Specifications
23
RG SERIES ENGINEERING GUIDE
Hot Gas Reheat Dehumidifi cation Overview
Hot Gas Reheat Dehumidification Overview
Dehumidification - The Need for Reheat
With tighter construction and more and more ventilation air
being introduced into buildings, there is more need now than
ever for proper humidity control. Ensuring dehumidification
can provide consistent employee comfort, a reduction in mold
liability, a reduction in cooling costs. Reduced humidity also
provides an improvement in indoor air quality (IAQ) thru lower
humidity levels which can reduce allergen levels, inhibit mold and
bacterial growth, and provide an improved computer environment.
ASHRAE 90.1 speaks of an acceptable humidity range in all
commercial buildings.
Typical Reheat Applications
Reheat can be used wherever moisture is a problem. Schools,
high latent auditorium and theaters, makeup air units*, and
computer rooms are typical applications. Although reheat equipped
water source heat pumps (wshp’s) can condition limited amounts
of outdoor air, the percentage of this outdoor air should never
exceed 50% of the return air to the unit limiting the mixed return
air temperature to a minimum of 50°F. When cold entering air
conditions are anticipated, hot gas bypass option should be
considered to prevent air coil freeze up.
*A dedicated outdoor air system (DOAS) should be investigated for
100% outdoor air applications.
The Design of Reheat Equipment
Hot gas reheat can help maintain specific humidity levels and
neutral air in a building. ASHRAE recommends a relative humidity
range of 30-60% with levels greater than 65% making mold growth
a possibility. The dehumidification relative humidity set points of
57% (on) and 52% (off) are recommended. During reheat the
leaving air temperature (LAT) will approximate neutral air. The
included chart (Leaving Air Temperature vs. Entering Water and Air
Conditions Chart) shows the LAT vs entering water temperature
(EWT) to the unit at differing entering air conditions. At 86-90°F
EWT the unit will provide nearly neutral air.
Moisture Removal Capacity
The amount of moisture removal may be calculated by subtracting
the sensible cooling capacity from the total cooling capacity in
the equipment performance data of the specifications catalog or
submittal data. An example is shown below:
Model RG*048, 1275 cfm, 12 gpm, 90°F EWT
TC – SC = LC
46.5 – 34.6 MBtu/h = 11.9 MBtu/h
Where TC = total cooling capacity, SC=sensible capacity, LC=latent capacity
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Leaving Air Temperature vs. Entering Water and Air Conditions Chart