Natural-Gas Flue-Gas Water Dew Point
The WATER dew point of natural-gas combustion products -- the flue temperature at which water vapor starts to condense in a vent, chimney, or heat exchanger. Methane makes 2 H2O per fuel molecule, so wet flue gas is ~19% water at stoichiometric, diluting as excess air rises: at 15% excess air the water fraction is 2/(1 + 9.52 x 1.15) = 16.7%, a ~127 mmHg partial pressure, giving (via the Antoine relation) a dew point of ~134 F (139 F near stoichiometric, lower with more excess air). This is why a non-condensing furnace/water heater must keep flue gas above ~135 F to the vent terminus -- run a masonry chimney or single-wall vent colder and the flue gas condenses to mildly acidic water that corrodes steel and spalls masonry (the reason for a liner or a Category-I vent table). A condensing appliance drops below it on purpose to harvest latent heat and drains the condensate. The water dew point (a sulfuric-acid dew point is far higher, for oil/high-sulfur fuel); shifts with fuel composition, combustion-air humidity, and altitude. A screen; the appliance listing, NFPA 54 vent tables, and the AHJ govern.
Formula and source
lambda = 1 + excess_air_pct/100; water_fraction = 2 / (1 + 9.52 x lambda); p_h2o_mmhg = 760 x water_fraction; dew_point_c = 1730.63 / (8.07131 - log10(p_h2o_mmhg)) - 233.426 (Antoine, water); dew_point_f = dew_point_c x 9/5 + 32.
Natural-gas flue-gas water dew point from methane combustion stoichiometry and the Antoine water-saturation relation, by name; the appliance listing, the NFPA 54 (National Fuel Gas Code) / manufacturer vent-sizing tables, and the AHJ govern.
Audience
This tile is built for hvac and the adjacent professions in the HVAC group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.