Dew Point & Condensation Risk

Surface condensation check for pipes, ducts, and building surfaces
Air Conditions
Psychrometric Results
Dew Point
—
°F
Wet Bulb
—
°F
Humidity Ratio
—
gr/lb
Enthalpy
—
BTU/lb
Condensation Risk — Surface Temperature Check
—
—

Common Surface Temperatures
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About This Calculator

This dew point calculator takes a dry-bulb temperature and relative humidity and returns the dew point, wet bulb, humidity ratio, and enthalpy of the air, then checks the result against common HVAC surface temperatures to flag where condensation is likely. It is built for quickly screening pipes, ducts, glazing, and building surfaces for sweating and moisture problems.

Dew point is the single most useful number for condensation control: any surface colder than the dew point will collect water. The tool reports the margin between room temperature and dew point, grades the risk, and tags surfaces such as chilled-water pipe and cold supply duct so you can confirm insulation or dehumidification before moisture causes damage or mold.

Formula & Method
Dew point (Magnus)α = ln(RH÷100) + a·T ÷ (b + T)
thenTdp = b · α ÷ (a − α), a = 17.625, b = 243.04 (T in °C)
Humidity ratioW = 0.62198 · Pw ÷ (P − Pw)
Enthalpyh = 0.240 · T + W · (1061 + 0.444 · T)

Dew point is computed with the Magnus approximation (a = 17.625, b = 243.04), a standard form consistent with ASHRAE Fundamentals Chapter 1 psychrometrics. Vapor pressure Pw = RH · Pws uses a Magnus-type saturation pressure, humidity ratio follows W = 0.62198 · Pw ÷ (P − Pw) at standard sea-level pressure, and wet bulb uses the Sprung approximation. Condensation is flagged whenever a surface temperature falls at or below the calculated dew point.

Frequently Asked Questions
What is dew point and why does it matter in HVAC?
Dew point is the temperature at which air becomes saturated and water vapor begins to condense. Any surface at or below the dew point will collect moisture. In HVAC it sets where you will get condensation on chilled-water pipes, supply ducts, cold glazing, and uninsulated equipment, so it drives insulation thickness and dehumidification targets.
How is dew point calculated from temperature and relative humidity?
This tool uses the Magnus formula. It computes alpha = ln(RH/100) + a times T divided by (b + T), with a = 17.625 and b = 243.04 and T in degrees Celsius, then dew point equals b times alpha divided by (a minus alpha). The result is converted back to degrees Fahrenheit. This is a standard, accurate approximation for typical building air conditions.
When is there a condensation risk on a surface?
Condensation occurs whenever a surface temperature is at or below the air dew point. The calculator flags high risk when the dry-bulb-to-dew-point margin is under 5 degrees Fahrenheit, moderate risk under 15 degrees, and low risk above that, and it checks common surfaces such as 44 degree and 50 degree chilled-water pipe, 55 degree glass and supply duct, and warmer slab and wall temperatures.
How do I prevent condensation on chilled water pipes?
Keep the pipe surface above the room dew point with continuous closed-cell or fiberglass insulation sized for the design dew point, and seal the vapor barrier so humid air cannot reach the cold surface. Where dew points are high, lower the space humidity with dehumidification rather than relying on insulation alone.
Related Tools
Psychrometric Calculator Wet Bulb / RH Humidity Ratio Enthalpy Calculator Coil Leaving Air
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.