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I live in a house in Ohio and I have four appliances powered by natural gas: furnace, water heater, dryer, and kitchen range.

Of these, only the furnace and water heater are vented to the outside of the house (chimney stack). In this context, venting refers to a flue that directs combustion byproducts outside the house as opposed to a vent for waste water or the vent that directs hot air and lint away from the dryer.

While I am no building contractor, I have been in quite a few houses in my life and I have always seen this same configuration when it comes to appliances powered by natural gas here in the U.S.

Aside from "because code says so" is there a reason why furnaces and water heaters require venting while dryers and ranges do not? Are there cases where building code may allow not venting furnaces or water heaters, or require venting for dryers or ranges?

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    My dryer certainly vents the combustion products outside, along with it's exhaust heat and moisture. I've never seen a dryer that vents it's combustion products inside, though I admit to not having seem many dryers outside of southern California. – Fake Name Jan 5 '15 at 11:56
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    This question is flawed in that dryers do all require exhaust vents. And most gas ranges are vented as well (though as stated, they aren't typically used the same amount as the other appliances so there's less of a risk) – DA01 Jan 5 '15 at 19:10
  • I'm pretty sure most gas dryers vent the gas combustion products via the "hot air" vent, typically routed through the side of the house. One should not run a gas dryer with the hot air not vented outside (and it's far from ideal to run an electric dryer that way). – Hot Licks Jan 5 '15 at 21:49
  • Good point, I had not thought about the dryer venting its combustion products out with the hot air and lint, but it makes perfect sense. – user4302 Jan 5 '15 at 21:52
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    I took my dryer apart today and saw firsthand that the burner vents into the drum, which then goes out the dryer to the duct and outside. So I guess it does vent to the outside. – user4302 Jan 11 '15 at 4:32
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Byproducts of combustion of natural gas are carbon dioxide and water vapour. Unfortunately natural gas isn't pure methane, it has other components (called condensates by the petroleum industry). You can see these other components by watching a burning gas flame - methane burns blue, other components burn yellow, orange, red, etc. These other components contribute to indoor pollutants.

Overall, it's a matter of volume, or amount of gas burned every day: - furnace is 100,000 BTU or more - HWT is 40,000 BTU and up - stove burner is 3,000-7,000 BTU ("power burners" could be up to 12,000 BTUs)

Besides the volume of gas that is burned by the appliance, the HWT and furnace tend to run for longer periods every day. Besides exhaust venting, building codes also require combustion air source for these appliances.

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  • Note that a non-blue flame can also be a sign of incomplete (ie. carbon-monoxide-producing) combustion of methane, usually because of insufficient oxygen. – Mark Jan 5 '15 at 21:44
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Your gas dryer vents its combustion products outside along with the moisture from your clothes, so it is vented to the outside.

Your oven doesn't vent out mostly for sake of having limited combustion. There are ventless heaters available. However you run it, though, any natural gas burning device will create water vapor and carbon dioxide. If it malfunctions, you also risk it producing carbon monoxide. Stoves and ovens get by because they have relatively low BTU output and in a residential setting relatively low use. In a commercial setting, you'd achieve exterior venting with a ventilation hood.

When you get to devices with higher output such as a clothes dryer or furnace, you really want that out of the house. The impracticality of venting your stove, though, outweighs the relatively small amount of downside.

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    > In a commercial setting, you'd achieve exterior venting with a ventilation hood. --- My apartment and my parent's house both have ventilation hoods over their electric stoves, intended for preventing the kitchen from filling with smoke from food. Is this not common? – Random832 Jan 5 '15 at 14:36
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    It is common in modern housing for residential electric stoves to have vent hoods too. I haven't ever lived anywhere without one. I was aiming for emphasis that a large gas stove's ventilation system is one of those hoods. – Jeff Ferland Jan 5 '15 at 16:01
  • Almost all stoves have a vent fan for getting rid of smoke (usually from frying bacon). If you were the least bit worried you could just turn it on while cooking and it will vent well enough. – Joshua Jan 5 '15 at 18:33
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    !?! Bad design. Mine (gas stove) vents out the roof. – Joshua Jan 5 '15 at 18:38
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    @Michael while common, it's not preferred by any means. Most new houses are (rightfully so) built so the hood vent exhausts to the outside of the home. – DA01 Jan 5 '15 at 19:12
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This appears to be a simple misunderstanding of how a gas dryer works. The gas dryer creates heat by burning air and the gas together, then blowing the heated air and combustion mixture through the clothes and then out the dryer vent.

In other words, the combustion gases are vented to the outside, per code, along with the moisture from the clothing. Since most gases used in a household setting are clean burning, there is no noticable "smoke" residue left in the clothes, but the product of combustion goes through the clothes with the rest of the air it heated, and then out the vent. If you use a gas with significant byproducts, you would find residue on your clothing.

Gas dryers are not permitted to vent indoors for this reason.

In this context, venting refers to a flue that directs combustion byproducts outside the house as opposed to a vent for waste water or the vent that directs hot air and lint away from the dryer.

In this context, the vent that directs combustion products is the vent that directs hot air and lint. They are one and the same for gas dryers.

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All gas dryers must vent outside or carbon monoxide would build possibly killing you. Since electric ones don't produce CO, some people vent them indoors for heat but the moisture could cause mold in the vented area. So, vent all driers outside to be safe.

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