Air Sealing and the CAZ

February 25, 2013
March/April 2013
A version of this article appears in the March/April 2013 issue of Home Energy Magazine.
Click here to read more articles about HVAC

After I described how air can get sucked down the chimney and vent connector and flue, and out the draft diverter of the gas water heater, and into the basement, the mother, who was now wondering if this was the cause of her child’s sickness, said to me, “Why doesn’t everyone know this? Why aren’t the public health organizations educating everyone about this danger?” I replied, “Indeed.”

A. Tamasin Sterner
is president and chief coach of Pure Energy Coach, LLC, and The Pure Energy Center, based in Lancaster, Pennsylvania, and Lavina, Montana. (A. T. Sterner)
  • Let’s start with the hows. Under certain conditions, flue gases (I like to call these gases fumes) might not vent properly and leave the building when:
  • the chimney is partially or totally blocked;
  • the temperature in the room is close to the outside temperature, as it sometimes is in the summer;
  • the chimney is too large for the combustion appliance, as in the case of an orphaned water heater;
  • there is inadequate air for both proper combustion and proper draft; or
  • something inside the building is causing the air to be sucked down the chimney. This could be an exhaust fan or dryer; a return duct leak, like an uncovered filter slot; or other suction from the attic through an unsealed chimney chase.

The left picture shows that smoke is not going up the draft diverter and into the flue (flue gas spillage). The right photo shows the smoke going up the draft diverter and into the flue. This is good. (Tamasin Sterner)

In the left photo tape was used by a technician to cover the holes they drilled. The holes are in the wrong place. In the same photo, a hand drills a hole in the right place for testing the CO level in the flue gasses. The right photo shows a tech drilling in the right place for draft testing. (Tamasin Sterner)

What? You mean incomplete air sealing in an attic can cause backdrafting of combustion appliances floors away from the attic? Indeed.

Here’s how. When a bypass, such as a chimney chase, isn’t air sealed in the attic, the stack effect can suck air out of the basement. Since most of our heating and water-heating systems are in the basement, their flue pipes can be the source of the makeup air that is being sucked out of the basement. This can be very dangerous!

Besides the stack effect, attic fans can suck air through open air leaks, and the air could be coming from the flue. Even closed heating or cooling supply registers can cause suction on the combustion appliance zone (CAZ) through unsealed bypasses because the return is still open!

BPI standards are very clear about this issue. They essentially state:

Thou shall not add attic ventilation without first verifying that the attic is fully air sealed from the living space. And thou shall not add attic insulation without first verifying that the attic is fully air sealed from the living space.

This is because adding ventilation without a solid air barrier between the attic and the living space will make the house air and heat leak out faster, because the attic will be colder in the winter. If the house air is carrying moisture with it, the moisture will condense on the cold roof deck and cause wood rot and maybe mold. Also, unbalanced house pressures can cause natural-draft heaters and water heaters to backdraft fumes. The fumes may contain CO. CO is very dangerous; in fact, it can kill you. If the CAZ isn’t fully separated from the attic, the CAZ can be under so much suction pressure that the combustion appliances can backdraft.

BPI expects technicians to verify an effective air barrier between the living spaces and the attic spaces by doing visual inspections, and by using a blower door, zone pressure testing with a manometer, and smoke. Using an infrared camera in the attic when the house is under slight positive pressure from the blower door (air is being blown into the house) can also be a very effective way to find remaining air leaks.

All of this puts understanding the house as a system into perspective, doesn’t it?

The days of doing a task without seeing the big picture are over. Why now? Because starting in May 2013, many more atmospheric-draft water heaters will be orphaned due to the new federal standards for the installation of direct-vent heating systems. These orphaned water heaters are weak when left alone, and the proper venting of their fumes can be easily overcome by even minor suction on the room where the water heater is located—the CAZ. The fear caused by knowing this is what keeps me up at night.

My therapist once told me, “Tamasin, you tend to compartmentalize things—you focus on a task and get it done and then go on to another task. You might feel better if you blended the work.” I took that to heart and focused on trying to see the big picture—to have a hawk’s-eye perspective when contemplating issues.

So now that I can fully view the impact of incomplete air sealing between the attic spaces and the rest of the building, I’m motivated even more to guarantee that contractors

  • really understand what house as a system means;
  • have the skills and tools and materials to fully air seal attics from the rest of the building; and
  • are actually doing the air sealing, and verifying the sealing is complete.

learn more

Pure Energy Coach
10 N Bausman Dr.
Lancaster, PA 17603
Tel: 717-293-8990

Watch Pure Energy's YouTube video, Pure Energy Coach: Zonal Pressure Diagnostics House.

Postwork inspections are critical. This is one reason contractors test out at the end of each day they air seal or insulate. Checking that combustion appliances still draft fast enough and strong enough is what the combustion safety tests are meant to do.

Quality control inspections have been my company’s bread and butter since 1987. We have always been motivated to perform high quality QC inspections because we have that hawk’s-eye view. As you can see from this article, there are many reasons why a contractor should do a good job of air sealing bypasses that can suck air out of the CAZ.

A. Tamasin Sterner is president and chief coach of Pure Energy Coach, LLC, and The Pure Energy Center, based in Lancaster, Pennsylvania, and Lavina, Montana.

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