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Air-conditioning condensation can be a stain pain

House Doctor | Gary Dymski

July 28, 2005

My home has central air-conditioning. Because of a condensation problem with the air-conditioning unit, I am getting rust stains on the cement on one side of the house. What can I do to remove the rust stains?

-- Dino Marchese, Malverne

Before you remove the stains, why don't you fix the condensation problem? Condensation is created by the unit as it dehumidifies and cools indoor air. This condensation drains through a plastic tube and into a floor drain. Often a small condensate pump is used to send the water through the tube to the drain. There are two possibilities: either the tube is clogged, causing water to back up, or the pump is defective.

Examine the tube for leaks or clogs. If there aren't any, check the pump. Fill the pump pan with water. If the pump does not start, it could be broken or not receiving power. Check electrical connections.

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As for the rust stains, try using trisodium phosphate substitute in warm water, or a tile and grout cleaner containing phosphoric acid. Brush the solution into the stain, let it set, then rinse with water. As a last resort, try diluting muriatic acid with water (about 10 parts water to 1 part acid) and apply it to dampened concrete with a soft-bristled brush. Scrub gently.

Be careful with the muriatic acid - it's extremely caustic; always add acid to water to avoid splashing. Wear a respirator, goggles and rubber gloves - and cover your arms and legs. Be sure the area is well ventilated, and have a neutralizing agent - baking soda or garden lime - on hand in case of spills.

One more thing: Avoid using any acid before or after trisodium phosphate. The mixture creates a gas. Rinse the area with water; let it dry before trying another cleaner.