Latent Heat Calculator
Latent heat in HVAC is the energy associated with moisture removal (or addition) to the air, manifested as the condensation of water vapor onto the cooling coil (or evaporation...
Formula
Source: Engineering Toolbox, ASHRAE Fundamentals | Last reviewed: June 8, 2026
Examples
0 BTU/hr
= 2720 BTU/hr
- cfm = 400
- delta_W = 10
400 CFM removing 10 grains/lb moisture = 2,720 BTU/hr latent
0 BTU/hr
= 24480 BTU/hr
- cfm = 1200
- delta_W = 30
1,200 CFM at 30 grains/lb moisture removal
0 BTU/hr
= 1700 BTU/hr
- cfm = 500
- delta_W = 5
500 CFM light dehumidification = 1,700 BTU/hr latent
Where is this used?
Swimming pool HVAC: high latent loads from evaporation require dedicated dehumidification.
Humidity control in museums and data centers: maintaining stable relative humidity.
Fresh air pretreatment: outdoor air in humid climates carries significant latent load.
In hot-humid climate HVAC design (Gulf Coast US, Southeast US, Florida, Houston, Miami, New Orleans; tropical regions worldwide; high-humidity Asian cities like Bangkok, Manila, Mumbai), the latent load dominates the cooling load in many buildings because of the high outdoor air humidity ratio and the high internal latent gains from occupants, cooking, showering, and process operations.
A 1,000 CFM outdoor air system in 95°F / 75% RH summer conditions at a Gulf Coast hospital has a latent load of about 47,000 BTU/hr (0.68 × 1,000 × (134 − 50) = 0.68 × 1,000 × 84 = 57,120 BTU/hr with the higher ΔW) — a large fraction of the total cooling load for that air system, requiring precision humidity control per ASHRAE 170 (40–60% RH in operating rooms).
Pharmaceutical manufacturing facilities with humidification control (cleanroom humidity typically maintained at 45 ± 5% RH per EU GMP Annex 1) require the latent heat calculation for the chilled water coil sizing and the regeneration heating load of the desiccant dehumidifier, with the latent load often exceeding the sensible load for the small cleanroom volume.
Indoor swimming pool dehumidification is one of the most demanding latent load applications: a 1,000 ft² pool at 82°F water temperature and 84°F / 60% RH indoor air has an evaporation rate of about 100 lb/hr of water, corresponding to a latent load of 106,100 BTU/hr — requiring a dedicated pool dehumidification unit (Calorex, Desert-Aire, Therma-Stor) sized for the latent load with sensible heat recovery from the exhaust stream.
Industrial drying applications (food dehydration, paper drying, ceramic drying, pharmaceutical fluid bed drying) use the latent heat calculation to size the desiccant dehumidifier (rotary or solid desiccant wheel) and the process air heater, with the moisture removal rate directly determining the desiccant regeneration energy.
Compressed air desiccant dryer sizing uses the latent heat calculation to size the desiccant bed and the regeneration heater (a 1,000 SCFM compressed air dryer at 100 psig might need to remove 5 lb/hr of water from the air after the aftercooler, requiring a desiccant bed sized for the moisture loading and a regeneration heater rated for the latent load plus the sensible regeneration load).
Frequently Asked Questions
What is a typical humidity ratio for HVAC calculations?
At standard conditions (75°F, 50% RH), the humidity ratio is about 65 grains/lb. After cooling to 55°F saturated (typical supply air), it drops to ~64 grains/lb. The ΔW is small for recirculated air but can be 80-120 grains/lb for outdoor air on a humid summer day.
Why does latent heat matter for cooling?
In humid climates, latent cooling can be 30-50% of total cooling load. An undersized or poorly designed cooling coil may satisfy temperature but leave the space clammy and uncomfortable due to inadequate moisture removal. This is why coil selection must consider both sensible and latent capacity.
How do I convert relative humidity to humidity ratio?
This requires psychrometric calculations involving temperature, barometric pressure, and saturation pressure. Use a psychrometric chart or calculator. As a rough guide, at 75°F: 50% RH ≈ 65 grains/lb, 60% RH ≈ 78 grains/lb, 40% RH ≈ 52 grains/lb.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.