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Nortek Global HVAC Receives Patent for A/C Hot Gas Reheat Circuit Modulation

Air conditioning systems, especially dedicated outdoor air systems (DOAS), can now eliminate frosting without interrupting the dehumidification cycle.


Nortek Global HVAC has been granted a U.S. patent for a method of controlling and stabilizing hot gas reheat circuits in air conditioning systems.

The method eliminates hot gas reheat circuit instability and the risk potential of freezing experienced by most current air conditioning reheat technology, especially on dedicated outdoor air systems (DOAS) when operating during cold and high humidity ambient conditions.

The current industry standard typically protects systems only mechanically with on/off thermostatic expansion valve (TXV) bypass diversions.

Reheat is required when the space has no internal load, however the outside air needs to be dehumidified and delivered to the space. The most critical reheat time is spring and fall rainy seasons with outdoor ambient temperatures between 56-66°F. Conventional direct expansion (DX) systems avert cold weather evaporator frosting through a coil frost-stat that deactivates the DX system when the evaporator coil temperature reaches 32°F. Other solutions utilize ineffective low ambient condenser fan control.

The patented Nortek method, which already appears on Reznor and Mammoth branded DOAS units, uses a DDC algorithm to prevent sub-cooling of the evaporator coil via a DDC-controlled modulating hot gas reheat TXV. Therefore, the DOAS’ dehumidification operation functions without coil frosting or inefficient air conditioning process interruption. The net result is stable and uninterrupted humidity control for the operation.

The method solves three following drawbacks associated with conventional DOAS:

  • Loss of cooling capacity when cycling between reheat and no reheat operation;
  • Freezing of the evaporator coil in 56-66°F ambient conditions;
  • Insufficient oil return due to poor refrigerant management that ultimately causes DX system failure.

For specifying engineers this methodology will provide a more effective dehumidification cycle and better, more reliable reheat and air comfort control for their customers’ DOAS units. Room humidity control remains stable regardless of outdoor ambient conditions.


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