All buildings that use a fossil fuel source for space or water heating, such as natural gas, liquid propane (lp), fuel oil or wood, have the risk of back drafting burned exhaust gasses into the structure.
These gasses contain elevated levels of carbon monoxide, which in higher amounts can cause health concerns or even death. A test called the combustion appliance zone (CAZ) test can determine if there is a possibility that this potentially catastrophic event may occur.
How the test is performed. The building being tested is placed under a negative pressure by operating all the exhausting devices within the building. Kitchen exhaust fans, bath fans, clothes dryers, and any other device that exhausts air from the structure are all started. A tool called a monometer measures the pressure within the building in comparison with the pressure outside the building. Doors within the structure are opened and closed to determine the worst-case condition in the room where the combustion appliances are located. The lowest pressure is recorded and will be compared to the type of combustion appliances in use within the structure.
Test results. What we are looking for in this test is the highest negative Pascal value achieved while all exhausting appliances are in operation. All fossil fuel appliances will back draft at a certain negative pressure.
Natural draft stand-alone gas water heater -2 Pa
Natural draft gas water heater vented with boiler or furnace -3 Pa
Natural draft boiler or furnace with damper (may include water heater) -5 Pa
Induced draft boiler or furnace (may include water heater) -5 Pa
Power vented boiler, furnace or water heater -15 Pa
Sealed combustion appliance -25 Pa to -50 Pa
When to test. It is a good idea to perform this test after any building shell improvements or additions are added to the home that may affect its “tightness”. Air sealing or simply adding additional insulation in an attic can change the performance of a home. Adding a bath fan or changing a range exhaust hood can also affect fossil fuel appliance venting. An oversized range exhaust hood may back draft appliance venting even in a poorly air sealed home.
You may also want to test the home if you have periodic or unexplained alarms from a carbon monoxide detector. A home may be placed under a worst-case pressure without the homeowner realizing, causing a CO alarm.
All new homes should be tested during the commissioning of mechanical systems. Commissioning is when the appliances and mechanical systems are operating for the first time and tested to be sure they are operating as designed. Often, testing on new homes is incomplete. Specialized tools and training are required for a complete commissioning.
Lastly, if you see debris or dust around the exhaust vent of a water heater, boiler, or furnace, this may be a sign of back drafting. This debris is the result of air being pulled backwards through the vent which deposits the debris near the exhaust vent of the appliance.
Homes that do not require testing. The only home that does not need a CAZ test is an all-electric home. A home with electric space heating and water heating systems cannot produce carbon monoxide. A home with sealed combustion heating equipment will have a lower risk, but I still suggest testing.
What to do if a CAZ test is failed. A failed test is the result of higher negative pressures in the home with regards to the type of venting of fossil fuel appliances. Bringing make-up air into the area where the combustion appliances are located is the logical fix. Another suggestion is to bring the make-up air in close to the vent that is exhausting the air. A large kitchen exhaust hood may have make-up air that enters under the range or cooking appliance it serves. The issue in Minnesota is the outdoor air temperatures during the winter months. Allowing -20-degree air to enter the home can create comfort and heating cost issues. Sometimes electric heaters are added to the ducts that supply the make-up air, warming the air as it enters the home.
A second option is to install sealed combustion equipment when building new or replace natural drafting equipment. Sealed combustion and power vented appliances are less likely to back-draft.
Another option is to locate fossil fuel burning appliances in a sealed room. This may become a code requirement for Minnesota in the future. If the room is sealed and isolated from the rest of the home and the outside, the area is unaffected by pressure changes from exhausting equipment located outside the dedicated mechanical room. This option would be more common in new construction but could be implemented in older homes that do not pass the CAZ test.
Reduce the negative pressure within the home by decreasing the size of the exhausting system(s). This option will require some testing and calculations by an HVAC specialist. Sometimes oversized exhaust fans are installed in homes based on how they look. These fans are designed for commercial kitchens and exhaust large amounts of air. Replacing the unit or permanently changing the amount of air it can move out of the home may reduce the negative pressure enough to pass the CAZ test.
Adding a speed control switch to control fan speed, thus limit the amount of exhaust air removed from the home. I am not a fan of this option; a person may forget or not know about the CAZ issue and operate the fan on a higher speed.
Cracking a window while using an exhaust fan. Again, not a fan. I prefer automatic solutions to CAZ problems within the home.
My CAZ experience. I am rarely asked to conduct a CAZ test. Most home owners and many builders are not aware that this test exists. Several years ago, I performed contract work for an area weatherization organization. The program I worked under was federally funded and we were required to test in and out every day, meaning we conducted several different tests at the start of the day, and re-tested at the end of the day to be sure we did not negatively change the performance of the home. Most of the homes we worked on were lower income clients and they continued to live in the home as we worked on them. We always made sure the customer was safe.
My recommendation regarding the CAZ test is to follow the When to test information in this blog and have your HVAC equipment serviced yearly by a trained HVAC technician.