A while back, I designed a HVAC system for a new elementary school in Salt Lake City. It was installed as specified and seemed to be performing just fine. Then, one day, the principal called and said she was feeling cold. It was cold outside (approximately 20°F), but not as cold as the winter design temperature, so the HVAC system should have been able to handle the load.

The system serving the principal’s office was fairly typical: a ducted fan coil above the ceiling with direct-expansion cooling, hot-water heating, minimum outside air only, direct digital controls tied into the building’s main control system, and overhead supply and return—nothing fancy. The office was located on the ground floor, with an east-facing exterior concrete block wall, one small double-paned window in that wall, and a T-bar ceiling with insulated metal roof above.

I was summoned to the principal’s office (D’oh!). When I arrived, I found the HVAC system performing correctly. I compared the space-temperature reading of the building automation system to that of a calibrated handheld instrument and found them in agreement: 70°F, right on setpoint. Still, the principal was feeling cold. Maybe she was extra-sensitive to temperature. Or feeling sick. Or just plain crazy.

It turned out she was none of those. After some further head scratching, we measured the surface temperature of the wall adjacent to her desk and found it to be much colder than 70°F. The principal was feeling cold because of the cold surface temperature of the wall. Further investigation revealed the rigid insulation between the exterior concrete block wall and the interior gypsum-board wall that was specified had not been installed. After the insulation was installed, the wall temperature increased, and the principal felt comfortable again. Detention averted.

The situation taught me thermal comfort is more than just dry-bulb air temperature. The 70°F air temperature we measured was not the operative temperature, which is more representative of thermal conditions than just air temperature. The cold wall made the operative temperature much lower than the air temperature.