88
22.2.7.
Humiditrol Dehumidification Operation - Dehumidification Demand
Upon a dehumidification only demand, the M3 unit controller activates various compressors depending on model for
reheating. At the same time, the M3 unit controller will use the solenoid valve(s) to divert hot gas from the compressor(s)
to the reheat coil.
The cooled and dehumidified air from the evaporator is then reheated as it passes through the reheat coil. The
de-superheated and partially condensed refrigerant continues to the outdoor condenser coil where condensing is
completed. The reheat coil is sized to offset most of the first and second stages of sensible cooling effect during reheat
only operation. This reduction in sensible cooling capacity extends compressor run time to control humidity when cooling
loads are light. The unit continues to operate in this mode until the dehumidification demand is satisfied. A heating
demand terminates reheat operation. The M3 unit controller relative humidity set point is set at the factory for 60% and
can be adjusted at the M3 unit controller or with the L Connection Network unit controller Software. For Network Control
Panel (Network Control Panel) applications, the humidity set point can be adjusted at the Network Control Panel. The
M3 unit controller also has an option for an external digital input for the dehumidification demand. This demand must
be provided from an external third-party controller.
For unit model specific details, refer to the unit installation instruction.
22.2.8.
Humiditrol Dehumidification Operation - Cooling Demand Only
The unit will operate conventionally whenever there is a demand for cooling and no dehumidification demand. The unit
can provide up to four stages of mechanical cooling in this scenario. Free cooling is only permitted when an economizer is
present, there is no demand for dehumidification and the outdoor air is suitable for this function.
22.2.9.
Humiditrol Dehumidification Operation - Cooling and Dehumidification Demand
Stage one cooling and dehumidification demand: The system activates various number of compressors depending
on model plus reheat. This provides approximately 75% humidity removal capacity plus 25% cooling capacity.
Stage two cooling and dehumidification demand: The system activates all compressors plus reheat. This provides
100% humidity removal capacity plus approximately 50% cooling capacity.
Stage three cooling and dehumidification demand: The system activates all compressors. If there is a fourth
compressor, it along with the reheat coil will cease operation. This provides approximately 75% of humidity removal
capacity and 75% cooling capacity.
Stage four cooling and dehumidification demand: The system activates all compressors. This will provide 100%
humidity removal capacity and 100% cooling capacity.
For unit model specific details, refer to the unit installation instruction.
22.3. CAV Units in Single-Zone Applications — Wired Thermostat or Third-Party Unit
Controller
22.3.1.
Unit Controller Operation (2-Heat / 2-Cool)
When using a two-stage heat/ cool thermostat or third-party temperature control with the M3 unit controller in the
thermostat mode, a packaged rooftop unit can provide up to two stages of mechanical heating and cooling operation. To
operate correctly, a thermostat or third-party temperature control must provide the following wiring connections to the M3
unit controller:
1. Ventilation demand
2. Occupied demand
3. Heating demand one
4. Heating demand two
5. Cooling demand one
6. Cooling demand two
In this set up, either the thermostat or third-party temperature control handling the rooftop unit staging and general
operation. The M3 unit controller functions primarily to determine unit error codes, provide diagnostic information and
maintain safe operation limits.
22.3.1.1.
Ventilation Demand
Upon receiving a ventilation demand from the thermostat or third-party temperature control, the M3 unit controller
instructs the supply fan to start operation. The supply fan runs at full capacity as long as a ventilation demand is present.
22.3.1.2.
Cooling Demand
Upon receiving a stage one demand for cooling from the thermostat or third-party temperature control, the M3 unit
controller activates the required number of compressors to provide 50% cooling capacity.
