Dew Point Temperature Calculator
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Calculate
Dry-bulb temperature of the air
Relative humidity of the air (0–100%)
Overview
Dew point temperature is the temperature at which moist air becomes fully saturated — the point at which water vapor begins to condense into liquid water. It is the single most actionable moisture metric in HVAC engineering because it defines a precise threshold: any surface in contact with air that falls below the dew point temperature will experience condensation. Unlike relative humidity, which changes whenever temperature changes, dew point changes only when the actual moisture content of the air changes. This makes it the preferred parameter for condensation risk assessment, building envelope analysis, and moisture control design.
In HVAC practice, dew point appears in virtually every moisture-related design decision. Cooling coil selection requires knowing the apparatus dew point — the effective surface temperature at which the coil must operate to remove latent heat. Supply air dew point determines whether condensation will form on cold supply ducts. Room dew point governs whether cold water pipes, basement walls, or exterior glazing will sweat. Outdoor design dew point drives the latent cooling load calculation for any air handling unit processing outdoor air.
This calculator derives dew point from any of four common input combinations — dry-bulb temperature with relative humidity, wet-bulb temperature, humidity ratio, or vapor pressure entered directly — using the Magnus approximation for saturation pressure and standard ASHRAE psychrometric formulations. It outputs dew point as the primary result alongside dew point depression, relative humidity, humidity ratio, vapor pressure, and enthalpy, and classifies the moisture condition against ASHRAE 55 comfort thresholds and condensation risk benchmarks.
Both Imperial (°F, gr/lb, psi) and Metric (°C, g/kg, kPa) unit systems are fully supported. All formulas use the corrected Magnus constant (0.08855 for Imperial, 0.61078 for Metric) consistent with the psychrometric equations used across this calculator suite.
How to Use This Calculator
Select input combination — choose from Dry-Bulb + Relative Humidity, Dry-Bulb + Wet-Bulb Temperature, Dry-Bulb + Humidity Ratio, or Vapor Pressure Only.
Enter dry-bulb temperature — in °C or °F (not required for Vapor Pressure Only).
Enter relative humidity — in %.
Enter wet-bulb temperature — in °C or °F.
Enter humidity ratio — in g/kg or gr/lb.
Enter vapor pressure — in kPa or psi.
Click "Calculate" — get dew point temperature, dew point depression (ΔT), relative humidity, humidity ratio, vapor pressure, and enthalpy.
Compare the dew point against the coldest surface temperature (pipes, ducts, glazing); insulate any surface that falls below it to prevent condensation.
Inputs & Outputs
Inputs
Outputs
Formula
Calculator Formulas
All calculations assume standard atmospheric pressure:
- Imperial: P_atm = 14.696 psi
- Metric: P_atm = 101.325 kPa
Source: ASHRAE Handbook—Fundamentals (2021), Chapter 1, "Thermodynamic Wet-Bulb and Dew-Point Temperature" — liquid-water and over-ice branches.
Humidity Ratio Is Carried Dimensionless
Humidity ratio is computed as a dimensionless mass ratio — kg of water vapor per kg of dry air, numerically identical to lb/lb — using the exact constant 0.621945. Display units are applied only at the end: × 1000 for g/kg, × 7000 for gr/lb. One physics path serves both unit systems, so switching the unit toggle cannot change the underlying answer.
Saturation Vapor Pressure (Magnus Approximation)
The phase is chosen by the temperature being evaluated:
Over liquid water (T ≥ 0°C / 32°F):
Metric: P_sat = 0.61078 × exp(17.625 × T / (243.04 + T)) [kPa]
Imperial: P_sat = 0.08855 × exp(17.625 × Tc / (243.04 + Tc)) [psi]
Over ice (T < 0°C / 32°F):
Metric: P_sat = 0.61078 × exp(22.587 × T / (273.86 + T)) [kPa]
Imperial: P_sat = 0.08855 × exp(22.587 × Tc / (273.86 + Tc)) [psi]
where Tc = (T_°F − 32) / 1.8
Note: 0.08855 = 0.61078 × 0.1450377 (base Magnus constant × kPa-to-psi conversion)
Phase Is Selected Per Temperature, Not Per Air State
Each temperature picks its branch independently:
- the humidity-ratio equation branch is selected by T_wb
- P_sat(T_wb) is evaluated on the branch belonging to T_wb
- P_sat(T_db), used for relative humidity, is evaluated on the branch belonging to T_db
At T_db = +2°C with T_wb = −3°C the humidity-ratio equation runs on the ice branch while P_sat(+2°C) is still computed over liquid water. Applying a single phase to the whole air state is a common and avoidable error — it shifts relative humidity by several percent.
Combination A: Dry-Bulb + Relative Humidity
Step 1: P_sat_db = Magnus at T_db, on the branch belonging to T_db Step 2: P_v = (RH / 100) × P_sat_db Step 3: Dew point (inverse Magnus)
Metric:
α = ln(P_v / 0.61078)
T_dp(°C) = 243.04 × α / (17.625 − α)
Imperial:
α = ln(P_v / 0.08855)
T_dp(°C) = 243.04 × α / (17.625 − α)
T_dp(°F) = 32 + 1.8 × T_dp(°C)
The Imperial inverse runs against the Imperial Magnus constant directly. Converting psi to kPa first and inverting against 0.61078 introduces a round-trip drift of roughly 0.015°F.
Step 4: Humidity ratio
W = 0.621945 × P_v / (P_atm − P_v) [dimensionless]
Step 5: ΔT = T_db − T_dp
Combination B: Dry-Bulb + Wet-Bulb
Step 1: Saturation humidity ratio at the wet bulb, with P_sat(T_wb) on the branch belonging to T_wb
W_sat_wb = 0.621945 × P_sat(T_wb) / (P_atm − P_sat(T_wb)) [dimensionless]
Step 2: Humidity ratio from the ASHRAE enthalpy balance. The branch is selected by T_wb:
Branch A — T_wb ≥ 0°C (32°F), saturation over liquid water
Metric: W = [(2501 − 2.326 × T_wb) × W_sat_wb − 1.006 × (T_db − T_wb)]
÷ (2501 + 1.86 × T_db − 4.186 × T_wb)
Imperial: W = [(1093 − 0.556 × T_wb) × W_sat_wb − 0.240 × (T_db − T_wb)]
÷ (1093 + 0.444 × T_db − T_wb)
Branch B — T_wb < 0°C (32°F), saturation over ice
Metric: W = [(2830 − 0.24 × T_wb) × W_sat_wb − 1.006 × (T_db − T_wb)]
÷ (2830 + 1.86 × T_db − 2.1 × T_wb)
Imperial: W = [(1220 − 0.04 × T_wb) × W_sat_wb − 0.240 × (T_db − T_wb)]
÷ (1220 + 0.444 × T_db − 0.48 × T_wb)
The two branches do not join continuously at the freezing point — evaporating liquid water and subliming ice are different energy balances. ASHRAE notes increased uncertainty in a humidity ratio determined from a wet-bulb reading taken close to 0°C.
Step 3: Vapor pressure
P_v = P_atm × W / (0.621945 + W)
Step 4: RH = (P_v / P_sat_db) × 100, with P_sat_db on the branch belonging to T_db Step 5: Dew point — same as Combination A Step 3 Step 6: ΔT = T_db − T_dp
Combination C: Dry-Bulb + Humidity Ratio
Step 1:
P_v = P_atm × W / (0.621945 + W)
Step 2: RH = (P_v / P_sat_db) × 100 Step 3: Dew point — same as Combination A Step 3 Step 4: ΔT = T_db − T_dp
Combination D: Vapor Pressure Only
Step 1: apply the inverse Magnus directly to the entered vapor pressure — against 0.61078 for kPa, against 0.08855 for psi.
Note: suppress RH, ΔT, and enthalpy — T_db not available
Which Saturation Temperature Is Reported Below Freezing
Below freezing the question "at what temperature does this air saturate" has two answers, and they are not interchangeable:
- the frost point, over ice — the temperature at which ice actually deposits on a surface
- the dew point, over supercooled liquid water — the temperature at which liquid condensate would form
This calculator reports the dew point over supercooled liquid water on both sides of 0°C, and labels it as such. That basis is used so the reported value round-trips with a dew point entered as an input elsewhere in the psychrometric set. The frost point is the higher of the two — roughly 1°F above the dew point near −5°C, about 2°F near −10°C, and close to 4°F near −20°C. If you are predicting frost on a coil, a window, or a duct surface, add that offset.
Enthalpy (Combinations A, B, C)
Imperial: h = 0.240 × T_db + W × (1061 + 0.444 × T_db) [BTU/lb]
Metric: h = 1.006 × T_db + W × (2501 + 1.86 × T_db) [kJ/kg]
(W dimensionless; equivalently W_gr/7000 or W_g·kg⁻¹/1000.)
Verification
- At RH = 100%: T_dp = T_db = T_wb, ΔT = 0
- At RH = 0%: T_dp → −∞, W = 0
- T_dp always ≤ T_wb always ≤ T_db
- T_dp unchanged by sensible heating or cooling at constant W
- Combination B and Combination C describe the same air: 28°C with a 21°C wet bulb gives W = 12.67 g/kg, and entering 28°C with W = 12.67 g/kg returns the same 17.7°C dew point
- The same physical state entered in Metric and in Imperial agrees to within 0.1% after conversion
Variable Reference
| Variable | Meaning | Units |
|---|---|---|
| T_db | Dry-bulb temperature | °F / °C |
| T_wb | Wet-bulb temperature | °F / °C |
| T_dp | Dew point temperature | °F / °C |
| RH | Relative humidity | % |
| W | Humidity ratio (dimensionless internally) | gr/lb / g/kg |
| W_sat_wb | Saturation humidity ratio at T_wb | dimensionless |
| P_v | Partial vapor pressure | psi / kPa |
| P_sat | Saturation vapor pressure | psi / kPa |
| ΔT | Dew point depression (T_db − T_dp) | °F / °C |
| h | Specific enthalpy | BTU/lb / kJ/kg |
| P_atm | Standard atmospheric pressure | 14.696 psi / 101.325 kPa |
What Is Dew Point Temperature?
Dew point temperature is the temperature to which air must be cooled, at constant pressure and constant moisture content, for saturation to occur and condensation to begin. At the dew point, the partial pressure of water vapor in the air equals the saturation vapor pressure at that temperature — meaning the air can hold no more moisture and any further cooling causes water to condense out of the air onto available surfaces.
The name derives from the familiar phenomenon of morning dew — overnight radiative cooling brings outdoor surfaces below the atmospheric dew point, causing water vapor to condense on grass, leaves, and other surfaces. In HVAC engineering, exactly the same phenomenon occurs on any surface that falls below the local air's dew point: cold water supply pipes sweat in humid summer air, supply air ducts condensate when installed in unconditioned humid spaces, and cooling coils remove latent heat precisely by operating their surface temperature below the entering air's dew point.
What distinguishes dew point from relative humidity as a practical metric is its absolute character. A given dew point corresponds to a fixed vapor pressure and a fixed humidity ratio, regardless of dry-bulb temperature. When air is heated from 50°F to 75°F, the dew point remains constant while relative humidity drops from approximately 85% to 40%. This invariance through sensible temperature change makes dew point the preferred parameter for condensation risk assessment — engineers compare a fixed dew point against a fixed surface temperature, without needing to track how temperature changes affect the calculation.
Key Facts
- Dew point is independent of dry-bulb temperature. When air is heated or cooled without adding or removing moisture, the dew point does not change. Only processes that add moisture (humidification, evaporation, infiltration) or remove moisture (condensation on a cooling coil below the dew point) change the dew point.
- ASHRAE 55 identifies a dew point of approximately 62°F (17°C) as the upper comfort limit for occupied spaces. Above this dew point, occupants perceive the air as uncomfortably humid regardless of dry-bulb temperature. Summer outdoor dew points in hot-humid climates frequently exceed 70°F (21°C), which is why mechanical dehumidification is essential in these regions.
- The apparatus dew point (ADP) of a cooling coil is the effective surface temperature at which the coil operates. For the coil to remove latent heat from the air, the ADP must be below the entering air's dew point. This is the fundamental constraint in cooling coil selection for humid climates.
- Dew point depression (ΔT = T_db − T_dp) is an intuitive field metric. A depression below 5°F (3°C) indicates near-saturation conditions and imminent condensation risk. A depression of 15–35°F (8–19°C) is typical of comfort HVAC conditions. A depression above 55°F (31°C) indicates very dry air typical of cold outdoor conditions or over-heated winter supply air.
- The Magnus approximation used in this calculator for dew point inversion is accurate to better than 0.1°C across the HVAC operating range of −40°F to 200°F (−40°C to 93°C). The Imperial constant 0.08855 = 0.61078 × 0.1450377 combines the base Magnus constant with the kPa-to-psi unit conversion.
- Frost point is the temperature at which ice forms rather than liquid water — it occurs below 32°F (0°C) and is slightly higher than the dew point at the same moisture content. At temperatures below freezing, surfaces experience frost formation rather than liquid condensation.
Applications
- Condensation risk assessment on building envelopes, cold water pipes, supply air ducts, and glazing systems — compare dew point against surface temperature to determine if condensation will occur
- Cooling coil apparatus dew point determination — the coil surface must operate below the entering dew point to condense moisture and remove latent heat
- Supply air duct condensation prevention in humid climates — determine the ambient dew point that duct insulation must protect against
- Building envelope interstitial condensation analysis — compare dew point at each plane within a wall or roof assembly against the actual temperature at that plane
- HVAC commissioning and field measurement — calculate dew point from dry-bulb and wet-bulb readings to verify supply air dehumidification performance
- Weather station and outdoor design condition analysis — convert ASHRAE climate data (design dry-bulb and coincident wet-bulb) into full moisture conditions for equipment sizing
Example Calculation
Imperial Example
Given: Combination A — Dry-Bulb + Relative Humidity
- T_db = 82°F, RH = 65%
- P_atm = 14.696 psi
Step 1 — Saturation pressure at 82°F (above freezing, liquid-water branch):
Tc = (82 − 32) / 1.8 = 27.78°C
P_sat = 0.08855 × exp(17.625 × 27.78 / (243.04 + 27.78))
P_sat = 0.08855 × exp(1.8078) = 0.08855 × 6.0970 = 0.5399 psi
Step 2 — Vapor pressure:
P_v = 0.65 × 0.5399 = 0.3509 psi
Step 3 — Dew point (inverse Magnus against the Imperial constant directly):
α = ln(0.3509 / 0.08855) = 1.3770
T_dp(°C) = 243.04 × 1.3770 / (17.625 − 1.3770) = 334.67 / 16.2480 = 20.60°C
T_dp(°F) = 32 + 1.8 × 20.60 = 69.08°F
Step 4 — Dew point depression:
ΔT = 82 − 69.08 = 12.92°F
Step 5 — Humidity ratio:
W = 0.621945 × 0.3509 / (14.696 − 0.3509)
= 0.218234 / 14.3451 = 0.0152148 lb/lb
W = 0.0152148 × 7000 = 106.50 gr/lb
Step 6 — Enthalpy:
h = 0.240 × 82 + 0.0152148 × (1061 + 0.444 × 82)
h = 19.68 + 0.0152148 × 1097.41 = 19.68 + 16.70 = 36.38 BTU/lb
Result summary:
- Dew Point (T_dp): 69.08°F
- Dew Point Depression: 12.92°F
- Relative Humidity: 65.0%
- Humidity Ratio (W): 106.50 gr/lb
- Vapor Pressure: 0.3509 psi
- Saturation Pressure: 0.5399 psi
- Enthalpy: 36.38 BTU/lb
- Status: LOW DEPRESSION — HIGH HUMIDITY (ΔT 5–15°F)
Metric Example
Given: Combination B — Dry-Bulb + Wet-Bulb
- T_db = 28°C, T_wb = 21°C
- P_atm = 101.325 kPa
Step 1 — Saturation humidity ratio at T_wb = 21°C. The wet bulb is above freezing, so P_sat(T_wb) is taken over liquid water and Branch A applies:
P_sat_wb = 0.61078 × exp(17.625 × 21 / (243.04 + 21))
= 0.61078 × exp(1.4018) = 0.61078 × 4.0624 = 2.4812 kPa
W_sat_wb = 0.621945 × 2.4812 / (101.325 − 2.4812)
= 1.543168 / 98.8438 = 0.0156125 kg/kg (15.6125 g/kg)
Step 2 — Humidity ratio (ASHRAE enthalpy balance, Branch A):
Numerator = (2501 − 2.326 × 21) × 0.0156125 − 1.006 × (28 − 21)
= 2452.15 × 0.0156125 − 7.042
= 38.2843 − 7.042 = 31.2423
Denominator = 2501 + 1.86 × 28 − 4.186 × 21 = 2501 + 52.08 − 87.906 = 2465.17
W = 31.2423 / 2465.17 = 0.0126734 kg/kg = 12.67 g/kg
Step 3 — Vapor pressure:
P_v = 101.325 × 0.0126734 / (0.621945 + 0.0126734)
= 1.284157 / 0.634618 = 2.0235 kPa
Step 4 — Saturation pressure at T_db = 28°C (above freezing, liquid-water branch):
P_sat_db = 0.61078 × exp(17.625 × 28 / (243.04 + 28))
= 0.61078 × exp(1.8208) = 0.61078 × 6.1766 = 3.7725 kPa
Step 5 — Relative humidity:
RH = (2.0235 / 3.7725) × 100 = 53.64%
Step 6 — Dew point:
α = ln(2.0235 / 0.61078) = 1.1978
T_dp(°C) = 243.04 × 1.1978 / (17.625 − 1.1978) = 291.13 / 16.4272 = 17.72°C
Step 7 — Dew point depression:
ΔT = 28 − 17.72 = 10.28°C
Step 8 — Enthalpy:
h = 1.006 × 28 + 0.0126734 × (2501 + 1.86 × 28)
h = 28.17 + 0.0126734 × 2553.08 = 28.17 + 32.35 = 60.52 kJ/kg
Result summary:
- Dew Point (T_dp): 17.72°C
- Dew Point Depression: 10.28°C
- Relative Humidity: 53.64%
- Humidity Ratio (W): 12.67 g/kg
- Vapor Pressure: 2.023 kPa
- Saturation Pressure: 3.773 kPa
- Enthalpy: 60.52 kJ/kg
- Status: MODERATE DEPRESSION — COMFORTABLE (ΔT 8–19°C)
Cross-check. Feeding 28°C together with W = 12.67 g/kg into Combination C returns the same 17.72°C dew point, and entering the equivalent Imperial state — 82.4°F dry bulb with a 69.8°F wet bulb — agrees to within 0.03% after conversion. If the two routes disagree by more than about 0.5%, the entered temperatures are the problem, not the calculation.
Sub-Freezing Example — Mixed Phase
This is the case that catches most implementations: the dry bulb is above freezing while the wet bulb is below it, so the two temperatures need different phases at the same time.
Given: Combination B — Dry-Bulb + Wet-Bulb
- T_db = 2°C, T_wb = −3°C
- P_atm = 101.325 kPa
Step 1 — Saturation humidity ratio at T_wb = −3°C. The wet bulb is below freezing, so P_sat(T_wb) is taken over ice:
P_sat_wb = 0.61078 × exp(22.587 × (−3) / (273.86 − 3))
= 0.61078 × exp(−0.25017) = 0.47560 kPa
W_sat_wb = 0.621945 × 0.47560 / (101.325 − 0.47560) = 0.0029330 kg/kg
Step 2 — Humidity ratio. The branch is chosen by the wet bulb, so Branch B (over ice) governs:
Numerator = (2830 − 0.24 × (−3)) × 0.0029330 − 1.006 × (2 − (−3))
= 2830.72 × 0.0029330 − 5.030
= 8.3025 − 5.030 = 3.2725
Denominator = 2830 + 1.86 × 2 − 2.1 × (−3) = 2830 + 3.72 + 6.30 = 2840.02
W = 3.2725 / 2840.02 = 0.0011523 kg/kg = 1.152 g/kg
Step 3 — Vapor pressure:
P_v = 101.325 × 0.0011523 / (0.621945 + 0.0011523) = 0.1874 kPa
Step 4 — Saturation pressure at T_db = +2°C. The dry bulb is above freezing, so this one is taken over liquid water, even though the equation branch above ran over ice:
P_sat_db = 0.61078 × exp(17.625 × 2 / (243.04 + 2)) = 0.70528 kPa
Taking P_sat(+2°C) over ice instead would give 0.71945 kPa and drag relative humidity down by about 2% — the phase belongs to the temperature, not to the air state.
Step 5 — Relative humidity and dew point:
RH = (0.1874 / 0.70528) × 100 = 26.57%
α = ln(0.1874 / 0.61078) = −1.1817
T_dp = 243.04 × (−1.1817) / (17.625 + 1.1817) = −15.27°C
ΔT = 2 − (−15.27) = 17.27°C
Result summary:
- Dew Point (T_dp): −15.27°C (over supercooled liquid water)
- Dew Point Depression: 17.27°C
- Relative Humidity: 26.57%
- Humidity Ratio (W): 1.15 g/kg
- Vapor Pressure: 0.187 kPa
- Saturation Pressure: 0.705 kPa
- Enthalpy: 4.90 kJ/kg
- Status: MODERATE DEPRESSION — COMFORTABLE (ΔT 8–19°C)
The reported −15.27°C is the supercooled-liquid dew point. The frost point for the same air is −13.62°C, about 1.7°C higher — that is the temperature at which ice would actually begin to deposit on a cold surface, and it is the number to use for frost prediction.
Standards & References
- ASHRAE Handbook — Fundamentals (2021), Ch. 1 Psychrometrics; Ch. 27 Moisture in Building Construction — Primary references for dew point formulations, saturation pressure equations, psychrometric relationships, and interstitial condensation analysis implemented on this page.
- ASHRAE Standard 55-2020 — Thermal Environmental Conditions for Human Occupancy — Identifies a dew point of approximately 62°F (17°C) as the upper humidity comfort limit for occupied spaces.
- ASHRAE Standard 62.1-2022 — Ventilation and Acceptable Indoor Air Quality — References sustained relative humidity above 70% as a condition that promotes mold growth and represents an IAQ concern.
- Magnus Formula (Alduchov-Eskridge, 1996) — Used for both saturation pressure and inverse dew point calculation. Constants 17.625 and 243.04 are optimized for the temperature range −40°C to +60°C.
- Psychrometer Coefficients (context, not the method used) — Combination B solves the ASHRAE enthalpy balance, not a psychrometer correlation. The two are often confused. The canonical Sprung form works in partial pressures and carries an explicit atmospheric-pressure factor: e = e_s(T_wb) − A × P × (T_db − T_wb). Coefficients of about 0.000653–0.000662 °C⁻¹ belong to well-aspirated psychrometers, while about 0.00077–0.00080 °C⁻¹ belongs to instruments in a naturally ventilated screen such as a Stevenson shield. Those coefficients apply to vapor pressure and do not transfer directly into the humidity-ratio equation.
- ASHRAE Handbook — Fundamentals (2021), Chapter 27: Moisture in Building Construction — Governs interstitial condensation analysis using dew point as the condensation threshold at each plane in a building assembly.
Limitations
- All calculations assume standard atmospheric pressure of 14.696 psi (101.325 kPa). At elevations above approximately 1,000 ft (300 m), lower atmospheric pressure increases humidity ratio for the same dew point. Dew point itself is independent of total pressure and remains valid at any altitude, but the associated humidity ratio and enthalpy values require altitude-corrected atmospheric pressure for accuracy.
- The Magnus approximation loses accuracy below approximately −40°F (−40°C) and above approximately 200°F (93°C). Frost point — the temperature at which ice rather than liquid water forms — deviates from the thermodynamic dew point by 1–3°F (0.5–1.5°C) at temperatures below 32°F (0°C).
- Combination B solves the ASHRAE enthalpy balance rather than a psychrometer correlation, so it is not tied to any particular instrument’s ventilation rate. A field reading taken with a real psychrometer still carries instrument-specific error: a naturally ventilated or screen-shielded sensor reads differently from a well-aspirated one, and the resulting dew point can differ by 1–3°F (0.5–1.5°C). Near 0°C the spread widens further — ASHRAE notes increased uncertainty in a humidity ratio determined from a wet-bulb reading close to freezing, and the liquid-water and over-ice branches do not join continuously there.
- Dew point calculated at a single state point does not account for spatial variation in moisture distribution within a space. In large or stratified spaces, the dew point may vary significantly with location.
Common Mistakes to Avoid
- Confusing dew point with wet-bulb temperature. Both are temperatures associated with moisture content but they are fundamentally different quantities. Wet-bulb is the equilibrium temperature of an evaporating water surface in the air — it lies between dry-bulb and dew point, and depends on all three psychrometric state variables. Dew point depends only on vapor pressure. They are equal only at 100% relative humidity.
- Assuming dew point changes when air is heated. Dew point is conserved through sensible heating and cooling — it changes only when moisture is added or removed. Air heated from 50°F to 80°F maintains exactly the same dew point; only relative humidity drops.
- Using relative humidity alone for condensation risk assessment. Relative humidity cannot determine condensation risk without knowing the dry-bulb temperature and the surface temperature. A dew point of 65°F at 80°F DB and 60% RH will condense on any surface below 65°F — regardless of what the relative humidity reading appears to indicate. Always use dew point for surface condensation assessment.
- Applying the wrong Imperial P_sat constant. The correct Imperial saturation pressure formula uses 0.08855 = 0.61078 × 0.1450377. Using only 0.1450377 (the unit conversion factor alone) produces P_sat values inflated by a factor of 6.89 and dew point results that are grossly incorrect.
- Comparing dew point against surface temperature without accounting for surface temperature depression due to airflow. In practice, supply air impinging on a surface at high velocity can lower the local surface temperature below the bulk surface temperature, increasing condensation risk.
- Treating the ASHRAE 55 dew point comfort limit of 62°F (17°C) as a hard code requirement. ASHRAE Standard 55 defines a comfort envelope that includes an upper dew point recommendation, but it is a thermal comfort standard, not a ventilation code.
Frequently Asked Questions
What is dew point temperature and why does it matter in HVAC?
What is the difference between dew point and wet-bulb temperature?
What is dew point depression and how is it used?
How does dew point relate to cooling coil design?
Why does dew point stay the same when air is heated?
What dew point should I design to for a comfortable occupied space?
Can I use this calculator for outdoor air dew point analysis?
Is this calculator valid below freezing temperatures?
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Calculate
Dry-bulb temperature of the air
Relative humidity of the air (0–100%)