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TECHNICAL TOOL / 07
Dehumidifier Sizing Calculator
Estimate the water entering the room, then compare the required removal rate with catalogue ratings.
ESTIMATE / ENGINEERING REVIEW REQUIRED
- Required 24-hour equivalent capacity
- 59.7 L/day
- US pints per day (PPD)
- 126.2 PPD
- Design removal rate
- 2.49 L/h
- Actual daily moisture load
- 50.0 L/day
Capacity includes your runtime and allowance. It is not the amount of water physically generated, and not a guaranteed catalogue model output at your conditions.
| Source | L/day |
|---|---|
| Outdoor-air infiltration | 49.06 |
| Additional door opening | 0.00 |
| People | 0.96 |
| Other evaporation | 0.00 |
| Crop water balance | 0.00 |
Outside air is drier on a humidity-ratio basis. The estimate conservatively takes no drying credit from it.
Check every active field. Moisture values must be nonnegative, water out cannot exceed water in, light hours must be 0.1–23.9, and all air states must be within range.
Candidate equipment for review
Catalogue ratings use the conditions stated beside each model. Ask for performance at your inlet temperature/RH, defrost behaviour, power, drainage or regeneration and installation limits. No automatic unit count is provided.
01 / THE MOISTURE BALANCE
How big a dehumidifier do I need?
Use the sustained water load and the hardest operating period. Floor area alone cannot tell you how much moisture enters from outside, people, processes or crops.
Air entering the space
Calculate outdoor and target humidity ratios at the same pressure. Use room-referenced incoming air volume and the indoor specific volume to obtain dry-air mass.
Water released inside
Add people, measured evaporation and the crop water balance. Structural drying, stored-product desorption and open-water evaporation require measured or separately calculated inputs.
Capacity at real conditions
Check lights-off temperature and low-RH performance. A rating at 30°C/80% RH cannot be treated as removal at 15°C/50% RH.
02 / METHOD & WORKED EXAMPLE
Every part of the estimate stays visible.
Δw = max(woutside − wtarget, 0)
Minfiltration = (V / vtarget) × ACH × 24 × Δw
Mdoors = (V / vtarget) × Ndoor/day × Δw
Mpeople = people × g/person/h × hours / 1000
Mcrop = irrigation − drainage − retained water
Mday = Minfiltration + Mdoors + Mpeople + Mother + Mcrop
qdesign = max(Mday / operating hours, qlights-on, qlights-off) × (1 + allowance / 100)
C24h = 24 × qdesign; PPD = C24h × 2.113
Minfiltration = (V / vtarget) × ACH × 24 × Δw
Mdoors = (V / vtarget) × Ndoor/day × Δw
Mpeople = people × g/person/h × hours / 1000
Mcrop = irrigation − drainage − retained water
Mday = Minfiltration + Mdoors + Mpeople + Mother + Mcrop
qdesign = max(Mday / operating hours, qlights-on, qlights-off) × (1 + allowance / 100)
C24h = 24 × qdesign; PPD = C24h × 2.113
V is m³; v is m³/kg dry air; w is kg/kg dry air. Liquid water is approximated as 1 kg/L. Daily door exchanges and other evaporation are averaged over 24 h. Occupancy is conservatively assumed to coincide with the higher crop load. Short door-opening or process peaks need a separate time profile; the calculation does not resolve those transients.
Example: 300 m³, target 25°C/50% RH, supply 30°C/80% RH, 101.325 kPa and 0.5 ACH. Infiltration is about 49.06 L/day. Two people at 60 g/h for 8 hours add 0.96 L/day: total 50.02 L/day. With occupancy overlapping the infiltration load, 24-hour availability and 15% allowance, required capacity is about 59.73 L/day (126.20 PPD). These are calculation assumptions, not a site survey.
Grow room dehumidifier calculator mode
Switch on grow room mode above. Enter whole-room irrigation, irrigation per plant × plant count, or irrigation per m² × canopy area. Subtract water actually removed from the room and net retained water. Water returned to a reservoir inside the room is not automatically a removed load; include any later reservoir evaporation. Daily water balance does not establish the light/dark release pattern: use measured crop and room trends.
Example: 80 L/day released, 18 h lights on and 75% released during light gives 3.33 L/h in both periods. If only 50% is released during light, the dark-period load rises to 6.67 L/h. Run separate cases when light/dark air temperatures or HVAC condensate removal differ.
Refrigerant or desiccant?
For this initial screening, room temperatures below 15°C or targets below 40% RH prompt a desiccant review; other conditions prompt a refrigerant review. These are routing heuristics, not validated equipment cut-offs. Catalogue candidates are filtered by the listed temperature range and bracket the requested rating where possible. They may still be unsuitable at the actual RH, airflow, pressure or installation conditions.
Air-property method:ASHRAE Fundamentals (2017), Chapter 1 · PsychroLib. The load balance is steady-state; it excludes initial pull-down time, building moisture, HVAC removal credits and heat balance unless you supply those loads separately.
FAQ
Commercial sizing questions
What size commercial dehumidifier do I need?
For a warehouse, factory or commercial growing room, start with the moisture entering from outdoor air and doors, then add people, processes and crop water release. Use room volume, actual incoming and target air states, available operating hours and the most demanding production or light-cycle period. The calculator reports a design removal rate and its 24-hour equivalent capacity; floor area alone cannot establish either.
Select equipment only after checking its removal at your temperature and RH, defrost behaviour, airflow, heat output, drainage or regeneration requirements and installation limits. Catalogue capacity at a different rating condition is not a confirmed commercial-room output or an automatic unit count.
NEXT / CONDITION REVIEW
Confirm capacity at your operating conditions.
Attach the downloaded input sheet and your measured light/dark or production-cycle data.
Review project requirements