Quick answer: An air conditioner removes heat in two forms: sensible heat (which lowers temperature) and latent heat (water vapour condensed on the cold coil). Most ACs spend about 70–80 % of their capacity on temperature and 20–30 % on moisture, and they are switched by a thermostat. So when a space needs little cooling but keeps gaining moisture (mild humid weather, many people, lots of outdoor air), the AC reaches the temperature setpoint and stops before the humidity comes down. The fix is either an AC system with humidity control (dehumidification mode, lower airflow, reheat) or a separate dehumidifier.
Quick answer
- Sensible = temperature; latent = moisture.
- Sensible heat ratio (SHR) of typical AC: about 0.70–0.80.
- If the room's moisture share is higher than the AC's, humidity rises.
Sensible vs latent cooling
| Sensible cooling | Latent cooling | |
|---|---|---|
| What it does | Lowers air temperature | Removes water vapour by condensing it |
| How | Air passes over a coil colder than the air | Coil surface is below the air's dew point; water drips into the pan |
| Controlled by | The thermostat | Nothing directly in a basic AC; it happens as a side effect |
Sensible heat ratio (SHR) = sensible cooling ÷ total cooling. An SHR of 0.75 means three-quarters of the cooling goes to temperature, a quarter to moisture.
Worked example: why a cool room can still be humid
An office on a mild, humid day:
- Sensible load (sun, lights, equipment, people): 4 kW
- Latent load (people, outdoor air): 4 kW, so the space needs an SHR of 0.50
The AC runs until the 4 kW of sensible load is met. With an SHR of 0.75, it removes only about 1.3 kW of latent load while doing so. The remaining 2.7 kW of moisture is about 4 kg of water an hour (roughly 95 litres a day) that stays in the air. The thermostat is satisfied, and the humidity climbs.
On a hot afternoon the sensible load is much higher, the AC runs longer and removes more moisture along the way, so the problem appears mostly in spring, autumn, rainy weather, evenings and at part load.
Oversized AC makes it worse
An oversized AC reaches the temperature setpoint in short bursts. Its coil hardly gets cold and wet before it stops, and when the fan keeps running the water on the coil evaporates back into the room. The US Department of Energy's research on HVAC sizing links oversized equipment with higher indoor humidity (US DOE, HVAC sizing research).
Ways to control humidity with AC
| Approach | How it helps | Limits |
|---|---|---|
| Fan on "auto", not "on" | Stops re-evaporation from the wet coil | Small effect |
| Lower fan speed / dehumidification mode | Colder coil, more water removed per hour | Still cools the room |
| Variable-speed compressors | Run longer at low speed with a cold coil | Equipment change |
| Reheat (hot-gas or other) | Dehumidifies, then reheats to avoid overcooling | Energy use; design-dependent |
| Dedicated dehumidifier | Controlled by humidity, independent of cooling | Adds some heat to the space |
| Dedicated outdoor air system (DOAS) | Dehumidifies ventilation air before it enters | Commercial-scale design |
The US DOE's research on advanced HVAC humidity control in hot-humid climates covers several of these strategies (US DOE).
Decoupled temperature and humidity control
The cleanest way to control both is to give each its own equipment and its own setpoint:
| Job | Equipment | Controlled by |
|---|---|---|
| Moisture (latent) | Dedicated outdoor air system (DOAS) or dehumidifier | Dew point or RH |
| Temperature (sensible) | Fan coils, chilled beams, radiant panels or the AC | Thermostat |
ASHRAE describes DOAS as a way to handle ventilation and latent load separately from sensible cooling. Decoupled systems avoid overcooling and reheat, and keep humidity steady at part load.
Temperature and humidity control units for rooms and processes
| Space | Typical solution |
|---|---|
| Offices, retail, hotels | AC plus DOAS or dehumidification mode |
| Labs, archives, museums | Precision air handling units with cooling, heating, humidification and dehumidification on tight setpoints |
| Data and electrical rooms | Close-control units; dew-point control to avoid condensation |
| Electrical cabinets and switchgear | Cabinet coolers or heaters for temperature, plus a cabinet dehumidifier for moisture; see heater vs ventilation vs dehumidifier |
| Process rooms (food, pharma, drying) | Dedicated dehumidifiers or desiccant units with separate temperature control |
Related: heater vs ventilation vs dehumidifier.
Targets
- Comfort and mold prevention: about 40–60 % RH; the US EPA recommends below 60 %, ideally 30–50 %.
- Temperature: set by comfort or process, separately from humidity.
Control the two with two setpoints, and make sure one controller coordinates the AC and any dehumidifier so they do not fight. See can you run an AC and dehumidifier at the same time.
In commercial buildings
Signs that the AC alone is not controlling humidity:
- RH above 60 % while temperatures are on target, especially in mild weather or after hours;
- condensation on supply diffusers, windows or cold pipes;
- musty smells, mold on ceiling tiles or stock;
- high occupancy or large outdoor-air rates.
For these spaces, a dedicated dehumidifier sized for the latent load at the room condition is usually the simplest fix. Our ceiling-mounted and ducted dehumidifiers and industrial dehumidifiers cover 26 to 960 L/day. If the AC itself may be faulty (drain, coil, airflow), see why your air conditioner is not removing humidity.
FAQ
Frequently asked questions
Does an air conditioner control humidity?
Partly. It removes moisture while cooling, but it is controlled by temperature, so humidity can stay high when little cooling is needed.
Why is my room cold but humid?
The AC reaches the temperature setpoint before removing enough moisture, often because it is oversized or the moisture load is high relative to the heat load.
What is a good sensible heat ratio?
It should match the space. Typical AC equipment is about 0.70–0.80; humid, crowded spaces may need 0.6 or lower, which requires dehumidification.
What is decoupled temperature and humidity control?
Separate equipment and setpoints for moisture (DOAS or dehumidifier, on dew point) and temperature (AC, fan coils or chilled beams, on a thermostat).
Should I use dry mode on my AC?
It removes more moisture per hour than cooling mode, but still cools. For steady humidity control without cooling, use a dehumidifier.
Sources
- US DOE, HVAC sizing research; Advanced HVAC humidity control in hot-humid climates.
- ENERGY STAR, Keep your cool and save money this summer.
- US EPA, A Brief Guide to Mold, Moisture and Your Home.
