Q: Last December, we replaced our 25-year-old whole-house air conditioner and furnace because of problems with the furnace. All was comfortable through winter and spring. But in early July, we started noticing stickiness and clamminess, and it got worse. We called the installer, and he insisted the system was working and the installation was fine.

We began to see mildew on coats in our front closet — so bad that I had to throw some away. I replaced our basement dehumidifier, but found no evidence of any water leaks. Then, I discovered mildew on den furniture that flanked an A/C vent, which also was covered in mildew. Now I have mildew on my dining room hutch, two jam cupboards and on items stored in a basement that had been fine for the 22 years we have lived here.

Is the weather causing this? The new air conditioner? The installation?

Herndon, Va.

A: There could be a few different causes, or a combination of them.

Most likely, your new heating and air-conditioning system is oversize. That might seem like a good thing, because it helps your house heat up quickly in winter and cool down fast in summer. But because the cooling phase of a heating and air-conditioning system also dehumidifies, an oversize system cools a house so quickly that it shuts off before it has removed enough moisture.

Add the astoundingly wet weather over the past months, a fan speed that might be too low, ducts that might be too narrow and a few other possible problems, and you have the ingredients for creating the ideal mildew habitat.

The Air Conditioning Contractors of America (acca.org ), a trade association, has a protocol called Manual J that contractors can use to calculate the proper size unit. It depends on many factors, including construction details, the number of people likely to live in the house (based on the number of bedrooms plus one), the proportion of window space to floor area, the window orientation and the duct system.

But many contractors still wing it and base unit size on the floor area. Or, for replacement systems, they just match the size of the old units.

Donald Prather, technical services manager for the ACCA, said there are plenty of reasons a new system might need to be smaller than the old one. “Most systems installed by builders had oversize units and undersize ducts,” he said. “And over the years, people added insulation, better windows and doors, plus took out old lightbulbs and put in fluorescents and then LEDs.” All these improvements reduce the size needed in an air conditioner.

Some experts advise a simple test to see whether you have an oversize unit: Time how long it runs on a hot day before it shuts off. If it’s less than 10 minutes, the unit is too big. Prather agreed that this is a valid test — provided a system is installed correctly. If the installer didn’t adjust the fan speed correctly, for example, you might be misled.

An oversize system is especially likely to lead to mildew problems if a contractor failed to adjust the new unit’s fan speed correctly, Prather said. If the mildew on air-conditioning vents is on the supply side (where cold air blows out into the room), the air flow is probably too low, and a higher fan speed might help.

You also can try to run exhaust fans, including a clothes dryer, less when the outdoor air is especially humid, because using a lot of indoor air causes that moist outside air to seep indoors through cracks, vents and other openings.

Cleaning the filter on the air conditioner also helps, because that keeps the condensing coil cleaner so it dehumidifies better. And check that nothing plugs the tubing where the condensed water drains outside.

If these simple fixes aren’t enough, decide whether to call in a consultant (not the installer you already hired) or go shopping for one or more additional dehumidifiers.

If you opt for hiring a consultant, get one who has the diagnostic tools to do a whole-house evaluation of heating and air-conditioning needs. Two organizations that provide training and links to qualified professionals near any Zip code are the Building Performance Institute (bpi.org ) and the Residential Energy Services Network, or Resnet (resnet.us ). At the BPI site, search for certified building analysts. At Resnet, look for an energy rater/auditor. Both sites also list trained contractors.

If you want to go the dehumidifier route , adding one or more upstairs or investing in a whole-house system could make a huge difference. Dedicated dehumidifiers are more reliable than depending only on what’s offered by an air conditioner, Prather said.

The federal government’s Energy Star program offers shopping advice and ratings of specific products. A whole-house dehumidifier might cost a couple thousand dollars, including installation. A Whynter 70-pint portable unit that you just need to plug in and switch on costs $246.64 at Home Depot. It’s Energy Star-rated and covers up to 3,800 square feet. To get rid of the water it strips from the air, you can empty the bucket manually, or send the water out automatically by connecting to a drain pipe or using a hose and the built-in pump that lifts water up to 15 feet vertically.

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