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Duct reheat coils are used with hot water or steam for general heating applications. They are relatively easy to install.
There are usually upstream coils that regulate the leaving air temperature to a design intermediate level. The duct coil takes the air from that intermediate level to its final desired temperature.
As the name implies, these reheat coils are generally designed to be installed into ducts. These coils may be installed in the ceilings of office buildings, schools, and more.
Duct Reheat Coils are available in standardized sizes, are in stock, and are ready to ship.
Both do the same job — lift air from an intermediate temperature to the final design condition — but they behave differently enough that the choice usually follows the building’s existing heating plant rather than the coil itself.
Hot water reheat coils circulate heated water through the tubes and modulate capacity by changing flow rate or supply water temperature. That suits zone-level control, where the point is holding a room within a degree or two rather than dumping heat. They are simple to pipe, they tolerate part-load operation indefinitely, and control valve selection does most of the work.
Steam reheat coils condense steam inside the tubes and give up latent heat, which moves considerably more heat per coil pass. For the same duty a steam coil is usually physically smaller — useful where a ceiling plenum has no room to spare. The cost sits on the piping side: steam needs proper trapping, venting and condensate drainage, and a coil left holding condensate will erode from the inside or split if it ever sees cold outside air.
If the building already has a hot water loop, a hot water reheat coil is nearly always the lower-friction answer. Where steam is already distributed, the reverse holds.
Sizing a reheat coil is a shorter exercise than sizing a primary coil, because the temperature rise is small and known. Four numbers decide it: airflow through that branch in CFM, the entering air temperature leaving the upstream coil, the leaving air temperature the zone needs, and the supply water or steam temperature available. Those give you the required capacity, and from there the rows and fin spacing.
Installation is where duct reheat coils are easier than their upstream counterparts, and where the details still bite:
Our standard duct reheat coils are stocked in common sizes and ready to ship. Where the duct is an odd dimension or the connections have to land somewhere specific, we field-measure and build to match, so the coil drops in without reworking the duct.
Reheat coils live downstream, close to the space they serve. In practice that means branch ducts and terminal boxes: VAV boxes with a reheat coil in the discharge, fan-powered boxes, and standalone duct-mounted coils feeding a zone or a group of rooms.
The buildings where this shows up most are the ones with many small zones served by one central air handler — office floors, schools, hospitals, laboratories and hotels. The air handler dehumidifies the whole supply down to a low dew point, and each zone’s reheat coil brings its own air back up to what that room actually needs. Without the reheat stage you either overcool half the building or give up humidity control.
Where the upstream coil is a chilled water coil, the two get sized as a pair — our guide to chilled water coils covers how that side is specified.
A duct reheat coil takes air that has already been cooled or dehumidified to an intermediate temperature and brings it up to the final design temperature for that zone. It is the last stage of temperature control, which is why reheat coils sit in branch ducts and terminal boxes rather than in the central air handler.
Hot water reheat coils circulate heated water through the tubes and modulate output by changing flow or water temperature, which suits close zone-level control. Steam reheat coils condense steam instead, delivering more heat per pass from a smaller coil, but they need trapping and condensate drainage that a hot water coil does not.
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