Reheat Boxes

A reheat box is not a VAV box. There is no damper. Air flows through it at a constant rate, delivered by whatever is upstream: an AHU, a fan coil, or a rooftop unit serving a constant-volume system. The reheat box has one job: take that constant stream of cold supply air and warm it up before it enters the space, when the space asks for heat.

You see them most in applications where you have to maintain a fixed airflow regardless of the thermal load. Labs are the classic example. A fume hood needs constant exhaust, so the supply side has to match with constant air delivery. You cannot throttle the damper down when the room gets comfortable; you will upset the pressurization. So the air flows constantly, and the reheat coil handles any heating the zone needs without touching the volume.

Hospitals use them for similar reasons: isolation rooms and procedure spaces have strict pressurization requirements where airflow volume is non-negotiable. Perimeter corridors and other high-ventilation spaces sometimes get reheat boxes too, particularly in older buildings where the original design was constant-volume throughout.

The coil itself is either hot water, with a modulating valve controlling heat output, or electric. Hot water gives you smooth, proportional control. Electric reheat is simpler to install and common in buildings without a boiler loop, but it does not modulate: each element is either on or off. A two-stage electric box gives you three states: off, stage 1, or both stages, and the control is stepped, not smooth. Some higher-end applications use SCR (silicon-controlled rectifier) electric heat, which fires in rapid pulses to simulate proportional output; that does modulate, but it is the exception, not the rule. From the controller's perspective, the point list is minimal: zone temperature in, discharge air temperature in, heating valve command (analog for hot water, binary for each electric stage) out. That is the whole picture.