Quick answer
Latent heat is the energy absorbed or released when water changes between vapor and liquid without a change in temperature; in HVAC, it is the moisture part of the cooling load.
When your AC coil condenses water out of the air, it removes latent heat. A thermometer does not show it, but you feel it as clamminess when it is not removed.
Quick facts
The key facts about Latent Heat, with sources:
- What it changes
- Moisture content (water vapor ↔ liquid), not temperature
- Grain
- 1/7,000 of a pound of water vapor 1
- Total cooling
- Sensible (1.08 × cfm × ΔT) + latent (0.68 × cfm × Δgr) 1
- Humidity effect
- Higher RH shifts coil work to latent, shrinking the temperature drop 2
- Oversizing effect
- Oversized ACs may not reach comfortable humidity 3
Key takeaways
- Latent heat is the energy in water vapor; removing it dries the air.
- Your AC removes it by condensing water on the indoor coil.
- Estimate it with 0.68 × cfm × change in grains of moisture 1.
- Humid climates need equipment and airflow set for the latent load 1.
- Oversized systems often leave latent heat behind, so homes feel clammy 3.
What latent heat means
Boil a pot of water and the burner keeps adding heat, yet the water stays at boiling temperature. That hidden heat goes into turning liquid into vapor. It is "latent," meaning hidden. The reverse is also true: when vapor condenses back to liquid, it gives that heat up.
In a home, water vapor comes from people, showers, cooking, plants and outdoor air. To get rid of it, an air conditioner must condense it. That means pulling out the latent heat stored in the vapor. The indoor coil does this when it runs colder than the air's dew point.
Load calculations treat this separately from sensible heat, the temperature part. Green Building Advisor notes equipment must meet "not only the total cooling load but also the sensible and latent individually" 1.
How an AC removes latent heat
- Humid room air → coil colder than the dew point
- Water vapor condenses on fins → releases its latent heat into the refrigerant
- Water drips → pan → condensate drain
- Refrigerant carries that heat outdoors with the sensible heat
How the 0.68 formula works
- Moisture in air is measured as a humidity ratio, in grains of water per pound of dry air 1.
- There are 7,000 grains in a pound 1.
- Measure (or estimate) grains entering and leaving the coil, and take the difference.
- Latent BTU/h ≈ 0.68 × cfm × grains difference 1.
- Add the sensible part (1.08 × cfm × ΔT) for total cooling 1.
Like 1.08, the 0.68 factor assumes standard sea-level air and needs adjusting at altitude 1.
Sources of latent load in a home
| Source | Why it adds moisture | How to cut it |
|---|---|---|
| Outdoor air leaking in | Humid outdoor air carries vapor | Air sealing; controlled ventilation |
| People | Breathing and sweating | Unavoidable; part of every load |
| Showers and baths | Steam | Run bath fans that vent outdoors 5 |
| Cooking | Boiling and steaming | Range hood vented outdoors 5 |
| Clothes dryers | Moist exhaust | Vent to the outside 5 |
| Basements and crawlspaces | Moisture from soil | Drainage, vapor barriers, dehumidifier |
Worked example: total cooling from both formulas
Example only. A 3-ton system on a humid day, using the published sensible and latent factors 1. Moisture values are illustrative, as a tech would read from a psychrometric chart.
| Step | Math | Result |
|---|---|---|
| Airflow, humid climate | 3 × about 400 cfm per ton 1 | 1,200 cfm |
| Moisture entering coil | Illustrative | 78 grains per lb |
| Moisture leaving coil | Illustrative | 62 grains per lb |
| Latent removal | 0.68 × 1,200 × 16 1 | 13,056 BTU/h |
| Temperature drop | Illustrative | 18°F |
| Sensible removal | 1.08 × 1,200 × 18 1 | 23,328 BTU/h |
| Total | 13,056 + 23,328 | 36,384 BTU/h, about 3 tons |
| Latent share | 13,056 ÷ 36,384 | About 36% |
Over a third of this system's work goes to drying the air. Water removed per hour: 1,200 cfm × 60 × 0.075 lb/ft³ × 16 grains ÷ 7,000 ≈ 12.3 lb, or about 1.5 gallons an hour 1. That is the water you see at the drain outlet.
Where you will see the term
- In Manual J reports, as the latent cooling load.
- In manufacturer performance tables, where total minus sensible capacity equals latent capacity.
- On dehumidifier specs, as pints per day removed.
- In thermostat settings such as "dehumidify" or "overcool."
Getting latent removal right
Why systems fall short on latent removal
The coil needs time to get cold and wet before it removes much moisture.
DOE notes oversized air conditioners may not achieve comfortable humidity, which pushes homeowners to lower the thermostat or add a dehumidifier 3.
DOE's field-study review found equipment oversized compared with Manual J in 31% to 93% of systems studied 4.
Field readings can also mislead. HVAC School points out that higher indoor humidity means more latent removal and a smaller temperature drop 2. A tech who judges by temperature drop alone may call a working system weak.
Where cost comes in
Latent heat has no price of its own, but removing it uses energy, and failing to remove it leads to costly fixes: dehumidifiers, mold cleanup or a replacement AC. Equipment with variable-speed blowers and compressors handles latent loads better and usually costs more up front.
Compare options with the replacement calculator, and see dehumidifier installation for add-on costs.
How designers handle latent load
- Calculate the latent load in Manual J, including outdoor air and occupants.
- Pick equipment whose latent capacity at design conditions covers it.
- Set airflow lower per ton in humid climates 1.
- Choose two-stage or variable-speed equipment for long, low runs.
- Add a dehumidifier where latent loads stay high in mild weather.
- Seal ducts and the shell to cut humid air leaks.
Keeping moisture removal working
Signs latent load is not being met
- RH above 60% with the AC running 5.
- Cool but sticky rooms.
- Condensation on supply vents or cold surfaces.
- Musty smells.
- Short AC cycles of a few minutes. See short cycling.
Standards that use it
Latent heat is physics, not a regulated value. It appears in ACCA Manual J, which calculates sensible and latent loads, and Manual S, which checks equipment against both. Many states adopt the International Residential Code, which points to these manuals for sizing; adoption varies.
Efficiency ratings like SEER2 measure total cooling, so they do not show how well a unit removes moisture. Ask for latent capacity at your conditions from the maker's data.
Latent heat vs sensible heat
| Feature | Latent heat | Sensible heat |
|---|---|---|
| What it changes | Moisture | Temperature |
| Felt as | Clamminess | Warm or cool |
| Formula (standard air) | 0.68 × cfm × Δgrains 1 | 1.08 × cfm × ΔT 1 |
| Helped by | Lower airflow, longer runs | Higher airflow, more capacity |
| More detail | This page | Sensible heat |
Common misconceptions
- Myth Colder air is drier air.
- Reality Cooling air raises its RH unless water is removed.
- Myth A bigger AC handles humidity better.
- Reality Oversized units can leave moisture behind 3.
- Myth A small temperature drop means the AC is failing.
- Reality On humid days, more work is latent 2.
- Myth Latent load only comes from outdoors.
- Reality People, cooking and showers add plenty 5.
Climate and latent load
Humid Southeast and Gulf states carry the biggest latent loads, so slower airflow, longer runs and sometimes a dehumidifier matter most there 1. Dry Southwest climates have small latent loads and can run more air per ton.
Mixed climates see latent load spike in spring and fall, when cooling runs are short. Those mild, muggy days are when a single-stage system struggles most, because the thermostat is satisfied after a few minutes and the coil never gets wet long enough to drain.
When it matters to you
It matters if your home is cool but clammy, if you live in a humid climate, or if you are replacing an AC. Ask the installer for the latent load from Manual J and the latent capacity of the proposed system.
Next step: read about what maintenance does for performance and check sizing with the AC sizing calculator.
Questions about Latent Heat
What is latent heat in air conditioning?
It is the energy tied up in water vapor. An air conditioner removes it by condensing moisture on the cold indoor coil, which dries the air. Latent heat removal does not show up on a thermometer, but you feel it as less clamminess.
How do you calculate latent heat in HVAC?
For standard air, latent BTU per hour ≈ 0.68 × cfm × the change in grains of moisture per pound of air. A grain is 1/7,000 of a pound. Add the sensible part, 1.08 × cfm × temperature change, for total cooling.
Why does my house feel humid when the AC is on?
The AC may be oversized and shutting off before removing much moisture, airflow may be set too high, the fan may be set to On, or humid air may be leaking in. A technician can check sizing, airflow and fan settings.
What is latent capacity?
It is how much moisture-removal capacity a unit has, in BTU per hour, at given conditions. It equals total capacity minus sensible capacity. Manufacturer performance tables list both. Humid climates need more latent capacity.
Does lowering the fan speed improve dehumidification?
Usually, yes. Lower airflow lets the coil run colder, so it condenses more water. A building-science guide suggests about 400 cfm per ton in humid climates. Too low can freeze the coil, so a technician should set it.
Is latent heat the same as humidity?
Not exactly. Humidity is the amount of water vapor in air. Latent heat is the energy that water vapor holds. Removing humidity means removing latent heat, which is why moisture counts as part of the cooling load.
How much water does an AC remove?
It varies widely with humidity, airflow and run time. In our worked example, a 3-ton system removing 16 grains per pound at 1,200 cfm pulls about 12 pounds of water an hour, about 1.5 gallons. You can see it at the drain outlet.
Do variable-speed systems remove more latent heat?
They often do, because they can run longer at low speed with a colder coil. Many have a dehumidify setting that slows the blower. Results depend on setup and the home's moisture sources.
Why is 7,000 used in moisture math?
There are 7,000 grains in a pound. Grains are a small unit that makes moisture numbers easy to work with. The 0.68 factor in the latent formula already includes this conversion for standard air.
Can a heat pump remove latent heat?
Yes. In cooling mode a heat pump works exactly like an air conditioner, so its indoor coil condenses moisture and removes latent heat. In heating mode it adds only sensible heat. Variable-speed heat pumps often include a dehumidify setting for humid summers.
Sources
- Green Building Advisor, Converting Heating and Cooling Loads to Air Flow Needs, retrieved .
- HVAC School, What Should the Air Delta T Be? (Air Temperature Split), retrieved .
- U.S. DOE Energy Saver, Central Air Conditioning (archived copy, 2025), retrieved .
- U.S. DOE Building Technologies Office, Residential HVAC Installation Practices: A Review of Research Findings (June 2018), retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The 0.68 and 1.08 factors, the grain definition and the airflow guidance are Green Building Advisor's; the oversizing finding is DOE's. The worked example uses illustrative grain and temperature readings, labelled as such, and the published factors.
Suggest a correction. We fix errors and say what changed.