Quick answer: A ducted dehumidifier connects to supply and return ducts, so it can sit out of sight above a ceiling or in a plant room and dry one or several rooms. Size it in two steps: capacity for the moisture you must remove, and airflow at the duct pressure your layout creates. The total pressure loss of ducts, bends, grilles and filters must stay within the unit's external static pressure, or the airflow and drying capacity fall.
Quick answer
- Three layouts: standalone ducts, connected to the building air handler, or treating outdoor air before it enters.
- Check capacity (L/day at your conditions), airflow (m³/h) and external static pressure (Pa) together.
- Keep supply and return far apart so dry air crosses the whole room.
Our ceiling-mounted and ducted dehumidifiers range from 26 to 960 L/day, with external static pressure from 40 to 200 Pa depending on the model.
Three ways to connect a ducted dehumidifier
| Layout | How it works | Best for | Watch out for |
|---|---|---|---|
| Standalone | The unit has its own return and supply ducts to the rooms it serves | Rooms that need humidity control when the HVAC is off: archives, pools, stores, grow rooms | Supply and return placed too close together |
| Connected to the air handler | The unit takes part of the return air and discharges into the supply side of the building's air handler | Buildings with good existing air distribution and a moisture problem in summer | Fan interaction and control coordination with the HVAC |
| Outdoor-air pre-treatment | The unit dries ventilation air before it enters the building | Buildings where outdoor air is the main moisture source | Undersizing for the most humid outdoor days |
Choose the layout from where the moisture comes from. If most of it arrives with ventilation air, dry the outdoor air. If it comes from inside (people, pools, plants, processes), dry the room air.
How to size capacity and airflow
Step 1: capacity. Add up the moisture load in kg/h or L/day: people, processes, plants, open water and outdoor air. Then pick a unit whose capacity at your room condition covers it. Ratings at 30 °C/80 % RH are higher than what the unit removes in a cooler, drier room; see how much water a dehumidifier collects in a day.
Step 2: airflow. The water removed equals the mass of air passing through the unit times the drop in moisture content across it:
water removed (kg/h) = airflow (m³/h) × air density (about 1.2 kg/m³) × moisture drop (kg per kg of dry air)
Example: an RYDZ-10S moves 2,500 m³/h, about 3,000 kg of air an hour. To remove 10 kg/h (240 L/day), it must lower the air's moisture content by about 3.3 g per kg of dry air. If the room needs more removal than the unit can achieve at that airflow, choose a larger unit or add a second one.
Step 3: air distribution. The ducts must deliver dry air across the whole space. Place supply grilles to throw air along the room and returns on the opposite side, and check that no corner is left stagnant.
The external static pressure budget
External static pressure (ESP) is the pressure the unit's fan can provide to push air through everything outside the unit at its rated airflow. Add up every loss at your design airflow and compare it with the unit's ESP.
| Item | Where the loss comes from |
|---|---|
| Straight duct | Length, size and duct type; flexible duct loses more than rigid |
| Fittings | Bends, branches, transitions; often as much loss as the straight runs |
| Grilles and diffusers | Supply and return terminals |
| Filters | Include an allowance for dirty filters, not only clean |
| Dampers and accessories | Volume dampers, fire dampers, silencers |
Rule: total losses ≤ the unit's ESP at the required airflow. If they exceed it, the airflow drops, the unit removes less water, and the air may not reach the far rooms. Shorten runs, enlarge ducts, use fewer bends, or choose a model with a higher ESP.
ESP of our ducted models
| Model | Removal (30 °C/80 % RH) | Airflow | External static pressure |
|---|---|---|---|
| RYDZ-26DM | 26 L/day | 260 m³/h | 40–60 Pa |
| RYDZ-58DM | 58 L/day | 550 m³/h | 40–60 Pa |
| RYDZ-90DM | 90 L/day | 1,000 m³/h | 50–80 Pa |
| RYDZ-138DM | 138 L/day | 1,300 m³/h | 50–80 Pa |
| RYDZ-7S | 168 L/day | 1,800 m³/h | 100 Pa |
| RYDZ-10S | 240 L/day | 2,500 m³/h | 100 Pa |
| RYDZ-12S | 288 L/day | 3,000 m³/h | 120 Pa |
| RYDZ-22S | 480 L/day | 5,600 m³/h | 150 Pa |
| RYDZ-30S | 720 L/day | 7,400 m³/h | 200 Pa |
| RYDZ-40S | 960 L/day | 10,000 m³/h | 200 Pa |
The smaller DM models suit short duct runs to one or two rooms; the S models handle longer runs and multiple zones.
Zoning: one unit or several?
One ducted unit can serve several rooms only if they need similar conditions. A single sensor controls the unit, so a dry room and a wet room on the same unit will not both be on target. Use separate units, or motorized dampers with their own sensors, when rooms have different loads or targets.
Drainage, access and noise
- Drainage: a gravity drain with the trap and fall shown in the manual, or a condensate pump with an overflow alarm. Insulate drain lines in cold spaces.
- Access: leave room to open the filter and electrical panels and to clean the drain pan.
- Noise: mount the unit on anti-vibration hangers and use flexible duct connectors; keep it away from quiet rooms.
Commissioning checklist
- Measure airflow at the unit and at each grille; compare with design.
- Measure temperature and humidity at the return, the supply and in each room.
- Test the humidistat, alarms and any interlock with the building HVAC.
- Check drainage under full load and the overflow cut-out.
- Record the readings as a baseline for later maintenance.
FAQ
Frequently asked questions
What is a ducted dehumidifier?
A dehumidifier with duct connections for return and supply air, so it can dry rooms away from where it is installed.
Is a ducted dehumidifier better than a portable one?
For permanent humidity control in commercial buildings, yes: it is out of the way, drains continuously and can serve several rooms. Portable units suit temporary jobs.
Can a ducted dehumidifier connect to existing HVAC ductwork?
Yes, in the "connected to the air handler" layout, but the fan pressures and controls of both systems must be coordinated by the HVAC designer.
How long can the ducts be?
As long as the total pressure loss at the required airflow stays within the unit's external static pressure. Larger ducts and fewer bends allow longer runs.
Sources
- ASHRAE Handbook—Fundamentals, Duct design; Psychrometrics.
- ASHRAE Handbook—HVAC Systems and Equipment, Dehumidification equipment.
