Recently I read an article about draining sediment from water heaters, and it recommended that the water supply to the heater be shut off first. I have been draining a bit of water from my heaters from time to time for years, but I have never shut off the water. I just thought that the water pressure helped to blow out sediment. How does shutting off the supply improve the process?

I'm glad to respond to your questions about draining sediment from a water heater. If the article didn't mention it, I want to point out that when you turn off the water supply line to the tank, it's important to open the hot side of a household faucet (preferably one nearest the tank) to let air into the water tank to help it drain properly.

First let's take a moment to explain how the tank works. The main water supply (cold) is carried through a plastic ("dip") tube directly to the bottom of the tank where the heating process takes place. As the water heats, it rises to the top. Knowing this, we can proceed; however, I'm going to dance a bit with my response because I was unable to find a straight technical answer to your question, "should you turn off the water or not?" I decided instead to go through the process on a 2-year-old gas unit that's never been drained to see firsthand what happens.

Turning off the water supply to the tank (and opening the hot side of a household faucet) prevents incoming cold water from stirring up the sediment at the bottom of the tank. The first bucket I drained off had a heavy amount of sediment (milky colored) in the bucket. The second bucket also contained sediment, but not as thick.

Next I turned off the household faucet, turned on the water supply to the tank, and drained a third bucket. This time the pressure from the drain valve was high and sediment was stirred up in the bucket. I drained one more bucket and even though sediment was visible, the water was clearer.

While this maintenance could be performed either way, my conclusion is that this should be done both ways and in a certain order. First, turn off the water so it doesn't disturb the sediment in the bottom of the tank allowing the bulk of it to drain in the first couple of buckets drawn off. Then turn on the water so the incoming water will agitate the sediment and help flush it out when the drain valve is open to draw off a couple more buckets.

I place the decision on your shoulders, but for the heck of it, try the two-step method and let me know what you found out.

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C.R.S., Inc. · Spokane, Washington · USA