Pocket HVAC
What is dew point?
Dew point is the temperature air has to cool to before the water vapor in it starts to condense. It depends only on how much moisture is in the air, so it's a direct measure of humidity, unlike relative humidity, which changes every time the air warms up or cools down.
Dew point is why things sweat: any surface colder than it collects condensation. That's how an evaporator coil pulls moisture out of the air, and it's also why uninsulated ducts and suction lines drip in a humid attic or crawl space.
How it's calculated
Saturation pressure: ASHRAE eqns 5 (over ice) and 6 (over water) Humidity ratio W = 0.621945 × pw ÷ (p − pw) Wet bulb: ASHRAE eqns 33 and 35, solved by bisection Dew point: eqns 5 and 6 solved for the temperature where pws = pw Enthalpy h = 0.240 t + W (1061 + 0.444 t) Btu/lb, or 1.006 t + W (2501 + 1.86 t) kJ/kg Pressure at elevation z: p = 14.696 (1 − 6.8754×10⁻⁶ z)^5.2559 psia
Sources
- ASHRAE Handbook—Fundamentals (2017), chapter 1, Psychrometrics: equations 3, 5, 6, 20, 26, 30, 33 and 35
- Meyer & Thevenard (2019), PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air
If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.
Checked by Paul Williams · Updated
Questions
What is the dew point at 75°F and 50% humidity?
At 75°F and 50% relative humidity the dew point is 55.1°F. Any surface colder than that, like a supply duct or a cold water line, will sweat. At 70°F and 40% the dew point is 44.6°F.
Why does dew point matter for HVAC?
It tells you which surfaces will condense: uninsulated ducts and pipes in an attic or crawl space, windows, and the coil itself. A coil only removes moisture where its surface is below the air's dew point.
Does elevation change the dew point?
Not when you start from relative humidity: the dew point depends only on how much water vapor pressure the air has. From a wet bulb reading it does, because the wet bulb equation depends on air pressure, so enter the elevation for wet bulb readings above sea level.
