Reheat Boxes
Lesson 2.3
What Is a Reheat Box
A reheat box is not a VAV box. There is no damper. Air flows through it at a constant rate. 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. Hospitals use them for similar reasons: isolation rooms and procedure spaces have strict pressurization requirements.
The coil itself is either hot water (with a modulating valve) or electric. Hot water gives you smooth, proportional control. Electric reheat is simpler to install but does not modulate: each element is either on or off.
The exception is SCR (silicon-controlled rectifier) electric heat. An SCR controller fires the heating elements in rapid pulses to simulate proportional output. From the BAS side it looks like an analog output commanding 0-100%.
The Sequence
Three states. Airflow is constant throughout. The only variable is how much the heating coil fires.
Heating: ZN-T below heating setpoint. For hot water: the controller opens the valve and modulates it to chase a discharge air temperature setpoint (typically 85-95 degrees F). For electric: the controller switches stages on.
Satisfied: ZN-T between setpoints. The heating valve closes fully. Cold supply air flows through the box unchanged.
Unoccupied: The valve closes. Setpoints widen. The reheat box only wakes up if the space drops to a low-limit temperature (typically 55 degrees F).
No cooling from this box: A reheat box has no way to actively cool a space. If the zone is too warm, that's an upstream problem.
What to Watch For in the Field
- DAT or zone temp: the valve chases one of them: Know which one your sequence is chasing before you start troubleshooting.
- Electric reheat stages do not modulate: Look at the stage-on time delays in the sequence; if they're too short the box will short-cycle.
- These boxes get hot fast: If airflow ever stops and the reheat valve is stuck open, you'll see DAT climb fast. Most sequences include a high-DAT limit.
- The upstream system sets the ceiling on cooling: If the space is too warm and the reheat box is fully off, the only fix is more cold air or colder air from upstream.