to itself so much! People are much more sensitive to cold drafts than they are to warm drafts. As a result the same velocity of air may produce complaints of cold drafts while cooling in the summer but no complaints when heating in the winter.
Vertical Temperature Difference
Vertical temperature difference between feet and head typically occurs in heated buildings. Warm air is less dense and tends to rise. Therefore, a warm air supply tends to rise, leaving the lower portion of the space cooler.
Also, many buildings in cool climates have a poorly insulated floor slab- on-grade, which makes for a cold floor and cool air just above the floor.
The variation in air temperature from feet to head is generally acceptable as long as it does not exceed 5°F.