Latent load
The portion of a cooling load bound up in moisture rather than temperature, from occupants, cooking, showers, and infiltration. A coil selected against total capacity alone can satisfy the sensible portion of a load quickly while removing far less of the latent portion than the space actually generates.
Also called latent heat load
What is latent load in cooling?
Latent load is the part of a cooling load spent removing moisture rather than lowering temperature, generated by occupants, cooking, showers, and infiltration. It is a distinct figure from sensible load, and a coil has to be checked against both.
Latent load: how it works
A cooling coil removes moisture only once its surface is wet and cold enough for condensation to be steady, which takes continuous run time to reach. Oversized sensible capacity satisfies a thermostat's temperature call and cycles the coil off before that steady, moisture-removing phase has run for long, leaving latent load only partly addressed even when the total tonnage was correctly matched.
Where you meet the term
- In a Manual J load calculation, published separately from sensible load rather than folded into one total figure.
- In a humid climate or high-occupancy building, where latent load can be a large share of the total cooling requirement.
- In a complaint of a clammy space at the correct thermostat reading, which is latent load under-addressed despite sensible load being satisfied.
How to check it
- Read the load calculation's latent figure separately. A load calculation that reports only a single total BTU figure has not stated latent load distinctly, which is needed to check coil selection against it. Technician
- Check coil run time against design cycling. A coil satisfying temperature within a short cycle spends less time in its moisture-removing phase, which is the mechanism behind a latent shortfall even at matched tonnage. Technician
Latent load: what it is not
- Latent load is not proportional to sensible load by a fixed ratio across every building. Occupancy, climate, and moisture sources set it independently.
- Oversizing total capacity to add margin does not add latent capacity in proportion. It shortens cycles, which more often reduces effective moisture removal.
Latent load questions
What corrects an under-addressed latent load without oversizing?
Three paths exist without touching sensible capacity: add a standalone dehumidifier, hold the existing coil at a lower fixed airflow so it stays wetter longer, or bring reheat control into the existing equipment.
Does latent load change between heating and cooling?
Latent load as a named calculation applies to cooling. A heating load calculation does not carry an equivalent latent component in the same sense, since heating does not remove moisture from the air.
Why does latent load matter more in a humid climate?
Outdoor design wet bulb sits higher in a humid climate even on a moderate dry-bulb day, raising the moisture the ventilation and infiltration portion of the load carries into the space.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Manual J residential load calculationACCA description of Manual J as the residential load calculation procedure that sizes heating and cooling equipment from the building's construction, insulation, window area and orientation, air leakage, local design temperatures, and internal gains, producing a room-by-room and whole-house load in BTU per hour that equipment is then matched to rather than sized by floor area alone.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.