Natural-Gas Flue-Gas Water Dew Point

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.

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.

The stoichiometry (CH4 + 2 O2 -> CO2 + 2 H2O) and the Antoine saturation equation are public physics; the excess-air level is the combustion setting.

Estimate. AHJ and licensed professional govern.

Field names used by the API: excess_air_pct, water_vapor_pct, dew_point_f

• Methane combustion + Antoine water fraction = 2/(1 + 9.52*lambda); dew point from the Antoine water-saturation relation on the water partial pressure at 1 atmcombustion stoichiometry + NIST Antoine constants
• Venting consequence non-condensing appliances must stay above ~135 F to the terminus or the vent condenses acidic water (liner / Category I table); condensing appliances drop below on purposeNFPA 54 / appliance listing