Sensible vs Latent Cooling Load: What an AC Must Remove

ByMuzaffer Caner Ankaralı

An air conditioner must handle two different parts of a building’s cooling demand. Sensible cooling lowers air and surface temperatures. Latent cooling removes water vapor. At one stated design condition, the two rates add to the total cooling load. These building loads are separate from the sensible, latent, and total capacities delivered by selected equipment.

Sensible, latent, and total cooling use the same rate basis

Solar gain, conduction through the enclosure, warm ducts, lighting, and appliances are common sensible gains. People, cooking, bathing, ventilation, infiltration, and damp materials can add moisture and therefore latent load. A room-by-room Manual J load calculation separates these components for the building. The relationship is:

Total cooling rate: Q̇total = Q̇sensible + Q̇latent

All three terms must use the same basis and units, such as Btu/h or kW, at the same indoor and outdoor design condition. Btu is an amount of energy; Btu/h is a rate. The HVAC Btu guide explains that distinction. The U.S. Department of Energy also treats total cooling load as the sum of sensible and latent load in its HVAC equipment-sizing guideline.

Sensible heat ratio must identify whether it describes load or capacity

Sensible heat ratio is:

SHR: SHR = Q̇sensible / Q̇total

SHR is dimensionless. A building-load SHR is calculated from the building’s sensible and total loads. An equipment-capacity SHR is calculated from the equipment’s sensible and total capacities at a stated operating condition. They are not interchangeable. Equipment selection compares the load split with manufacturer expanded performance data at the applicable airflow, entering-air condition, and outdoor temperature. Nominal tonnage alone does not provide that comparison, as the DOE sizing guideline explains.

Humidity ratio describes moisture on a dry-air mass basis

Relative humidity is temperature-dependent, so RH alone is not a moisture mass or latent-load rate. Psychrometric calculations commonly use humidity ratio W, defined as the mass of water vapor divided by the mass of dry air:

Humidity ratio: W = 0.621945 × pw / (p − pw)

Here, W is kg water/kg dry air or lb water/lb dry air, pw is water-vapor partial pressure, and p is total barometric pressure. When dry-bulb temperature and RH are known, pw = RH × pws(Tdb), with RH written as a fraction and pws the saturation vapor pressure at dry-bulb temperature. These definitions and ideal-gas relationships are given in ASHRAE Handbook—Fundamentals, Chapter 1: Psychrometrics.

Dew point determines where a cooling coil can condense moisture

Dew-point temperature is the saturation temperature corresponding to the air’s existing humidity ratio and pressure. Air can lose sensible heat to a coil surface that is cooler than the air. Latent removal occurs only on the wet portions of the coil where the local surface temperature is below the dew point of the air passing over that surface. Water vapor then condenses and must drain away. ASHRAE Handbook—HVAC Systems and Equipment, Chapter 23 explicitly distinguishes dry coil area from wet, dehumidifying coil area on this basis.

This is more precise than saying an evaporator is always below dew point. Refrigerant temperature, fin and tube surface temperatures, airflow, entering conditions, coil geometry, and load can produce dry and wet regions on the same coil. A cold refrigerant value does not prove that every part of the air-side surface is actively condensing moisture.

Cooling changes temperature, humidity ratio, and relative humidity differently

Before condensation begins, air may cool with little change in humidity ratio. Because cooler air has a lower saturation vapor pressure, RH can rise even when the water-vapor mass stays nearly constant. Once a wet coil surface is below the local dew point, both dry-bulb temperature and humidity ratio can decrease. This psychrometric path explains why a home can reach the thermostat setpoint while still feeling damp; see why a house feels humid at 72°F.

Airflow and runtime affect the sensible-latent capacity split

Indoor airflow, entering dry-bulb and wet-bulb conditions, outdoor temperature, compressor stage, coil match, and refrigerant controls influence delivered capacity. Within approved equipment limits, changing airflow can change leaving-air temperature and the sensible-latent split. The actual result must come from manufacturer performance data, not a guessed percentage.

Runtime matters because the coil must cool, moisture must condense, and retained water must reach the drain. Short cycles can end a temperature call before sustained dehumidification develops, although runtime alone does not prove oversizing. The AC short-cycling guide describes the additional evidence needed.

Design reports should show load and capacity in matching columns

A defensible report identifies sensible, latent, and total building load; indoor and outdoor design conditions; ventilation assumptions; airflow; and matched-system sensible and total capacity. Commissioning then checks delivered airflow, external static pressure, drainage, refrigerant operation, control sequence, indoor dew point, and room delivery. ACCA Manual P provides the psychrometric framework for room, coil, and air-system processes.

Frequently Asked Questions

Is latent load the same as relative humidity?

No. RH is a temperature-dependent condition. Latent load is a moisture-related energy rate under stated conditions.

Does one SHR describe a building all year?

No. Solar, internal, ventilation, infiltration, and moisture gains change by hour and season. Equipment SHR also changes with operating conditions.

Should a homeowner lower blower speed to remove more moisture?

No. Airflow changes require the equipment-approved range, manufacturer data, static-pressure and airflow evidence, and post-adjustment safety checks.

Revision history

  • August 21, 2026 — Technical evidence update: Added heat-rate equations, SHR definitions, humidity-ratio variables and units, dew-point and wet/dry coil distinctions, and claim-level ASHRAE, DOE, and ACCA references. No earlier revision entries were created.

Sources and verification

Final sizing, airflow, electrical, refrigerant, and control decisions must follow the exact matched-equipment instructions and applicable local requirements.

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