Latent Load vs. Sensible Heat: The Difference That Makes or Breaks HVAC Sizing
The thermostat hits 72°F. The house still feels like a swamp. That’s a latent load problem — and it’s exactly the thing a square-footage estimate ignores entirely.
Sensible heat is what raises dry-bulb temperature — the number on the thermometer. Latent heat is what’s tied up in moisture — water vapor suspended in the air. An air conditioner does both. Sizing for one while ignoring the other is how you end up with a house that’s 72°F and 65% relative humidity — technically at setpoint, practically miserable.
Where each load comes from
Sensible gains are straightforward: solar radiation through windows, conduction through walls and roofs, heat from people, lights, appliances, and hot outdoor air. Latent gains come from:
- Outdoor air infiltration and ventilation — in Miami, incoming air at 90°F and 75% RH carries a large moisture load that has to be condensed out before it can be exhausted as water.
- Occupants — each person generates roughly 200–300 BTU/hr of latent heat through respiration and perspiration. Four occupants add 800–1,200 BTU/hr of latent load before cooking or showering.
- Cooking, showers, plants — all significant moisture sources the load calculation must account for.
- Envelope leakage — in a leaky older home in a humid climate, infiltration latent load alone can dwarf everything else.
Why oversizing kills latent performance
When you oversize a cooling system, you make the humidity problem worse. An air conditioner removes moisture only while the evaporator coil is cold and the system is running. A properly sized system runs long, steady cycles — moisture gets removed, the house dries out. An oversized system hits the sensible setpoint fast and shuts off. The coil never stays cold long enough to pull meaningful moisture out. The house reaches 72°F. Humidity stays at 62%. The homeowner turns the thermostat to 68°F trying to feel comfortable. The system short-cycles faster. The humidity gets worse.
How Manual J separates the two loads
A proper Manual J load calculation calculates sensible and latent cooling loads separately for every room. The outdoor humidity at design conditions — expressed in grains of moisture per pound of dry air — drives the latent calculation. At Miami’s design conditions (91°F DB / 77°F WB), outdoor air carries roughly 130 grains/lb of moisture. At Atlanta’s conditions (92°F DB / 75°F WB), it’s closer to 105 grains/lb. That difference matters for every cubic foot of outdoor air entering the building.
| Factor | Affects sensible | Affects latent |
|---|---|---|
| Solar gain through windows | ✓ Primary driver | Minor |
| Conduction through walls/roof | ✓ Yes | Minor |
| Infiltration/ventilation air | ✓ Yes | ✓ Major in humid climates |
| Occupancy | ~75 BTU/person sensible | ✓ ~200 BTU/person latent |
| Outdoor humidity (design grains) | Minor | ✓ Drives the entire latent calculation |
What this means for equipment selection
Once Manual J produces separate sensible and latent loads, Manual S equipment selection has to match both. A unit’s sensible heat ratio (SHR) — the fraction of total cooling capacity that goes toward sensible cooling — varies with operating conditions. Two units with identical total capacity at AHRI conditions can have very different SHRs at your actual design day. That’s the check Manual S performs. That’s the check that doesn’t happen when equipment is selected by tonnage alone.
We calculate sensible and latent loads separately for every residential and light commercial project — because in a humid climate, the latent number is often what controls the equipment selection.
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