Can you guess what caused the damage to the below flue cap? Before we identify the culprit, a few words about the importance of caps. Chimney or flue caps are indispensable for protecting the integrity of your chimney interior and maintaining air flow and draft. They do so by preventing the infiltration of rain and snow into your flue, which can cause water damage, and by preventing (or minimizing) the entry of animals, who bring with them lots of flue-clogging nesting debris.
This month a homeowner in St. Paul had their power company out to do a routine pre-season check on their boiler. To their surprise, it was red-tagged and shut down, after a determination that their boiler flue was clogged. This left the homeowner without heat in the cold of a Minnesota November. They called several companies seeking to have a boiler flue cleaning, but because of the cold and wet conditions, and the pitch and height of their roof, they were repeatedly turned down. Their roof's pitch was 10/12, and a 32-ft ladder fully extended was required just to get to the roof line. From there it was another 15 feet to the top of the chimney.
When our technician arrived on the job, he first read through the notes from the gas company, who had indicated there was a backdraft from the flue, presumably due to a blockage, to such an extent that some of the plastic components on the top of the water heater had melted (the water heater and boiler shared a flue).
The technician began by disassembling the portion of the flue located in the basement, where the flue enters the chimney stack, expecting to immediately find a blockage there—but there was none. This area is typically where clogs are found—at the elbow where the flue goes from horizontal to vertical after entering the brick. Animals frequently nest in this area due to its warmth. But this occurrence is extremely rare when a cap is in place over the flue, which was the case with this home. All of this left our technician a bit stumped.
To further his investigation, he climbed the roof for a closer look. On reaching the top, he observed that a very determined animal had gnawed through the screening of the aluminum chimney cap. On removing the mangled cap and peering down the chimney, our technician spotted the problem: an enterprising squirrel had constructed a nest a quarter of the way down the flue, causing a complete obstruction. This would have blocked the exit of dangerous carbon monoxide produced by the boiler, and thus the red-tagging by the gas company.
Cell-phone images taken from on-high have their limitations, but here's a bit of a closer look:
The technician began the job of removing the obstruction by using a Viper GFK Chimney Cleaning System, which features a coiled rod—65 feet fully extended—to which brush heads of various sizes and materials (poly or steel wire) can be attached. He inserted the poly brush into the flue from above, attempting to push through the nest. His assistant was below, capturing the falling mess—the smaller debris with an industrial vacuum, the larger debris with his hands. After managing to get the poly brush all the way through the nest and down to the elbow—almost 50 feet from the top of the chimney to the basement—the tech switched to the steel brush to finish the job (the poly brush was successful in removing only about a quarter of it). After 45 minutes, they managed to remove the entirety of the nest and ended up filling a 5-gallon bucket with nesting debris, mostly consisting of large sticks and grass clippings.
On completion, daylight was observed from down below, entering through the top of the flue. When a lighter was held at the opening, the flame was pulled toward it, indicating proper draft. The homeowner was assured that the flue was clean and clear of debris and that it was safe to call the gas company and get the appliance turned back on. Before leaving, the technician placed a new type B gas vent cap on top of the flue.
Unfortunately, there isn't really a huge insight to reveal here. In our technician's almost 20 years of experience, he had never observed a capped flue that suffered such an obstruction. (Uncapped flues are another story altogether.) By ensuring that each of your flues has a cap, you reduce the chances of animal intrusion to almost zero. Checking the fit and condition of your cap occasionally, or having someone else do it, will virtually guarantee your flue remains unobstructed, and elevate you to super-homeowner status.
Many thanks to our technician Ben S for lending his expertise to this article.