Knowledgebase: HVAC > Terminal Equipment
Terminal Unit Reheat Coil Not Meeting Setpoint
Posted by Richard Raustad, Last modified by Linda Lawrie on 20 April 2010 05:14 PM
Q: I'm wrestling with E+ to properly heat a zone served by a VAV fan with a hot water heating coil. The AHU main heating coil meets its setpoint, but the reheat coil does not heat that air enough to satisfy the zone heating setpoint.

A: When Autosizing equipment, it is very important to review the inputs to the Sizing objects. By telling the simulation that the system heating set point is 40C (the temperature leaving the main heating coil during sizing), and the zone heating set point is 43.3C, the reheat coil will size very small (3.3C delta T; 43.3 leaving the zone terminal unit - 40C leaving the main duct ). The reheat coil is operating at the maximum water flow rate in your simulation.

Sizing:Zone,
North, !- Zone Name
43.3, !- Zone Heating Design Supply Air Temperature {C}

Sizing:System,
AirLoop_AH1, !- AirLoop Name
40, !- Central Heating Design Supply Air Temperature {C}

When I changed the system sizing to 30C, the main heating coil was still able to meet a large portion of the load, while the reheat coil was sized larger and able to meet the remainging load.

Sizing:System,
AirLoop_AH1, !- AirLoop Name
30, !- Central Heating Design Supply Air Temperature {C}

Alternatively, you could leave the system heating supply air temp at 40C and raise the zone heating set point to 53.3C, this also worked (I tried them both).
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