Understanding Dew Point Temperature for HVAC Control

On Dew Point Temperature

(https://ja.wikipedia.org/wiki/湿り空気線図)

Recently, I've had more opportunities in my work to update controllers from HVAC manufacturers and central monitoring systems (PCs). Since I often introduce (substitute) PLCs in place of the controllers,
I have increasingly had to decipher the functions from existing instrumentation diagrams and implement that logic into the PLCs.
In that process, I frequently need to calculate the "dew point temperature."

For various reasons, I haven't been directly involved in design myself, so I decided to write this article as a summary of what I learned about the "dew point temperature."

What is Dew Point Temperature in the First Place?

Dew Point Temperature:

The temperature at which water vapor contained in the air begins to cool and turn into water droplets (condensation).
It refers to the threshold line where condensation begins to occur.

When and Where is it Used?

It is mainly used for humidity control.

Dehumidification Control

Let's assume the air in this graph is the air at that location.
Air contains moisture, but there is a limit to how much it can hold.
The boundary line between the region where it can hold moisture and the region where it cannot hold any more is the "saturation curve."
Once it crosses the saturation curve, moisture overflows, and the air becomes fully saturated with water content.
Strictly speaking, the saturation curve cannot be physically crossed.

Now, assume we are at point A (where the temperature is high, and the absolute humidity is also high).
Since it feels muggy, we cool the air temperature.
As the temperature drops, it eventually reaches a point where it hits the boundary line, point B.
This point is called the "dew point temperature."

If you cool the air further from there, the amount of moisture the air can hold (absolute humidity) decreases, and eventually, the moisture overflows.
Therefore, as you cool the air, the absolute humidity of the air follows the saturation curve downward.
This is how the absolute humidity of the air is lowered.

As a premise, there are two types of humidity:
"Absolute Humidity" and "Relative Humidity."

Absolute Humidity

The "amount of moisture contained in the air" itself.
The "weight" of how many grams of water vapor are mixed into 1 kg of air.

Relative Humidity

The ratio of "how much is contained now compared to the saturation (limit)."
The "congestion rate," representing what percentage of the "capacity (maximum amount)" the air can hold at that temperature is currently filled.

The sensation of being muggy and the values shown by thermo-hygrometers are relative humidity.

By lowering the absolute humidity mentioned earlier,
the amount of moisture in the space itself decreases, so the muggy feeling disappears.
Since the relative humidity also drops as a result (the image being that the absolute humidity is reduced, or diluted),
the room becomes more comfortable.

In this way, by setting a target value for the dew point temperature,
during dehumidification, the air is cooled to reach that setting, thereby controlling the absolute humidity.

Humidification Control

When humidifying, the process follows a similar diagram.
Assuming the current state is point A in the diagram, since the air is bone-dry, we start by setting point B as the dew point temperature target.
The humidifier will continue to operate to reach point B.
However, as mentioned earlier, the amount of moisture the air can hold is determined by the temperature (dry-bulb temperature).
Therefore, in an actual field implementation, we also configure the heat source—such as a heater or hot water coil—to ensure the temperature remains above point B.
If the air handling unit (AHU) starts and humidifies immediately, the air's moisture-holding capacity is low, causing condensation. Think of it like a cup overflowing with water.
For this reason, a common implementation is to stop humidification for XX minutes during the AHU startup, and only begin humidification once temperature control has brought the air close to point B.

Why Control by Dew Point?

One might wonder, "Why not just control humidity based on indoor relative humidity?" However, the choice depends on the intended use.
It depends on whether you control using absolute humidity or relative humidity.

Indoor Humidity Control (%)

Primary Target: Humans (focus on comfort)
Key Metric: Relative humidity (sensory perception)
Typical Locations: Offices, commercial facilities, homes
Accuracy/Stability: Moderate (affected by temperature fluctuations)

• Note:
  Even if the actual moisture content in the air does not change, the percentage value will fluctuate significantly with even a 1°C change in temperature. The PLC reacts to this "apparent change" by triggering humidifiers or dehumidifiers, making control unstable.

Dew Point Temperature Control (°C)

Primary Target: Equipment/Products (focus on quality maintenance)
Key Metric: Absolute humidity (physical water mass)
Typical Locations: Factories, cleanrooms, data centers
Accuracy/Stability: Extremely high (not affected by temperature)

• Note:
  Even if the temperature changes slightly, the dew point temperature value remains steady as long as the moisture content (absolute humidity) remains constant. In other words, the PLC only operates "when the moisture content truly changes," which suppresses unnecessary hunting (control oscillation) and allows for precise management.

Control via dew point temperature provides higher accuracy.
You choose the method during the design phase based on whether you are catering to "humans" who do not require such extreme precision, or "equipment" that is sensitive and requires meticulous accuracy.

Calculation Method (Magnus Formula)

While "dew point temperature sensors" exist in the market, some design philosophies require calculating it within the controller at the field site.
The necessary inputs are:
Temperature sensor: Dry-bulb temperature
Humidity sensor: Relative humidity

The formula is below:

--st

Magnus Formula

1Result := Temperature + 243.5
2Result := 17.67 * Temperature
3Result := 2Result / 1Result
4Result := EXP (3Result)
5Result := 6.112 * 4Result
6Result := 5Result * Humidity / 100.0
7Result := 6Result / 6.112
8Result := LN (7Result)
9Result := 243.5 * 8Result
10Result := 17.67 - 8Result

DewPointTemperature := 9Result / 10Result

I made this into a tool since I use it quite often to verify values.
Dew Point Temperature Calculator

So, this was a memorandum for me to avoid forgetting these points.