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Is it safe to use the same outside vent outlet for a gas dryer and gas hot water heater? The 3" diameter, 18" long, water heater duct joins the 4" diameter, dryer duct in a Y right at the wall with 2" shared duct to the outside. The alternative is 15' of 3" duct to the outside for a stand-alone water heater vent and the dryer to a separate 4" diameter, 10' long duct to the outside.

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NO

Dryers need separate ventilation because lint gets into their ventilation system and builds up there -- in a shared vent, this'd cause excess backpressure that the water heater can't tolerate, leading to backdrafting, CO, and an unexpected visit to the ER. Furthermore, a gas dryer is a forced draft appliance, which aggravates the backdrafting issue and will send you to the hospital with the CO detector ringing in your ears well before lint buildup will.

Atop that, Section M1502.2 of the International Residential Code prohibits what you are asking about:

M1502.2 Independent Exhaust Systems

Dryer exhaust systems shall be independent of all other systems and shall convey the moisture to the outdoors.

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    I believe there is also another code about requiring different vent paths outside individually for any gas venting appliance. (for the same reason, avoiding backdrafting) – noybman Sep 21 '17 at 1:34
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    @noybman -- you can have shared vents for pure atmospheric draft appliances – ThreePhaseEel Sep 21 '17 at 1:35
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    Also, "10.4.4 A clothes dryer exhaust duct shall not be connected into any vent connector, gas vent, chimney, crawl space, attic, or other similar concealed space." NFPA 54(2009)(Fuel Gas Code):10.4.4. For completeness, "A clothes dryer exhaust duct shall not be connected into any chimney connector, vent connector, chimney, or vent." NFPA 211(2013)(Chimneys, Fireplaces, Vents...):10.7.3.4. Only living CO victims get a ride to the E.R. Not everyone has a CO detector. – Upnorth Sep 21 '17 at 4:59

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