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HUMIDI-MIZER
ADAPTIVE DEHUMIDIFICATION
SYSTEM
Units with the factory-equipped Humidi-MiZer option are ca-
pable of providing multiple modes of improved dehumidifica-
tion as a variation of the normal cooling cycle. The design of
the Humidi-MiZer system allows for two humidity control
modes of operation of the rooftop unit, utilizing a common
subcooling/reheat dehumidification coil located downstream of
the standard evaporator coil. This allows the rooftop unit to op-
erate in both a dehumidification (Subcooling) mode and a hot
gas Reheat Mode for maximum system flexibility. The Humi-
di-MiZer package is factory-installed and will operate whenev-
er there is a dehumidification requirement present.
The Humidi-MiZer system is initiated based on an input from a
discrete input from a mechanical space or return air humidistat.
Humidi-MiZer Modes
DEHUMIDIFICATION MODE (SUBCOOLING)
This mode will be engaged to satisfy part-load type conditions
when there is a space call for cooling and dehumidification. Al-
though the temperature could have dropped, decreasing levels of
the sensible load in the space, the outdoor and/or space humidity
levels can be higher. A typical scenario could be when the out-
side air is 85°F (29°C) with 70% to 80% relative humidity (RH).
Desired sensible heat ratio (SHR) for equipment in this scenario
is typically from 0.4 to 0.7. The Humidi-MiZer unit will initiate
Dehumidification mode when the space temperature and humid-
ity are both above the temperature and humidity setpoints and
will attempt to meet both setpoint requirements.
Once the humidity requirement is met, the unit can continue to
operate in normal cooling mode to meet any remaining sensi-
ble capacity load. Alternatively, if the sensible load is met and
humidity levels remain high the unit can switch to Hot Gas Re-
heat mode to provide neutral, dehumidified air.
REHEAT MODE
This mode is used when dehumidification is required without a
need for cooling, such as when the outside air is at a neutral
temperature, but high humidity exists. This situation requires
the equipment to operate at a low SHR of 0.0 to 0.2. With no
cooling requirement calling for dehumidification, the Humidi-
MiZer adaptive dehumidification system will turn on both
compressors and open the two hot gas bypass valves allowing
refrigerant flow to the Humidi-MiZer coil to reheat the unit's
supply air to a neutral temperature.
As the hot bypassed refrigerant liquid (gas or two-phase mix-
ture) passes through the Humidi-MiZer coil, it is exposed to
the to the cold supply airflow coming from the evaporator coil.
The refrigerant is subcooled in this coil to a temperature ap-
proaching the evaporator leaving air temperature. The liquid
refrigerant then enters a Thermostatic Expansion Valve (TXV)
decreasing the air pressure.
The refrigerant enters the TXV and evaporator coil at a tempera-
ture lower than the temperature in the standard cooling opera-
tion. This lower temperature increases the latent capacity of the
evaporator. The refrigerant passes through the evaporator turn-
ing it into a superheated vapor. The air passing over the evapora-
tor coil becomes colder than it would during normal operation.
As this same air passes over the Humidi-MiZer Reheat Coil, it
will be warmed to the neutral supply air temperature.
Humidi-MiZer System Components
The Humidi-MiZer uses the standard unit compressor(s), evapo-
rator coil and Round Tube-Plate Fin (RTPF) condenser coil. Ad-
ditional refrigeration system hardware includes a subcooler/re-
heat coil and solenoid valves. On 48HC models, the evaporator
coil includes a TXV as a standard feature. Units with Humidi-
MiZer FIOP also include a factory-installed head pressure con-
trol system (Motormaster I) to provide proper liquid pressure
during reheat modes. Unique controls include a Reheat Relay
Board (RHB), evaporator coil freezestat, secondary low pressure
switch and a low outdoor temperature lockout switch (LTLO).
Units with two refrigeration circuits include a solenoid valve,
TXV, freezestat and low pressure switch for each circuit. See
Fig. 15.
RH2.A
Fig. 15 — Humidi-MiZer Valve Locations
Subcooler/Reheat Coil
The Subcooler/Reheat Coil is mounted across the leaving face
of the unit's evaporator coil. The coil is a one-row design with
two separate circuits.
Operating Sequences
The Humidi-MiZer system provides three sub-modes of opera-
tion: Normal Cooling (see Fig. 16), Subcooling Reheat1 (see
Fig. 17) and Hot Gas Reheat2 (see Fig. 18).
The Reheat1 and Reheat2 modes are available when the unit is
not in a heating mode and when the Low Ambient Lockout
Switch is closed.
When there is only a single cooling demand (thermostat Y1 alone
or with thermostat Y2), one or both circuits will operate in Re-
heat2 mode. Both solenoids are energized in both circuits. See
Fig. 18, Hot Gas Reheat Schematic for system refrigerant flow.
When there is both cooling demand (thermostat Y1 demand)
and dehumidification demand, circuit 1 will operate in Reheat1
mode (Subcooling, Fig. 17) and circuit will operated in Re-
heat2 mode (Reheat, Fig. 18). In Reheat1 mode, the 3-way so-
lenoid valve is energized, opening the reheat coil to the refrig-
eration flow path providing sub-cooling to the liquid before it
enters the TXV. See Tables 2-4 for Humidi-MiZer connections,
modes, and troubleshooting.
11
RH1.B
RH1.A
RH2.B
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Troubleshooting
