In my recent article, “Hidden Problems Can Cause Carbon Monoxide”(The News,October 10, page 10), I discussed, among other things, the combustion air system. This system is made up of the venting to the furnace enclosure, which allows fresh air to be drawn from the outside of the structure to the inlet of the heat exchanger; the heat exchanger itself; and the flue system, which allows the combustion products being generated by the furnace to exit the structure to the outside. In this article, I’d like to cover more about the discharge side of the combustion air system, the flue.

Natural Draft Basics

Most noncondensing residential furnaces use natural draft to drive the combustion process. Even 80%-AFUE furnaces with combustion blowers are natural-draft furnaces. The combustion blower in an 80% furnace assists the combustion gases, but the flue still operates like a natural-draft furnace.

The natural pressure differential created by heating the combustion gases in the furnace is what pushes the flue gases up and out of the flue.

As combustion gases are heated, they expand. This makes them less dense than the colder air entering the furnace, so they weigh less. They also exist at a lower pressure than the colder inlet air. Since the combustion gases are at a lower pressure, the higher-pressure inlet gases displace them, pushing them up and out of the flue. For the flue to operate properly, the outlet gases must always be at a lower pressure than the inlet air so that this pressure differential can be maintained.

When this pressure differential is diminished by the flue pressure becoming too positive or the combustion air inlet becoming too negative, air stops drafting through the furnace. That’s when CO is created. And if the furnace is not drafting, the CO ends up staying in the residence.

Depending on the outdoor temperature, the pressure differential needed to drive the flue gases up and out of the furnace is somewhere between 1 and 5 Pascals (0.004 to 0.020 in. of water column [wc]). What this means is, as flue gases leave the furnace, they must be at -1 to -5 Pascals relative to atmospheric pressure.

A Pascal is about the weight of a Post-it® note. So you can see that the pressure differential needed to be maintained between the inlet and outlet of the furnace’s combustion air system is very delicate and can be easily disturbed. Anything that raises the pressure in the flue or lowers the pressure in the combustion air inlet system can interrupt the flow of air through the furnace and create carbon monoxide.

Remember, in order for the flue to work properly, it must be able to maintain the flue gases at a negative pressure relative to the pressure of the combustion inlet air at the entrance to the furnace heat exchanger. There are basically two things that can cause the pressure in the flue system to become too positive:

1. The flue is restrictive, causing the pressure in the flue to rise; or

2. The flue is dissipating the flue gas heat, causing the pressure to increase in the flue (remember that the heat is the reason the gases are at a negative pressure in the first place).

Flue Restrictions

Let’s take a look at the more common things that can cause the pressure to rise in the flue as a result of a restriction.

  • Flue pipe is too small for the amount of combustion gases generated by the furnace.
  • The most common variables for which the flue must be sized are Btu capacity of the furnace, the horizontal and vertical length of the flue, and the number of elbows and type of vent connector (the part of the flue that connects the furnace to the main flue pipe).

    Designing the flue size is too involved for this article. My opinion is that any installer who installs a flue should be able to design it. Designing a flue is a relatively easy task and flue design instructions are readily available. Some manufacturers supply design instructions in their furnace installation books. Your gas utility may have a flue design workbook, or you can get one from the American Gas Association (AGA).

  • There are too many elbows.
  • The design tables for flue sizing assume the flue will have two elbows. If there are more, this could be a possible red flag. Flues can have more than two elbows, but this should be taken into consideration in the design stage.

  • There is inadequate rise.
  • Horizontal flue runs should have a rise of 1/4 in. per 10 ft of horizontal run. This promotes the flow of flue gases and prevents condensates from trapping in the flue.

  • Consider the horizontal/vertical ratio.
  • There should always be at least as much vertical pipe as there is horizontal pipe. Too much horizontal pipe will create a restriction.

  • There is improper flue termination.
  • The flue termination should always be elevated above the roof an appropriate amount and be kept away from vertical walls (Figure 1).

  • The flue cap is missing.
  • Flue caps play a more significant role than might be expected. Of course they keep the rain and critters out, but they also prevent down drafts in the flue pipe, as well as creating a venturi effect that assists the gases in their escape.

  • There are flue obstructions.
  • This is an easy one. There should be no obstructions in the flue. By the way, some installers who become frustrated by the difficulty in mating double-wall flue pipe bend in the inner lining to defeat the troublesome locking mechanism to ease assembly. This can cause a flue system to malfunction from restrictions and/or because the dead insulative air space between the flue pipe walls has been compromised.

  • The vent connector is installed improperly.
  • The vent connector should exit the furnace straight for at least 18 in. before any turns are made. This allows the flue gases to gain some velocity before meeting any resistance to flow.

    Heat Dissipation

    Some of the more common things that can cause a flue to dissipate too much heat are the bent double-wall lining (mentioned above) and:

  • Flue pipe is too big for the furnace.
  • Excessively large flues reduce the velocity of the combustion products, allowing them more time to dissipate heat through the pipe walls. Flues that were acceptable for natural-draft furnaces are often too large for the new, 80% induced-draft furnaces. This is because the new furnaces draw more heat out of the combustion gases before they enter the flue.

    These gases are cooler, so they can tolerate less heat loss before they begin to condense acid. Another thing that happens commonly in the field is that when an old furnace gets replaced with a new one, it is often downsized. If the flue pipe is not redesigned and reinstalled, it ends up being too large for the application.

  • Single-wall flue pipe was used in an unconditioned space.
  • Double-wall vent should always be used in unconditioned spaces to help retard heat loss.

  • New furnaces were vented into unlined chimneys.
  • Unlined chimneys suck heat like crazy, causing the pressure to increase in the chimney. A secondary problem to this is that the cooler combustion products in the chimney condense and leave acid behind that attacks the chimney structure. (Don’t you hate it when a chimney falls on your head?)

    Your local codes overrule any guidelines in this article. There are too many variables when designing a flue system than can be covered in this article.

    An installation supervisor could teach himself flue design in a couple of hours and teach his installers in the same amount of time. Considering the consequences of a malfunctioning flue, it seems like a good investment.

    Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

    Sidebar: CO Article Correction

    Information was added to the CO article [“Hidden Problems Can Cause Carbon Monoxide,” October 10, page 10] that needs to be clarified. Cracked heat exchangers don’t cause CO. This is a common misunderstanding in our industry. Cracked heat exchangers cause one of two problems:

    1. Pushing more air into the combustion chamber; or

    2. Pulling more air into and through the combustion chamber.

    Although both of these situations, for other reasons, are a hazard, in either case, the amount of secondary air is increased, which leans the flame out and reduces CO.

    Publication date: 11/26/2001