Straight Dope | The Straight Dope | Chicago Reader

Straight Dope 

In a gas oven you can cook a turkey for five, six hours, and the oven is not vented to the outside. But run your gas furnace for any time at all without a vent and somebody is gonna die. Huh?

--DPeter6857, via AOL

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.

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 the vent is blocked, the oxygen supply is quickly depleted, resulting in lots of carbon monoxide and a bunch of asphyxiated folks.

But you're Joe Skeptic. You're saying, hmm, if adequate oxygen is the key, what if I just bring in a fresh-air supply for the furnace and bag that costly chimney?

Good effort, doofus, but no. Oxygen's gotta circulate to combust the gas efficaciously. In a gas range this is accomplished by local convection. The much greater oxygen demands of a furnace require a chimney.

Perhaps you've never considered the miracle of the chimney. High time you began.

It is of course true that a chimney enables waste gases to escape, but this doesn't convey the ingenuity of the thing. A well-constructed chimney fosters draft, whereby a column of heated exhaust gases is channeled energetically 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 chimneys on outdoor barbecue pits. The fact that they keep the smoke out of your eyes is incidental.)

A proper draft is so strong that the chimney need not be sealed at the point where it exits the furnace in order to do the righteous work of exhaust-gas removal. Often, in fact, there is an opening or gap of some kind. Don't worry, it's so fresh air can be pulled in, not so toxic gas can get out. As long as the toxic gas can escape, there isn't any toxic gas. It's only when it can't that there is.

With the recent deluge of hurricanes and tropical storms in the Atlantic, I couldn't help but wonder--why do most weather systems move from west to east over North America, but hurricanes and tropical storms in the Atlantic move from east to west?

--Ed in Massachusetts

Because they start in the tropics, you silly goose, where the prevailing winds, aka the trade winds, are out of the east. Lucky for Columbus and a million other mariners in the age of sail. Maybe not so lucky for you.

Just to give you the big picture--you know how urgently I want you guys to get clear on the concept--the basic flow of winds in the North Atlantic in hurricane season is clockwise. The center of this circular flow is something known as the "Bermuda high," which in the summer months typically parks itself in the mid-Atlantic somewhere between 30 and 35 degrees north latitude.

For complex geophysical reasons having to do with the rotation of the earth, the tropical winds in the hurricane-spawning region south of the Bermuda high basically blow west. Once a hurricane gets up to speed, it may continue due west across Central America and out to the Pacific. More commonly, it may circle around the Bermuda high, first northwest and then north. Later, after having leveled or at least threatened various points of interest on the eastern seaboard, the hurricane or what's left of it heels over to the northeast and east and back out to sea.

You get the picture, I'm sure of it. And all done without a Telestrator.

Is there something you need to get straight? Cecil Adams can deliver the Straight Dope on any topic. Write Cecil Adams at the Chicago Reader, 11 E. Illinois, Chicago 60611; E-mail him at; or visit the Straight Dope area at America Online, keyword: Straight Dope.

Art accompanying story in printed newspaper (not available in this archive): illustration by Slug Signorino.

Support Independent Chicago Journalism: Join the Reader Revolution

We speak Chicago to Chicagoans, but we couldn’t do it without your help. Every dollar you give helps us continue to explore and report on the diverse happenings of our city. Our reporters scour Chicago in search of what’s new, what’s now, and what’s next. Stay connected to our city’s pulse by joining the Reader Revolution.

Are you in?

  Reader Revolutionary $35/month →  
  Rabble Rouser $25/month →  
  Reader Radical $15/month →  
  Reader Rebel  $5/month  → 

Not ready to commit? Send us what you can!

 One-time donation  → 

Agenda Teaser

Performing Arts
July 15
Bitchin Bajas Constellation
July 23

Popular Stories