Quick answer: Plan the drain route before choosing a pump. If a drain point lower than the unit's outlet is within reach and the pipe can keep a steady fall, gravity drainage is the simplest and most reliable option. Use a built-in or external condensate pump when the discharge point is higher or far away. Size it for the peak water flow at the required total head (vertical lift plus pipe, fitting and check-valve losses), not the daily average. In unattended spaces, add a high-water alarm or a shutdown interlock so a failed pump can't flood the floor.
Quick answer
- Route first: gravity if you can, pump if you must.
- Pump duty = peak flow at total head (lift + losses), on the pump curve.
- Overflow alarm or interlock for unattended sites.
How much water
| Unit rating | Average flow at rating | Notes |
|---|---|---|
| 90 L/day | about 3.8 L/h | Portable/commercial |
| 240 L/day | 10 L/h | Industrial floor unit |
| 480 L/day | 20 L/h | Large industrial unit |
| 960 L/day | 40 L/h | Largest single units |
The actual flow depends on the operating condition, and peaks occur during pull-down after wet events. A pump empties its reservoir in bursts, so the instantaneous flow is higher than the average.
Gravity, built-in pump or external pump
| Option | Use when | Watch for |
|---|---|---|
| Gravity drain | Lower drain point reachable with steady fall | Sags, rises, blockages, freezing |
| Built-in pump (if the model offers one) | Moderate lift; single unit | Check head and flow on the datasheet |
| External condensate pump | Higher lift, long route, several units | Pump curve at total head; alarm contacts |
Work out total head
Total head = vertical lift + friction losses in pipe and fittings + check-valve loss + discharge pressure
Example (illustrative): lift of 4 m to a drain above a ceiling, 20 m of hose, several bends and a check valve. Add the losses from the hose and fitting data at the pump's flow, then pick a pump whose curve gives more than the peak flow at that head.
Traps, backflow and overflow
- Follow the equipment manual for traps; units with negative pressure at the drain need a trap to stop air being drawn in.
- Fit a check valve on pumped lines so water doesn't flow back into the reservoir.
- Use a high-water switch to alarm or stop the dehumidifier if the pump fails.
- Don't connect to drains that can back up into the unit.
Route checklist
- Drain elevation and discharge point (approved by the site)
- Route length, bends, hose or pipe size
- Freezing exposure; insulate or heat-trace
- Vibration and damage protection
- Access for cleaning the tray, trap, hose and pump
Maintenance
- Clean trays, traps and pump reservoirs; algae and dust block them.
- Test the high-water alarm.
- Check hoses for kinks and the outlet for blockage.
- See the maintenance checklist.
For small enclosures, see condensate drainage for battery cabinets. Our industrial dehumidifiers list drain connections and options on their datasheets.
FAQ
Frequently asked questions
Do I need a commercial dehumidifier with a pump?
Only if gravity drainage to a lower point isn't possible; a pump adds a component that needs maintenance and an alarm.
How high can a condensate pump lift water?
It depends on the pump; read the flow at your total head from its curve.
What happens if the condensate pump fails?
Without an alarm or interlock, the reservoir overflows; add a high-water switch that stops the dehumidifier.
Sources
- ASHRAE Handbook—HVAC Systems and Equipment, Dehumidification.
