If you’re not interested in the details, I’ll spare you. The short answer is:

55°F (or 12.5°C for the SI folks out there) corresponds to the dew-point temperature under what is considered comfortable normal room temperature and humidity levels (75°F and 50% RH). And by setting the supply air temperature to this value, engineers basically are guaranteeing that they aren’t going to have humidity issues in the space.

Okay. Now for the more in-depth look at this.

This is a question I often ask of the students/interns that have worked with me:

“What should the supply air temperature be to cool a space and why?”

Most of the time, I will initially get blank stares back. Not many young engineers have given that much thought to the temperature of the air that comes out of their air conditioning units. Most will realize that it has to be less than the room temperature and probably greater than 32°F, considering then you’re going to start freezing moisture.

From here, I’ll usually transition into typical humidity control in air handling units. An important point that I’ll make next is this:

Temperature sensors are generally more reliable and robust than humidity sensors.

This is not a knock against manufacturers of humidity sensors. It’s simply a more difficult property to measure.

For a person to be comfortable in a space, both the temperature and humidity need to be in particular ranges (there are other factors, like radiation, but temperature and humidity are generally the most important to get right). We can control temperature pretty easily; if we need to cool the space, we can either lower/raise the supply temperature or adjust the flow of the hot/cold air. But what about humidity? Do we put a humidifier and then a dehumidifier in the air handling unit?

No. Most of the time we don’t control to a humidity setpoint. So for humidity control, we normally don’t have any active actuator or mechanism to actively maintain a constant humidity level.

But we still need to maintain comfortable conditions in the space! We can not allow the humidity to go too high not only for comfort but also for other moisture-related problems like mold. Going in the opposite direction isn’t as bad. You can send drier air and the occupants normally won’t complain as much. Obviously, there is some lower bound at which your skin dries out and there can be other side effects, so engineers can’t take too much freedom in this. Where I live, Dallas, it’s rare that the outdoor conditions cause severe issues with dry air.

I’ll then ask my students, “How does a dehumidifier work?”. Some will have an answer and some won’t. Besides some fancy techniques, the best we can normally do is cool the air to the dew point temperature and start to condense liquid water out. Some engineers think (and say) of this as squeezing the moisture out of the air.

So if you ignore for the moment spaces that have large latent heat loads and focus on spaces like office buildings, the humidity in the space is directly controlled by the supply air humidity. So the question “what is the highest level of humidity I can send into the space and still be comfortable?” naturally arises. “Comfortable conditions” have been shown in numerous experiments to exist in a fairly small window of temperatures and humidity levels (at least in my mind I think it’s pretty small considering what our ancestors had to live through). A reasonable, comfortable condition can be taken at 75°F and 50% RH. If you crunch the numbers (assuming standard sea-level atmospheric pressure!), the dew point temperature, the temperature at which you will start dehumidifying the air, is the magic 55°F.

So when the supply air is sent to the space at 55°F, we are using thermodynamics to guarantee that the space will not have humidity at levels higher than what is comfortable. If the outdoor air humidity levels are dry, with a dew point temperature < 55°F, that’s fine too, we’ll just send a little bit drier air.

We can then size our air handling unit to send the required flow at 55°F to meet our design cooling loads and our unit is on its way to operation!

So this is why if you would walk into any random air handling unit in the United States and check the supply air temperature setpoint, there’s a good chance that it is at 55°F. Now whether this is the most energy-efficient or best way to do it, that’s a whole ‘nother topic which we’ll have to explore in a future post!