| Functionality | Factory Testing | Ease of Installation | Ease of Maintenance | Energy Efficiency |
AAON • 2425 South Yukon Avenue • Tulsa, Oklahoma 74107 • (918) 583-2266 • Fax (918) 583-6094 • www.aaon.com
Unitary Equipment
Humidity Solution
Modulating Hot Gas Reheat
The
Another Breakthrough from the Leader in HVAC Innovation
Humidity Problems?
The Solution: Modulating Hot Gas Reheat
Humidity Problems?
In recent years, there has been a move towards improving indoor air quality. This increased recognition and study of indoor air quality has pointed to moisture as one of the main causes of poor indoor air quality. High humidity can cause many problems including mold growth, condensation, wood rot, paper deterioration, increase in sickness and allergic reactions, and a variety of other indoor air quality and physically damaging issues.
In many geographic locations and in some building applications, the standard moisture removal ability of the cooling system cannot match the humidity control demand since it has been designed to satisfy a dry bulb temperature setpoint. When the setpoint has been satisfied, the supply fan continues to provide fresh air while the cooling system is shutoff until the temperature rises to some level above the setpoint. During that time all moisture removal has been lost.
Previous rooftop unit designs have tried to address this situation by taking a standard design and adding a 3-way solenoid valve to bypass hot refrigerant gas to a reheat coil that is placed after the cooling coil. The 3-way valve is usually controlled by a humidistat in the space that allows the unit to continue the moisture removal process after the dry bulb thermostat has been satisfied. This solves some moisture related problems, but a number of other problems may continue, including:
• Poor control of the amount of reheating
• Uncomfortable discharge air temperature swings during operation
• Unacceptable discharge air temperatures and poor temperature control when used in make up air applications, especially 100% outside air.
The Solution
The ideal solution would be to have a unit design that has the ability to continue the moisture removal from the space after the dry bulb temperature setpoint has been satisfied but with a controlled amount of reheat to meet the actual dehumidification requirement.
Modulating Hot Gas Reheat
AAON has addressed all of the above problems with a unique design incorporating a reheat coil, a modulating reheat hot gas valve, a modulating condenser hot gas valve, and an electronic controller.
The advantages of the design are many:
• The reheat option can be ordered factory installed complete with the electronic controller.
• If the dry bulb thermostat is satisfied and the humidistat is still calling for moisture removal, the control valves will start to modulate the amount of hot refrigerant gas passing through the reheat coil.
• A factory provided supply air temperature sensor field mounted in the supply air ductwork provides input to the electronic controller for reheat temperature control.
• The valve positions are controlled to provide a specific supply air temperature or a specific reset supply air temperature that are set on the control board. The temperature setpoint used is determined by a field provided 0-10 VDC reset signal.
• Since the amount of hot refrigerant gas passing through the reheat coil is modulated, the system delivers only the amount of reheating that is required for space comfort.
• Occupant comfort is uniform and consistent because there will be no drastic swings in the supply air temperature that are inherent with on/off solenoid valve control systems.
Minimum Work, Maximum Performance
• Refrigerant piping, valves, and reheat coil are all factory installed and wired to the controller.
• Controls are factory configured and ready to accept inputs from the field mounted, factory provided supply air temperature sensor in the ductwork, supply air temperature and reset supply air temperature DIP switch setpoints, field wired humidistat and optional field provided 0-10 VDC supply air temperature reset signal.
Outside Air? No Problem
To meet today’s standards for ventilation units must be designed to handle large amounts of outside air. With its enhanced latent capacity, the modulating hot gas reheat option can handle large amounts (up to 100%) of outside air. When used in make up air applications the modulating hot gas reheat option can dehumidify and still maintain neutral supply air temperature.
Cooling and Heating
During cooling and heating operations, the system will respond to the dry bulb temperature being controlled. A call for cooling or heating always overrides a call from the humidistat for dehumidification.
Reheat Operation
When the dry bulb temperature is satisfied but the humidistat is not satisfied, the compressors will continue to operate. The supply air temperature will be controlled to the setpoint selected and set on the control board. The AAON modulating hot gas reheat
option provides a controlled supply air temperature at all times.
Designed for Higher Humidity Locations
Rooftop units from other manufacturers are not able to meet the high humidity requirements of many areas. AAON rooftop units with the modulating hot gas reheat option, however, allow increased moisture removal ability based on the space requirements and the ambient conditions.
Contact your local AAON representative to find out more about this, and other, innovative solutions from AAON.
Rooftop Unit Refrigerant Piping
A Simple and Effective
Modulating Hot Gas Reheat System
| Functionality | Factory Testing | Ease of Installation | Ease of Maintenance | Energy Efficiency |
AAON • 2425 South Yukon Avenue • Tulsa, Oklahoma 74107 • (918) 583-2266 • Fax (918) 583-6094 • www.aaon.com
The Unitary Equipment Humidity Solution
Competitive System Comparison
|
Modulating Hot Gas Reheat | Carrier MoistureMiser™ | Lennox Humiditrol® | |
| Design Approach | Modulating valves to divert only the required amount of hot refrigerant gas. | On/Off solenoid valve to divert warm refrigerant liquid | On/Off solenoid valve to divert hot refrigerant gas |
| Hot Gas Reheat On Demand | Yes | No | Yes |
| Controlled Amount of Reheat | Yes | No | No |
| Dehumidification Without a Cooling Demand | Yes | No | Yes |
| Can Be Used in a Make Up Air Application Up to 100% | Yes | No | No |
| Can Interface to a 0-10 VDC Control System | Yes | No | No |
AAON System Advantages
Normal Heating and Cooling
“AAON leads the way in hot gas reheat humidity control. Since its incorporation in 1987 AAON has offered On/Off hot gas reheat for energy efficient humidity control. In 1996 AAON began offering modulating hot gas reheat for precise energy efficient humidity control. Today AAON continues to improve and add to its humidity control solutions to provide complete comfort its customers.
UnitarySys • R58470 • 062707