Why are furnaces vented outdoors but ovens aren't?
In a gas oven you can cook a turkey for five, six hours, and the oven isn't vented to the outside. But run your gas furnace for any time at all without a vent and somebody is gonna die. Huh?
Now Peter. Your oven, relatively speaking, is little. Your furnace is big. Little things give us little problems. Big things give us big problems. It's not such a hard concept to grasp.
Given abundant oxygen, combustion of natural gas creates two major by-products: water vapor and carbon dioxide (two atoms of oxygen per atom of carbon). Lacking enough oxygen, however, you get carbon monoxide, with only one atom of oxygen per atom of carbon. Carbon dioxide is harmless. Carbon monoxide will kill you.
A gas range typically uses 10,000-15,000 BTUs of energy per hour. Most houses are sufficiently leaky that ample fresh oxygen can be drawn from outside to replace what's lost to combustion.
Not so with a furnace, which can use 100,000 BTUs or more. If the furnace isn't vented or if the vent is blocked, the oxygen supply is quickly depleted, resulting in lots of carbon monoxide and potentially mass asphyxiation.
You may ask: if adequate oxygen is the key, why don't I just bring in a fresh-air supply for the furnace and skip the costly chimney?
Nice try, but no. Oxygen has to circulate to burn the gas efficiently. In a gas range this is accomplished by local convection. A furnace, with its much greater oxygen demand, requires a chimney.
Perhaps you've never considered the miracle of the chimney. High time you did.
A chimney enables waste gases to escape, but that's only half the story. A well-constructed chimney also fosters draft, whereby a column of heated exhaust gases is channeled up a flue. This creates a partial vacuum in the firebox below and draws in fresh oxygen to feed the flames. (That's why you have a chimney on an outdoor barbecue pit — the fact that it keeps smoke out of your eyes is incidental.)
A proper draft is so strong the chimney needn't be sealed at the point where it exits the furnace. Often, in fact, there's an opening or gap. Don't worry, it's so fresh air can be pulled in, not so toxic gas can get out. Fact is, as long as the toxic gas can escape, there isn't any toxic gas. It's only when it can't that there is.