What Does a Flue Do?

Oct. 1, 2006
As we approach full swing of heating season in most parts of the country, our attention is turned to making heating systems work as they should. A common

As we approach full swing of heating season in most parts of the country, our attention is turned to making heating systems work as they should. A common problem at this time of year is ensuring safe operation of a furnace or water heater. One way we do this is by ensuring the nasty gases produced by combustion process are removed from the building to protect our customers from carbon monoxide (CO) poisoning.

Personal Protection

First things first. We’ve all read a hundred times that CO is a colorless, odorless deadly gas that poisons and kills people. Yet many of us have not knowingly experienced the symptoms of CO poisoning Safety pays. To refresh you, here’s a table showing CO levels and the corresponding symptoms that accompany timed exposure.

CO Level / Symptom or Standard
9 ppm / Maximum allowable short term exposure in a living area.
35 ppm / Maximum allowable CO level for 8 hours.
100 ppm / Time to take action and consider evacuation.
400 ppm / Headaches within 1-2 hours, life-threatening after 3 hours.
1600 ppm / Headache within 20 minutes. Death within 1 hour.
3200 ppm / Nausea within 5 minutes. Death within 30 minutes.
12,800 ppm / Death within 1-3 minutes.

Any questions? Please don’t take chances by not knowing the ambient level of CO around you.
The best and most cost-efficient way to protect yourself from CO poisoning is to carry a low level CO monitor in your tool bag. Get one that has a digital readout so you can check the level of CO present, and one that sounds an alarm at 10 ppm to notify you the moment you enter a hazardous situation. The cost of a good quality CO monitor can be less than $80.

In addition to being a cheap life preserver, when the alarm sounds the odds are very good your customer will become immediately aware of their need to own one, and you can extend them the opportunity and properly install it for them.


A flue is a path through which CO and other combustion byproducts can be carried from the combustion appliance to outdoors. CO is still plenty noxious outside, but if delivered to the right location, it soon becomes diluted to a harmless level.

There’s nothing magical about this sheet metal or PVC assembly. It either works all the time or it doesn’t. Endless engineering data is available proven by laboratory tests and backed up by pages in the code books, none of them guaranteeing the flue system will always work.

Who’s in the line of fire when something goes wrong? You are, or at least the person that holds your company contracting license is. More important in everyday life is that you should have a commitment to keeping yourself and your customers safe from CO poisoning.

In the big scheme of things, you need to understand three things:
• The basic principles of how a flue works
• How to design and install a venting system so it works
• How to test each venting system and document the test to prove that it works.

If you follow these three steps, you’ll sleep well at nights and enjoy the peace of mind that comes from having done this part of your job well.

What Does a Flue Do?

Here’s an interesting question I was recently asked by Captain CO himself, Jim Davis: “What size flue does a campfire need to draft?”

“None, of course,” was my answer.

“Why?” He asked.

“Because heat rises.” I answered. And, thinking I was really smart, I added, “Also because hot goes to cold.”

Flue gas temperatures are critical to proper function. Since combustion appliance discharge gas temperatures vary depending on the fuel/air ratio and mechanical issues, it’s critical to verify discharge temperatures and that the unit is not underfired. Another issue may be excess dilution air pulled into the draft hood, cooling the flue gasses and diminishing the heat rising effect.

As the conversation continued, we also discussed another simple principle of physics that’s critical when considering flue and chimney function. High pressure goes to low pressure. One key indicator of whether or not a flue works properly is the verification of a positive pressure in the mechanical area where the combustion appliance is located with reference to the outside pressure where the flue terminates.

Should there be a negative pressure in the mechanical room, it may cause the flue gasses to be pulled back down the flue and spill into the building. This may present an extreme hazard to the occupants.

It only takes a negative pressure of .015-in. w.c. to backdraft a flue. That’s fifteen hundredths of an inch of pressure! This negative pressure is often caused by return air duct leaks, or other fans (such as an exhaust fan or a dryer) operating in the envelope of a building. As you can see, it doesn’t take much to overcome the draft of a flue and create a hazardous situation.

Ideal flue pressure is a negative pressure between .01-in. and .02-in. of pressure measured about 18-in. past the draft hood or barometric damper. If draft pressure is too low, draft is nonexistent. If draft is too high, excess air from the room entering the draft hood may block the flue gasses from escaping the appliance.

The best and most reliable method of maintaining a positive pressure in a mechanical area is to pressurize it with a fan when the appliance is in operation. This can be accomplished by bringing on a combustion air fan when the equipment comes on. Another option is to install a small supply air register in the room or on the supply plenum, and measure its performance to verify that it adequately pressurizes the area with all the appliances operating.

Final Physics

One last physics visit concerning pressure and temperature. The greater the difference in pressure or temperature, the stronger the forces become.

So as the temperature between inside and outside increase, draft increases. On the other hand, as the temperature difference decreases, draft decreases. The law is true for pressure as well. The greater the pressure differences, the greater the draft and vice-versa.

Remember, too, that these laws never stop operating. This principle always remains in effect, long after we measure them or stop thinking about them. Also remember that there are many more laws of nature operating continually that we may be aware of at the moment when we’re testing. With experience, you’ll begin to see many more pressure and temperature effects operating on a system or building than you ever imagined.

In closing, understand that the flue does not create either the heat or the pressure. It simply provides a path through which they move.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, a training company specializing in measuring, rating, improving, and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a CO Poisoning Levels and Symptoms Table, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.