Every cooling load in a building is really two loads stacked on top of each other. One of them cools the air. The other one dries the air. If your engineer designed for the first and ignored the second, the building will hit its setpoint and still feel wrong. This note is about why those two loads exist, why the difference matters, and what to ask when a space is running cold and clammy at the same time.
The two loads, in plain terms
Sensible load is heat you can feel with a thermometer. Sun through a window. Bodies giving off warmth. Lights, computers, ovens, servers. All of it raises the air temperature. When the thermostat calls for cooling, this is what it is responding to.
Latent load is heat carried by water vapor in the air. It does not change the temperature you feel, but it changes the moisture content of the room. People breathing and sweating add moisture. Kitchens, showers, indoor pools, and outdoor air pulled in for ventilation all add moisture. A room can hold a lot of water vapor before it feels muggy, but once it crosses that line, occupants notice immediately.
An air conditioner removes both at the same time. Air passes over a cold coil, cools down (that is the sensible part), and if the coil is cold enough, water vapor condenses out onto it and drains away (that is the latent part). The ratio between the two is called the sensible heat ratio, or SHR, and it is set almost entirely by how cold the coil is.
Why buildings feel muggy at 72
The common failure looks like this. The building was designed for a hot summer day. The engineer sized the cooling equipment against a 97-degree design temperature and picked a unit large enough to handle it. On that day, the unit runs hard, the coil gets cold, and both sensible and latent loads come off the space.
But most of the year is not that day. On a mild, humid morning in April or October, the outdoor temperature is only in the seventies, but the outdoor humidity is very high. The building's sensible load is small because it is not hot outside. The unit only needs to run for a few minutes to hold the setpoint. During those few minutes, the coil never gets very cold, and it removes only a small amount of moisture.
Meanwhile the latent load is still there. People are still breathing. The kitchen is still running. Outdoor air is still coming in through the ventilation system. Moisture builds up in the space faster than the short cooling cycles can remove it. The thermostat reads 72. The room feels 78 with a swamp in it.
A concrete example
| Condition | Sensible load | Latent load | Sensible heat ratio |
|---|---|---|---|
| Peak summer (97°F outdoor) | High | Moderate | 0.80 to 0.85 |
| Mild humid morning (75°F outdoor, saturated) | Low | Still moderate | 0.50 to 0.60 |
| What a standard rooftop unit is built for | Both | Both | Fixed near 0.75 |
What actually fixes it
There are three levers, and any real solution uses more than one.
Right-size the equipment. Oversized cooling equipment is the number one cause of humidity complaints. A unit twice as big as the space needs will cycle on for two minutes and off for twenty. Short cycles remove almost no moisture. Correctly sized equipment runs long, gentle cycles that give the coil time to get cold and pull water out.
Cool the coil down. A cooling coil selected to leave the air at 55 degrees will dehumidify aggressively. A coil selected to leave the air at 63 degrees will barely dehumidify at all. The warmer coil looks efficient on paper because it uses less energy per hour, but the space it serves will be humid whenever the sensible load is low.
Add reheat, or add dedicated dehumidification. On buildings with high latent loads (healthcare, restaurants, veterinary, natatoriums, or any building with substantial outdoor air), the equipment needs to be able to overcool and then reheat, or run a dedicated dehumidification cycle. This is not a control setting. It is a piece of equipment that either exists in the design or does not. If it does not, no amount of thermostat programming will fix the humidity problem.
What to ask your engineer
- What is the design sensible heat ratio of the cooling equipment, and how does it compare to the space it serves?
- What is the coil's leaving air temperature at design conditions?
- Was the equipment sized for peak sensible load only, or was a wet-bulb design day evaluated?
- Is there any provision for dehumidification when the sensible load is low, and how does it operate?
- What ventilation airflow is being assumed, and how much moisture does that outdoor air carry?
If the answers are vague, the humidity problem is a design problem, not a maintenance problem. The building will not get better by adjusting the thermostat.
The one-line version
Temperature and humidity are two different loads with two different fixes. A building that ignores the latent side will feel wrong even when it looks fine on the temperature log.