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?
Best Answer
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.