When is reheat used in a VAV system?

• A. To further cool the air for humidity control
• B. When cooling air alone is not warm enough to meet space heating requirements; downstream heater raises air temperature
• C. To defrost the coils
• D. To heat the space independently of air supply

Answer: (B)

Reheat in a VAV system is used to raise the temperature of supply air after it has been cooled to meet the cooling load, when the space also requires heating. The VAV box controls how much air flows, delivering cooler air to handle cooling and humidity as needed. If the space heating setpoint is higher than the cooled air’s temperature, the air would be too cold without warming it. A downstream heater provides that temperature rise, delivering air at the right temperature while keeping airflow control intact. This setup lets you manage temperature independently of air volume, which is important when dehumidification is achieved through cooling but some spaces still need warmth. Reheat isn’t about cooling the air further, defrosting coils, or heating the space without using the supplied air; the air itself is heated after cooling to meet the space’s heating requirement.