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Q: I heard you mention on radio using Drano drain cleaner for cleaning water heaters. I didn't catch the formula, and I would like to have it so that I can clean my water heater. - Adrianne.

A: We did talk about the use of Drano to dissolve the mineral salts that build up at the base of most water heaters. However, since then, we have been informed by a professor of chemistry at Stanford University that this could be dangerous if proper precautions aren't taken. Before we relate our findings, here's what he told us in a letter:"Drano is lye (sodium hydroxide) plus aluminum metal chips. In the presence of water these chemicals react to create hydrogen gas. The gas evolution is the reason that Drano makes a boiling sound when used in a drain. It is possible that a whole can of Drano would be enough to form an explosive mixture with the oxygen of the air in the partially empty heater tank. A spark could initiate a terrible explosion. Please warn about this."

After we received this letter, we contacted Larry Weingarten, one of the country's foremost authorities on water heater maintenance.

He has been testing various dilutions of Drano for us and he has gotten some fantastic results. But he suggests that we should use pure lye as found in Red Devil drain cleaner (there's no aluminum) instead of Drano.

Weingarten recommends one cup of Red Devil drain cleaner to three or four gallons of water. Then, here's how he says we should do the job in seven steps.

- Turn off the gas and drain the water heater.

- Remove the cathodic anode (the hex nut at the top of the heater). This provides an opening at the top of the tank and allows you to check the anode at the same time. The anode should be about 5 feet long and about-inch in diameter and shouldn't look deteriorated. If it is, replace it (cost: about $9).

- Pour in one cup of Red Devil crystals (don't premix with water; lye in solution is highly caustic and very dangerous).

- Add three to four gallons of cold water to the tank and let it sit for about three hours.

- After three hours, add a quart of vinegar (any kind) to the tank and turn the water supply on again.

- When the tank is about one-third full, begin draining the solution out of the tank.

- Fill and flush the tank two more times, adding a quart of vinegar at the beginning of each fill-up. The vinegar will help to neutralize the lye and clean it from the tank.

Weingarten says that a clean water heater means a minimum of 5 percent savings in water heating costs, cleaner hot water, less noise and no rotten egg smell. You may not realize it, but if you have never cleaned your tank, the mineral salts that prompted this whole discussion probably exist in your water heater.