Reheat
Heating air after it has been cooled to remove moisture, so it can be supplied to a room at the right temperature without overcooling it. Reheat lets a system control humidity and temperature separately, at the energy cost of heating air that was just cooled.
Also called reheat coil, terminal reheat, dehumidification reheat
What is reheat in an HVAC system?
Reheat is heat added to air after a cooling coil has dried it, so the supply does not overcool the room. Reheating 500 cfm from 38°F to 55°F takes 1.08 × 500 × 17 = 9,180 BTU per hour, which is why recovered heat or mixing is used where it can be.
Reheat: how it works
To remove moisture, a coil cools air below the dew point the room needs, which can be far colder than the supply temperature the room's heat load calls for. If the room's sensible load is small, that cold air would overcool it. A reheat coil downstream warms the air to the supply temperature the load needs, while the moisture already removed stays removed.
Where you meet the term
- In a cleanroom or laboratory where a low humidity target forces supply air colder than the heat load needs.
- At night or at light process load, when dried makeup air alone would overcool the room.
- In an energy review, where reheat energy is traced back to the humidity target that requires it.
How to check it
- Look for heating and cooling on the same air stream. On the building automation screen, a cooling valve and a reheat valve open together on the same air stream show reheat in use, which is expected only when humidity control demands it. Owner check
- Measure temperatures across both coils. A technician measures air temperature leaving the cooling coil and leaving the reheat coil and compares both with the setpoints in the sequence of operations. Technician
Reheat: what it is not
- Reheat is not a fault. It is a deliberate step where humidity control needs air drier than the heat load alone would produce.
- Reheat is not required for every dehumidification design. Where the recirculating air still needs cooling, blending the dried air into it does the job the reheat coil would have done.
Reheat questions
Is reheat a waste of energy?
Reheat spends heat to undo cooling, so it is expensive when that heat is new. Recovered heat from a chiller condenser, a heat pump, or desiccant regeneration costs far less, and mixing cold dry air into recirculation air can avoid reheat at design load.
What is the difference between a reheat coil and a heating coil?
The coil can be identical. Reheat describes its job, warming air after dehumidification, while a heating coil in winter warms air because the room itself needs heat.
Why does a cleanroom need reheat in summer?
Holding a low humidity means cooling air colder than the room's heat load needs, and in a room with a small sensible load that air would overcool the space unless it is warmed again.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, the bin and degree-day methods used for seasonal energy estimation, the table of heat gain from occupants by activity including light bench work at 275 BTU per hour sensible and 475 latent, the standard-air sensible and latent heat factors, and the orifice flow relationship used to estimate airflow through openings and gaps from pressure difference.