Q: I have two questions. First, how do you install an air gap for the garbage disposal into an existing sink that didn't have one? The extra hole in the sink top has a soap dispenser installed in it. I do have disposal water that occasionally backs up into the dishwasher and/or blows out the other side of the sink.

Second, we bought a water heater, and when the plumber brought it to our house to install it, he checked out the old one and told us that we did not need a new water heater - we just needed to clean out the lime build-up of the old one.

We have drained and attempted to clean out the water heater by scraping and gouging the sediment. We even poured 25 gallons of hot vinegar into it and vacuumed out all that we could, but we still cannot get all of it. How are we supposed to get the rest of the lime out of this tank? Other than a slight lack of hot water, the only other problem is the pop-off valve seeps pressure once in a while. We just deal with the lack of hot water till it gets really bad, then we have to tear it all apart and do it all over again.

We have lived in this house for 1 1/2 years, and we have no idea how old the water heater is.

A: A simple definition of an air gap is that it is a free-air space between the end of a potable supply pipe and the top edge of a sewer drain or sink.

The reason your faucet is above the sink rim is so that your drinking water never contacts the stored water in the sink.

If a home's sewer pipe becomes clogged, the lowest drain opening becomes the flood level rim, and that could be a lower-level sink. To prevent back-up to the dishwasher, you can purchase an above- the-sink rim air gap for less than $20.

You might also try elevating the discharge hose from the dishwasher under the sink so that it has a loop in the hose before it enters the disposal port.

The hard water inside your plumbing fixtures will damage the water heater and leave deposits inside other fixtures.

What you need is a water softener at the plumbing supply entry pipe. The softener will remove minerals from the water and discharge the residue to a sewer opening through an air gap. Then purchase a new water heater because the one you have is using a lot of energy just to heat the minerals lining the tank. Also, stop probing the tank's liner or you will damage it.

Dwight Barnett is a certified master inspector for the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 268, Evansville, IN 47702.

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