There are rust and/or mineral nodes on the interior of the center flue. Water gets into them and the heat from the flue turns them to steam. The pressure forces the steam out of the node and when it contacts the water, it collapses back to water. Since steam has a lot of energy, the transition back to water releases that energy in the form of a mini-explosion which is the sound you hear.: My 40 gallon propane fired hot water heater has been making some new noises lately; I guess I can best describe them as crackling noises. The unit is about 10 years old and we live in an area with extemely hard water. We do have a water softener installed. Is there much likelihood that this noise is the result of hard water deposits inside the unit? If so, what is the best way to check it out? If not, what are some other possibilities?: Thanks,: Bob Langley : Winters, CA
