Conservation Consultants, Inc.

Appliance Venting-What You Don't See Can Kill You


The products of perfect combustion are primarily water vapor, carbon dioxide, and heat.  In the real world, residential combustion appliances such as boilers, furnaces and water heaters additionally produce Products of Incomplete Combustion (PICs), including potentially harmful carbon monoxide (CO).  It’s desirable to vent all these products outside of the home which is why combustion appliances all have flues piped to the outside.

Sometimes, for a variety of reasons, a flue won’t remove all the PICs, instead allowing some (or even all) of the PICs to come into the home.  This happens most often with older, natural (atmospheric) draft appliances which rely on the buoyancy of hot air and the “stack effect” to carry PICs up the flue and out of the home.  Depending on a variety of factors, including how tight or leaky the home is, the adverse effects of PICs can range from unnoticeable to unhealthy or even deadly -- every year some 500 persons die from non-fire related CO poisoning in the US.

Regardless, it’s important to make sure your combustion appliances are drafting properly, and also to have at least one CO detector installed in your home. As certified auditors, combustion appliance safety checks are part of our daily work. We use expensive, sophisticated equipment and specific protocols to help identify, verify, and quantify problems. But we also use our eyes because there are usually visual indicators of serious venting problems that anyone with a bit of an educated eye can and should look for.

                           Jonathan Measuring CO Levels                                                                                  Charlie Measuring the Draft 
                        For a Sealed Combustion Furnace                                                                                      of a Hot Water Boiler

There are two main venting problems: back drafting, characterized by periodic negative pressures in the combustion zone (usually the basement) that overpower natural draft and essentially suck combustion products out of the vent and back into the home and the more severe spillage, where the combustion appliance rarely if ever establishes proper draft.

What are the Causes of Spillage and Back Drafting?

Based on the thousands of homes CCI personnel have audited over the years, we've noted that improper installation or retrofitting is frequently to blame. For example:

  • Spillage or back drafting can happen with any natural draft appliance, but are most common with water tanks, which often have a relatively weak draft to start with. Installers should (but frequently do not) verify proper drafting during installation.
  • When upgrading to a new, sealed combustion furnace that employs it's own venting we see:
    • Water tanks becoming “orphaned” because they no longer have the furnace to assist with drafting.
    • Chimneys or vent stacks that become clogged or blocked with falling masonry or other debris.
    • Chimney penetrations where the old furnace used to vent left unsealed (photo).


  • Vent stacks having insufficient slope.

  • Vent stacks showing signs of corrosion, or as illustrated in the next photo, gaping holes!

  • Vent stacks of insufficient height that fail to conform to code. 

What Should You Look For?

Flue gases are moist, corrosive, and hot, so when there are serious venting problems, the top of the water tank, especially around the draft diverter, will show signs of moisture, become corroded, discolored, or even have grainy carbon/soot deposits on it.  In extreme cases, spilling hot flue gasses will melt the plastic water line surrounds.

Furnaces can have venting problems as well, and will display some of the same symptoms. If your furnace or water tank displays any of the telltale signs of venting problems, you should immediately have a combustion appliance safety inspection performed.  CCI includes combustion safety analysis as a component of many of our energy auditing services.  Contact a heating or plumbing contractor to fix the problem.

And don’t forget about that CO detector, too.  My preference is for the newer plug-in type that displays the CO level in parts per million (PPM) and, of course, runs off of the house current rather than a battery.

Jonathan Nadle, Senior Energy Auditor