Carbon Monoxide: Installing Flue Pipe Test Ports

By David Richardson


David Richardson

One of the most controversial topics you find among contractors, auditors, and inspectors is if it’s acceptable to install a test port in double wall pipe or PVC to perform combustion safety tests. Often, the disagreements come from a lack of understanding about what is really occurring and the importance of the test port. Let’s look at some of the concerns and why test parts are a necessary part of any installation.

The Risk

One of the main risk with installing a test port is concern it will allow CO to spill out of the venting system. There are other factors likely to contribute before a test port will. Consider natural draft equipment. You won’t find a bigger opening in the venting system than a drafthood. The same safety concern could also be raised for a gas oven. These appliances go one step further by dumping combustion byproducts directly into the home for occupants to breath. A properly installed test port will not allow flue gases to leak through it into a building.

Natural Draft Equipment

A test port is needed on natural draft equipment to obtain a draft pressure reading, not a flue gas sample. The draft test is separate and apart from the flue gases for combustion safety. The ability to test for carbon monoxide in a natural draft piece of equipment is usually much easier as a drafthood allows easy access to the flue gases inside the heat exchanger.

Induced Draft Equipment

When it comes to 80% furnaces, every double wall pipe manufacturer has written letters that say it’s okay to install a test port if it is sealed properly when finished. There are still some who don’t consider this enough even with this information available. Their argument is that the induced draft blower will blow CO out of the double wall pipe if the port isn’t made airtight. Some areas across the country focus so much on this detail that they must use short length screws when installing double wall pipe so it doesn’t pierce the inner wall of the vent.

An 80% furnace is considered a category one appliance. This means it relies on negative draft pressure to remove flue gases from the venting system. The assumption is made that the induced draft blower pushes flue gases out of the venting system. If this was the case, the venting system would need to be 100% airtight. Double wall pipe is the farthest thing from airtight you will find. The elbows and connections of the pipe will leak unless they are sealed with a high temperature silicone. These combined leaks add up to a lot more leakage than any test port.

The Clearance Factor

One purpose of double wall pipe is to allow a 1” clearance from combustible material. When a test port is installed into this pipe, it essentially turns it into a single wall connector. If there are combustible materials within 6 inches of the test port, replace the pipe. If not, then follow manufacturer recommendations and seal the port accordingly.

Those opposed to this really need to understand how important it is. The only way to confirm safe equipment operation is to install a test port in the flue and measure CO. Flame color and gas pressure just don’t provide the necessary information.

Condensing Equipment

With condensing or 90% equipment, a PVC flue is commonly used and vented to the outside either sidewall or through the roof. A simple method to CO test sidewall installations is at the termination outside, no test port needed. What about roof top terminations? In this configuration, you would need to install a test port in the PVC flue pipe.

Moisture leaking out the vent system is a valid concern and can be easily addressed by using a mechanical fitting tapped into the PVC pipe. A PVC plug and Teflon paste make this a sealed and watertight connection. You can assure the equipment is safe and not have to climb onto a roof to obtain the information.

A Good Indicator

An inspector should know when they encounter an installation where a test port has been installed in the flue for combustion testing, they are dealing with a professional concerned with safe equipment operation. It’s sad that many are hindered from doing the right thing by those who simply don’t understand. Take the time to help these individuals understand what is really going on. You might be surprised how they assist you in assuring your customers have the safest possible installations available.

The answers are out there and together we will find them. If you need any additional information on this material or have any questions, feel free to e-mail me

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