“Flue gas may be coming out of your flue instead of going up.”

One homeowner turned to Reddit’s r/Plumbing community after a two-year-old water heater started giving off a burning plastic smell.

What happened

In the post, the homeowner said the unit had been installed just over two years earlier.

They added that the installer’s two-year warranty covered only the copper connections and that water had pooled near the expansion tank, likely around one of those connections. The homeowner also wrote, “I just noticed that there was a burning plastic smell and looked at the top of the water heater and this is what I see. Why is it melting?”

After viewing the photos, several commenters said the damage looked concerning and also pointed to a possible leak.

Many responses concentrated on the exhaust system.

One commenter wrote, “Flue gas may be coming out of your flue instead of going up. Hold a lit match or lighter near it and see which way the flame goes. Also give us a picture of the full flue pipe.”

Another warned, “Back draft from the exhaust combustion. Venting issue. Carbon monoxide is nothing to play with. I'd address this immediately.”

The homeowner said there had not been any problem for two years, which made the sudden smell and scorching more alarming.

What people suggested

The most common advice was to check for backdraft, examine the full length of the flue, and make sure the roof vent was not blocked. Commenters also repeatedly asked for additional photos of the rest of the vent pipe.

Many said the issue needed immediate attention, citing the burning plastic smell, melted material near the draft hood, and water near the expansion tank as reasons not to wait.

For homeowners weighing replacement after a scare like this, Cala offers smart heat pump water heaters. The company says its customizable models can help reduce energy use by heating water only when needed. If a gas unit’s venting becomes a concern, that kind of alternative may be worth considering.

A system like Cala’s also addresses the same priorities raised in the Reddit post: safer everyday operation, dependable hot water, and lower running costs when a conventional unit becomes a hassle.

Related safety warnings

Similar warning signs have appeared in other water heater cases involving venting problems, odors, or flames that pointed to serious safety risks.

• One homeowner learned their flue was sloped incorrectly after plumbers warned about carbon monoxide risk.
• Another shut off a water heater after a chimney vent warning raised alarms.
• A burnt plastic odor prompted one homeowner to cut power immediately before the situation worsened.