| Scenario | Room | Indoor | Outdoor | Moisture | Unit | Output |
|---|---|---|---|---|---|---|
| Grow tent | 12 m² × 2.4 m | 24°C, 75% → 55% | 18°C, 65%, ACH 0.30 | Plants 2.0 L/day, Other 0.5 L/day | 20 L/day, factors 1.0 | Runtime ≈ 2–6 hrs (varies by leakage) |
| Small greenhouse | 30 m² × 3.0 m | 22°C, 80% → 60% | 15°C, 70%, ACH 0.80 | Plants 6.0 L/day, Other 2.0 L/day | 35 L/day, factors 0.9 | May need larger capacity or sealing |
| Propagation room | 20 m² × 2.7 m | 25°C, 70% → 55% | —, ACH 0.10 | Plants 1.5 L/day, Other 0.8 L/day | 25 L/day, factors 1.1 | Lower daily runtime at setpoint |
This tool estimates how long a dehumidifier needs to reach a target relative humidity while moisture continues entering the space.
pw = RH · pws
w = 0.62198 · pw / (P − pw) (kg/kg dry air)
ΔW = (w_initial − w_target) · m_dry (kg ≈ L)
Net = Cap_hr − Load
Runtime = ΔW / Net
- Choose your unit system and room input mode.
- Enter room size and indoor temperature.
- Set initial and target relative humidity levels.
- Add leakage using ACH, then optional outdoor conditions.
- Estimate plant transpiration and other evaporation sources.
- Enter your dehumidifier capacity and adjustment factors.
- Press Calculate runtime to view results.
- Use Download CSV for spreadsheets and logging.
- Use Download PDF for quick sharing and records.
- If results show “Not achievable,” reduce moisture or upgrade capacity.
Moisture drivers in grow spaces
Humidity rises when vapor enters faster than removal. Key sources include transpiration, wet media, open reservoirs, and door cycles. Leakage at 0.30 ACH can keep adding moisture when outside air is wetter than your target. Track sources weekly to improve estimates.
Setting targets by growth stage
Set RH by stage and disease risk. Propagation may run higher, while dense canopies often need lower RH. Many growers operate in 50–65% RH with stable temperature. If condensation appears, lower RH or reduce surface wetness and improve airflow.
Interpreting runtime versus maintenance hours
Runtime estimates hours to move from your initial RH to the target while moisture loads continue. Maintenance hours/day estimate the fraction of time needed to hold the target. If maintenance exceeds 18–20 hours/day, the unit is near its limit and RH can drift upward.
Capacity reality checks and factors
Rated capacity is commonly tested under warm, humid conditions. Cooler rooms or lower RH reduce output. The optional adjustment estimates this effect, and your manual factor lets you match measured drain volume. Use a 24-hour bucket test to calibrate the factor.
Example dataset for planning
Benchmark inputs with a small dataset. Keep temperature steady, read RH daily, and estimate moisture sources in liters per day. Compare scenarios to see when sealing, source reduction, or a larger unit delivers the biggest improvement.
- Example A: 28.8 m³, 24°C, 75%→55%, ACH 0.30, plants 2.0 L/day, other 0.5 L/day, unit 20 L/day.
- Example B: 90.0 m³, 22°C, 80%→60%, ACH 0.80, plants 6.0 L/day, other 2.0 L/day, unit 35 L/day.
- Example C: 54.0 m³, 25°C, 70%→55%, ACH 0.10, plants 1.5 L/day, other 0.8 L/day, unit 25 L/day.
FAQs
1) What ACH should I enter if I’m unsure?
Use 0.20–0.40 for a fairly sealed room. Use 0.60–1.00 for leaky spaces or frequent door opening. Adjust after observing how fast RH rebounds when the unit stops.
2) Why do I see “Not achievable”?
Your ongoing moisture load is higher than effective removal. The target cannot be reached. Reduce evaporation and leakage, or select a dehumidifier with higher daily capacity.
3) Should I include outdoor humidity?
Include it if outside air can enter through gaps, vents, or door cycles. Disable it only for truly sealed rooms with recirculation-only airflow.
4) How do I estimate plant moisture in liters per day?
Track water added minus runoff, averaged over several days. If possible, compare against measured dehumidifier drain volume to validate after canopy, light, or airflow changes.
5) Why does capacity change with temperature and RH?
Removal depends on cooling air below its dew point. Cooler air and lower RH reduce condensation potential, and coil frosting can further reduce performance. Ratings often assume warmer, wetter conditions.
6) Can I use pints per day from the label?
Yes. Choose “Pints/day” and enter the label value. The calculator converts to liters internally, so you can mix imperial capacity with metric room inputs.
7) What are quick ways to reduce daily runtime?
Cover reservoirs, reduce exposed wet surfaces, improve drainage, and seal leaks. Spread watering events and avoid standing water. These steps reduce moisture spikes and lower the required maintenance hours.