A ceiling vent cover. (istockphoto)

Question:We have seven-inch round ducts in the ceiling of our house, for heating and air conditioning. In a little-used room, we’ve never bothered to find ceiling diffusers for them. Now that we’d like to do something with that room, we haven’t been able to find any diffusers that fit ducts. Any suggestions?

— King George, Va.

Answer: Knowing the nuances of heating and air conditioning terminology is part of the solution. If you have a drop ceiling — the type often found in offices but rarely in homes except in basements — you do just need diffusers (the vents through which the air is pushed) that fit seven-inch ducts. Carrier makes one that would fit, called the “Airtec MV4 Ceiling Diffuser w/ 4 Way Grille/ Damper.” It fits ducts that are 6, 7 or 8 inches wide, but it requires a hole that’s 9¼ inches across. R.K. Payne (540-373-2020; www.rkpaynehvac.com) is a Carrier dealer that serves King George and surrounding areas.

More likely, though, the ceiling is drywall. What you need is actually a two-part solution: a boot that fits into the duct above the ceiling and has a rectangular opening on the room side that fits a register. “Diffusers are for drop ceilings, registers are for drywall,” explained Jessica Wooten, operations manager for CMR Heating & Air Inc., an HVAC contractor in Fredericksburg (540-775-5005; www.fredericksburghvac.com). You can see a picture of the boot by searching on Amazon.com for “ceiling plenum box 7-inch.”

Wooten recommends getting a free estimate for the materials and installation. Depending on whether everything is up to code and the installation is straightforward or complicated, the cost could range from $200 to $1,500, she said. If the estimate comes in at the high end, get a second opinion.

I have a Sunbrella fabric shower curtain in our guest bathroom. The curtain is white with large black swirls, but the bottom foot or so has pinkish watermarks. We’ve tried using Soft Scrub to eliminate the marks. They are a little less noticeable but are still there and easily seen. Any other suggestions?

— Arlington

Soft Scrub is a brand that makes a variety of cleaning products, some with bleach and others without. Bleach, the traditional chlorine-based type, is what you need to remove these stains, which are either algae or mildew. Mix your own cleaning solution using 1/4 cup dishwashing liquid, 1 cup bleach and 1 gallon water. Put on goggles and open a window if you are working inside. Spray the stained area and wait 15 minutes. Then scrub gently with a sponge, clean towel or soft-bristle brush. Rinse and let the shower curtain air-dry.

Or you can put the shower curtain in a washing machine. Use cold water and your regular mild clothes-washing detergent plus bleach. Take out the curtain as soon as possible after the spin cycle so the wrinkles don’t set. Lay the curtain flat and smooth out any wrinkles. The fabric should air-dry quickly. If any wrinkles persist, you can use an iron on a synthetic setting. “Some irons exceed the recommended temperature on the synthetic setting so test a small inconspicuous area before ironing the entire piece,” Leah M. Shearer, who does public relations work for Glen Raven, the company that owns the Sunbrella brand, said in an e-mail. “Do not use a steamer or iron set to the steam setting as this will degrade the fabric.”

Sunbrella fabrics are treated with a fluorocarbon finish to make them more water-repellent. If the shower curtain is more than five years old or if you need to use bleach frequently, Glen Raven recommends refreshing this finish by applying two thin coats of 303 High Tech Fabric Guard (www.303products.com). Or, you can just keep using the curtain until cleaning becomes too much of a hassle and then buy a new one.

Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.

The Checklist: Read Jeanne Huber’s month-by-month roundup of home-improvement tasks at washingtonpost.com/home.