Rust-colored water and soot around the burner are the two big signs your pool heater's heat exchanger is toast.

Yes, if you see either of those, it’s time to start shopping for a new heater — not just the exchanger.

The heat exchanger is the part that transfers heat from the gas flame to your pool water. Over time, corrosion, scale buildup, or thermal stress can crack it. When that happens, combustion gases leak into the water (that’s the soot and that weird chemical smell you might notice at the returns), or water leaks into the burner chamber (rust-colored water, puddles under the heater, or even steam/condensation where it shouldn’t be).

Another tell is the heater cycling on and off more than usual, or struggling to hold temperature — but that can also be a dirty filter or bad thermostat. The rust and soot are the dead giveaways.

Some people try to solder or patch a cracked heat exchanger. Don’t. It’s under pressure and thermal stress; a patch will fail within a season. Replace the whole heater or let a pro swap the exchanger if it’s still under warranty — but on older units, the labor often costs more than a new heater.

If your pool heater is more than 8-10 years old and shows these signs, don’t throw good money after bad. Just replace it.