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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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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.

Jeff Ferland
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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.

Adam Davis
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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.

Mark
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