SANDRA KOREN, New Bedford

A. The vents may be closed but not well enough to prevent water vapor from entering the ducts. The water vapor is in the house air that gets into the ducts. When this water vapor condenses, the water produced runs back through the vents. To stop this, either cover the vents themselves with plastic, and tape the plastic tightly to the ceiling. Or, for a better-looking seal, lift the cover and insert an inch-thick piece of Styrofoam to plug the duct tightly.

Any air left in the duct is likely to be dry enough to prevent condensation.

Another way to fix the problem: The ducts are already insulated, but you could wrap more insulation around them to try to prevent condensation of the water vapor. Finally, you could rearrange the ducts on the attic floor so they sag in the middle. This way, any water from the condensation will sit harmlessley in the bottom of the sag and stay there, until it evaporates eventually.

This answer is also for Fran DeLuca of Falmouth, who asked the same question.

Q. My hot-air heating system is 42 years old, in good shape, and has given me no trouble all those years. Now I want to put in air conditioning, using the same heating ducts. Should I vacuum the ducts before installing the A/C?

ED RICHARDSON, Winchester

A. Yes, but not by you. You can, but if you do, you will reach only about 1 to 3 feet of the duct, and even if you get that far, it will not be very effective or efficient. A professional uses brushes or other devices to wipe off what's on the interior walls of the ducts, and pull it out with powerful vacuums. And, if the ducts are 42 years old without having been cleaned, you don't even want to know what's in them.

So, do it before installing A/C, and ask the cleaning company about filters; maybe you'll opt for an electronic filter as well.

Q. I bought a large ceiling fan for a room 16 by 14 feet. It is 52 inches in diameter, but it doesn't seem to be cooling. Is it working properly?

C. B., Newton

A. It is working properly. Fans are not designed to cool the air but to circulate it. Make sure the fan is on its summer setting, blowing downward. Ceiling fans are supposed to blow downward in summer (to waft over your overheated body), and upward in winter, spreading warm air more evenly around the room. Fifty-two inches is the largest you can get, which is more than adequate in that room.

Q. I have a large air conditioner in a living-room window. It is big, heavy, and awkward to move, so I'd like to keep it there all winter. How can I make it weatherproof, as well as insulating on the inside against loss of heat?

RITA NEVULIS, Braintree

A. One good thing about storing an a/c in the window is that it is as good a storage space as any. To make it weatherproof, buy a plastic cover that you can put on the outside. To insulate on the inside, I suggest a large open box that you can set up against the window, covering the a/c with its edges butting against the window casing (frame). Weatherstrip the edges of the box with foam weatherstripping tape.

You have to make the box deep enough so it will clear the a/c while it sets against the casing. The box could be made of 1 x 2s, 1 x 3s, or 1 x 4s, depending on how far out the a/c sticks into the room. Cover one side of the box with thin plywood or hardboard, and paint or stain to match the woodwork. Fill the box with as much insulation as will fit.

You will have to pay attention to where the bottom sash of the window has been raised, because it is not weathertight at all. You could fill that space with insulation and cover it with anything decorative. The insulation to use is High R Sheathing or Thermax, which has the highest R factor per inch.

Q. All this continuing heat reminded me of your story some time ago about a portable air conditioner, one that does not have to be inserted in a window or through the wall. What is such a portable unit, how does it work and is it worth buying? And where?

B.R., Malden

A. I forget the name of it, but the portable unit is like any other except it is quite compact, mounted on wheels, and has an attached flexible duct that must be inserted in a window for exhausting the warm air outdoors. One place you might be able to find one is George Washington Toma TV & Appliance in East Weymouth. Call (617) 335-6435. I have not seen or read about them recently, but if they are still around, you should be able to find one at an appliance store near to you. But, because of the heat wave, you may find they have been sold out.

Q. I am having central air installed. Is it OK to close off certain rooms to try to save expenses?

V.C., Abington

A. Yes, just as you can shut off certain rooms when heating with warm air. It is not a matter of closing the door; you have to close the vent serving that room. Of course, it's a good idea to close the door anyway because cooled, ambient air will go into the room, wasting all that cool.

Q. My house is four stories high with no attic. A spiral staircase leads from the fourth floor to the roof, into sort of a penthouse. I am thinking of putting in a whole house fan, but where could I put it?

CINDY COLPACK, Boston

A. A possible place is the wall of that penthouse, if it is big enough. A better way is to build a small penthouse on the roof and mount the fan on one wall of the penthouse. Or, put the fan in the ceiling of the fourth floor and build the penthouse over the opening. Put vents in the four walls of the penthouse just below its roof, to protect the fan from the weather. The vents should be large enough to handle the volume of air from the fan.

Q. Those portable air conditioners that do not need to be put in a window, with a duct that goes outside. Are they OK?

FRANCIS CURLEY, Arlington

A. Portable A/C units are relatively new, and expensive, but work well enough, and have the advantage of movability from room to room where needed. They must have a duct going to the outdoors, to allow air exhaust and condensation to be safely disposed of. The best thing about them is that they are on wheels, and can be easily moved around. Anyone who has tried to mount or remove a 200-pound window air conditioner will appreciate what a godsend a portable unit can be.

Q. You wrote about a windowless or portable air conditioner recently, but I can't find any. I have looked at various places, including Home Depot, which said it never heard of such an air conditioner. Where can I find one and what brands are they?

SEVERAL CALLERS, from all over

A. One brand of the portable air conditioner is Toshiba, quite expensive (about $1,000), with a duct that must be inserted outdoors. It and other brands are sold in appliance stores, including George Washington Toma, 791 Broad St., East Weymouth, MA 02189, 781-335-6435 and Concept Equipment, Bay State Road, Cambridge, MA 02138, 617-868-1147.

Q. My window air conditioner makes a terrible racket even when it is not on; water dripping on it from rain and the roof make a pinging sound that is really annoying. I tried foam on the top as a cushion, but it deteriorates quickly. Asphalt roof shingles are sort of OK, but not good enough. What will be a good cover to quiet down that pinging?

BARB McCURL, Belmont

A. Aha! The same happens on my aluminum ladder stored against the house. I plan some day to put a cushion on it.

For your purposes, the asphalt roof shingle helps dull the sound, making a thud instead of a ping, and that is about as good as you can get. However, try a piece of thick, indoor-outdoor carpeting; it is softer than the shingle and should work a little better; at least enough to let you sleep at night.