Condensing Appliance Flue Condensate Rate
The condensate a high-efficiency condensing furnace, boiler, or water heater drains -- the number that sizes the drain, condensate pump, and neutralizer. Natural gas MAKES water (CH4 -> 2 H2O), ~9.4 lb per therm (100,000 BTU): a 100,000 BTU/hr appliance produces ~9.4 lb/hr of water vapor. A condensing appliance cools the flue below its dew point to recover latent heat, condensing a fraction (~0.8-0.9 at design return temps), so condensate = (input/100,000) x water-per-therm x condensing fraction / 8.34 lb/gal = ~0.96 gph per 100,000 BTU/hr, ~1.5 gph for a 150,000 BTU/hr boiler. The condensate is mildly acidic (pH ~3-5), so codes increasingly require a limestone/marble neutralizer before cast-iron/copper drains or septic, and the drain must be trapped, pitched, and freeze-protected. A sizing aid; the rated condensate output, the plumbing code (drain/trap/neutralizer), and the manufacturer govern.
Formula and source
therms_per_hr = input_btu_hr / 100000; water_produced_lb_hr = therms_per_hr x water_lb_per_therm; condensate_gph = water_produced_lb_hr x condensing_fraction / 8.34. Natural gas produces ~9.4 lb water per therm.
Condensing appliance flue condensate rate from the water of combustion (methane stoichiometry) and the condensing fraction, by name; the appliance's rated condensate output, the local plumbing code (drain, trap, neutralizer), and the manufacturer's instructions govern.
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