Stress
The high demand for HVACR services means some stress, particularly when it's very hot or very cold outside. Extreme temperatures strain systems and cause breakdowns at the worst times.
Let's face it, having the A/C go out in July at the annual ice sculpting convention in Phoenix is a problem. Kind of like throwing water on the Wicked Witch of the West, it's not a pretty sight.
Rotating on-call schedules helps manage stress. People need to have a life and some downtime outside of work. Under normal circumstances, doing HVACR work should mean stress levels on the low side of normal, meaning it won't be too hard to keep your cool.