EKS-HUM-015 / Moisture load and airflow
Latent Load
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Latent load is the rate of heat associated with a moisture phase change or a change in the water-vapour content of an air stream or space.
- Aliases
- latent heat load
QUANTITY, SYMBOL AND UNIT
Quantity / objectLatent heat load
SymbolQ_lat
SIkWkilowatt
SIkJ/hkilojoule per hour
IPBtu/hBritish thermal unit per hour
Definition
In humidity-control work, latent load links a water-vapour generation or removal rate to an energy rate. Crop transpiration, evaporation, occupants, wet processes, ventilation and infiltration can all contribute moisture that becomes a latent load.
The term is distinct from sensible load, which changes temperature without directly changing the air's water-vapour content.
Formula and variables
Q_lat = m_dot_w · h_fg Conditions: Use a latent heat value appropriate to the phase change and temperature range. Keep mass-rate and energy-rate units consistent.
| Symbol | Meaning | Unit |
|---|---|---|
Q_lat | latent heat load | kW |
m_dot_w | water-vapour generation or removal rate | kg/s |
h_fg | latent heat of vaporization | kJ/kg |
Relations and distinctions
Latent load and moisture load
Moisture load is commonly expressed as a mass rate of water. Latent load expresses the energy rate associated with that mass transfer.
Latent and sensible load
Latent load concerns moisture and phase change; sensible load concerns temperature change. Equipment and spaces can have both at the same time.
Engineering meaning
- Convert a credible moisture-generation rate into an energy rate when coordinating dehumidification and cooling duties.
- Separate latent and sensible requirements before using SHR to evaluate an air-treatment system.
- Use peak and time-dependent moisture sources rather than a daily average when they control equipment capacity.
Common misconceptions
MisconceptionLatent load is the same as the rated litres per day of a dehumidifier.
Accurate explanationOne is an energy rate and the other is a water-removal rate at stated rating conditions. They are related but not interchangeable without conditions and units.
MisconceptionCooling capacity automatically covers the latent load.
Accurate explanationA cooling coil removes sensible and latent heat in a ratio set by its condition and airflow; that ratio may not match the space load.
Application boundaries
- A latent-load value does not select equipment without entering-air conditions, target, airflow, operating schedule and part-load behaviour.
- The latent heat of vaporization varies with temperature; use a value suitable for the calculation range.
- Infiltration, ventilation and internal generation must be counted separately before being summed.
Verified example
Water generation converted to latent load
A process releasing 10 kg of water vapour per hour corresponds to about 6.81 kW of latent load when 2,450 kJ/kg is used as the illustrative latent heat value.
Basis: 10 kg/h × 2,450 kJ/kg ÷ 3,600 s/h = 6.81 kW. The selected h_fg must be reviewed for the real temperature range.Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, humidity parameters and moist-air relationships
Supports: Dew-point definition; Humidity parameters; Moist-air relationships -
ASHRAE / SRC-ASHRAE-F17-CH16
ASHRAE Handbook - Fundamentals, Chapter 16: Ventilation and Infiltration ↗
2017 SI edition / Ventilation load examples and sensible/latent heat equations
Supports: Outdoor-air sensible and latent load; Moisture-load calculation context
Maintenance record
Latent Load
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- July 31, 2026
- Terminology checked
- July 31, 2026
- Last updated
- July 31, 2026