On social media during hot days, videos and photos are circulating showing people venting warm air from portable air conditioners into chimneys or flues. At first glance, this might seem like a simple and clever solution. In reality, however, it is a very risky interference with the exhaust pathway.

A chimney is not a universal ventilation shaft. It is designed for the safe removal of flue gases from a specific appliance, such as a stove, fireplace, or boiler. Any other use can alter its function, damage its structure, and in extreme cases, endanger the safety of the house and its inhabitants.

Mobile air conditioners don't just expel hot air. They can also introduce moisture into the chimney along with it. This moisture can condense in the cooler flue path, soak into the masonry, and react with deposits left in the chimney after the heating season. The results can include damp patches, odors, degradation of plaster, worsening of the chimney's condition, and the formation or worsening of tar deposits.

Another problem is pressure. A chimney works on the principle of draft, while air conditioning pushes air with a fan. If air is pushed into the flue, it can seep through leaks into the building's structures or other spaces. The risk is even higher with older chimneys, unlined flues, or leaky connections.

The biggest problem may not appear right away in the summer. It can come in the winter when the stove or fireplace is lit again. A damaged, damp, or dirty flue can mean poorer draft, an increased risk of flue gases returning into the room, and the danger of igniting deposits in the chimney.

The verdict is simple: air conditioning belongs in a window, wall, or designated outlet. A chimney belongs to exhaust gases. Rather than risk damage to your home, fire, or health hazards for a few minutes of comfort, it's better to choose a professionally correct solution and use the exhaust path only for its intended purpose.

If someone has already tried a similar solution, they should have the chimney inspected by a professional before the next heating season.

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