Unitary Equipment Space Humidity

Evaluating the Ability of Unitary Equipment to Maintain Adequate Space Humidity Levels

The objectives of this project which was monitored by TC 7.6 Unitary Air Conditioners and Heat Pumps were to use simulations and life-cycle cost analysis to evaluate the ability of unitary equipment to maintain adequate space humidity levels and to develop design guidelines based on the results of the evaluation.  Humidity control is a significant issue for a variety of reasons, including occupant comfort, microbial growth, and physical damage to buildings and equipment.  Five types of commercial applications were analyzed:

  • Small office
  • Large retail
  • Classroom
  • Restaurant dining area
  • Small hotel guest room
  • Theater.

Analyzes were done for ten different cities representing varying climates.

A comparative systems analysis was performed for a range of humidity control systems including:

Case 0 – Conventional DX System (typical HVAC design practice)
Case 1 – Base DX System (good dehumidification design practice)
Case 2 – DX Design for Improved Dehumidification (modified coil, compressor, etc.)
Case 3 – Base DX System with Lower Airflow
Case 4 – DX System with Air-to-Air Heat Exchanger (AAHX)
Case 5 – DX System with Subcooling Reheat Coil
Case 6 – DX System with Fan Control to Drain Coil
Case 7 – DX System with Airflow Control Using Bypass Damper
Case 8 – Hybrid DX and Desiccant System (condition mixed air stream)
Case 9 – DX System with Enthalpy Recovery
Case 10 – DX System with Outdoor Air Preconditioning
Case 11 – DX Dual Path System (separate systems for outdoor and recirculated air)
Case 12 – DX Dual Path with Enthalpy Recovery
Case 13 – DX Dual Path with AAHX
Case 14 – DX and Desiccant Dual Path System (desiccant system for outdoor air)
Case 15 – DX System with Demand Controlled Ventilation
Case 16 – DX Dual Path System with Demand Controlled Ventilation.

The ultimate goal of this work was to develop design guidelines.  The following general principles may be posed from the results of this analysis:

  1. In nearly all cases, simple variations in the Base DX system (lower airflow, lower SHR) do little to improve humidity control but may be useful to save fan energy.  The exception to this rule is Standard 2004 ventilation rates with the Retail application in the most humid climates.
  2. Demand controlled ventilation (DCV) saves energy, but does little to improve humidity control in most cases.
  3. Semi-active humidity control systems (Case 5 Subcool Reheat and Case 7 and Coil Bypass) can help but often fall short, especially in the most humid climates.
  4. Certain applications, such as the Theater, Restaurant and Motel, in very humid climates have high humidity issues primarily at times when there is no sensible load on the coil due to cool moist outside air.  Only active humidity control systems (desiccants and reheat) can control humidity at such times.  Depending on the control settings, enthalpy wheels may not operate at such times, and therefore provide less benefit for humidity control.
  5. For all of the systems without direct humidity control (all cases except desiccant Cases 8 and 14 and reheat Case 17), system capacity vs. load profile is crucial.  The poor humidity control performance of many of these system options can be attributed primarily to a high percentage of hours operating at low part loads.  2-stage systems with a 60% stage 1 capacity help significantly, but do not overcome this issue.  Case 6 Base DX w/o Latent Coil Degradation represents the ideal in capacity staging where the coil never evaporates condensed moisture back into the supply air stream.
  6. For the Office, humidity control is not an issue.
  7. For the Restaurant, Theater, and Schools, systems with direct humidity control (desiccant Cases 8 and 14 and reheat Case 17) are the only systems which can provide adequate humidity control in the most humid climates.  In less humid climates, enthalpy wheel systems (Cases 9 and 12) can also provide adequate control.
  8. For the Motel, continuous operation and single-stage equipment result in excessive hours of high humidity.  Only Case 14 Dual path w/Desiccant provides adequate (or near-adequate) humidity control in the most humid climates.  Reheat and dual path systems can help significantly, and are sufficient in moderate climates.
  9. For the Retail Store, a wider range of options can be beneficial.
  10. The enthalpy wheel and DCV options generally provide equal or better humidity control compared to the base system, with significant energy cost and life cycle cost savings.  Significantly better humidity control (but not necessarily adequate control) is found in the Restaurant with the 2004 Standard, Retail with both standards, and School with both standards.  Worse humidity control is found in the Restaurant and Theater in certain locations.