K.S., Brookline

A. Well, it's longer than optimum, which is 8 feet. Any longer than that will reduce its efficiency, but if it cannot be helped, then put in a PVC pipe rather than one of those accordian-like flexible vents, and keep it level, or even sloping down toward the outside. And use short lengths so it can be taken apart to clean.

The reason for a rigid duct sloping down is: Water vapor can condense in the duct if it is in a cold basement; if the duct is flexible and sagging, water can collect in the low part of the sag, and in extreme cases can freeze, and block the duct. The dryer will keep trying to pump hot air through the duct, and a blockage could cause a fire.

Water in a rigid, sloping duct flows down through the outside vent, going harmlessly outdoors rather than back to the dryer or soaking lint in the duct, encouraging block.

And any kind of duct to the outdoors is better than spewing all that hot, moist air inside the house.

Q. My small house is on a slab, with heat and air conditioning coming from ducts in the ceiling. The paint is peeling on the ceiling around the duct openings. How can I prevent that?

PAT LYONS, Natick

A. What is happening: Moist air in the ducts is condensing right at the opening of the duct, which is cool, and spreading to the ceiling, where the dampness is causing the peeling. So, check the insulation on the attic floor to make sure it is snug against the ducts, and make sure the ducts themselves are well insulated, with no gaps. If the insulation seems flimsy (an inch or so thick), add another layer of duct insulation and make sure all seams and holes in the insulation are covered with duct tape or a similar tape that will keep them sealed. Then sand the peeling paint smooth and apply two thin coats of latex ceiling paint. You may have to repaint the entire ceiling. The paint also might have peeled because it was applied too thickly, and that is why two thin coats will perform better and resist peeling better than one or two thick coats.

Q. I've seen a lot of ads and commercials lately on cleaning hot-air ducts. Is it really necessary? My house is 12 years old and the ducts are used for heating and air-conditioning.

J.G., Hanover

A. Maybe yes, maybe no. Let's put it this way. Duct cleaning is a new industry, now that more efficient brushes and vacuums have been developed, and the people who do the duct cleaning would love to do it regularly, to keep the ducts nice and clean. After all, ducts do collect dirt and other contaminants that we really don't want in our lungs.

The dirt and other things that line the ducts are often stuck to the sides, and the air is not necessarily going to slough off all those contaminants as it goes slowly through the ducts. Besides, a good filter on the furnace will catch some of those contaminants.

On the other hand, if you looked into the ducts, you might be shocked to see what is there. If some of the contaminants are mold and mildew, and some people are highly allergic to such things, it would be a good idea to clean the ducts.

Therefore, after 12 years, it is a good idea to clean the ducts, but only to clean them; there is generally no need to install any kind of a liner in the cleaned ducts.

Q. The bedroom floors above my garage in the basement are very cold. The ceiling is insulated, but the hot air ducts, both delivery and return, are exposed on the ceiling in the garage. How can I make those bedrooms warmer?

ROB GOTT, Billerica

A. Any time hot air ducts, including the returns, are uninsulated and exposed to a cold space such as a garage, they will deliver darned little warm air to the bedrooms because the cold ducts will cool off the air before it gets to the rooms. So, insulate all ducts with two-inch-thick duct insulation with a vinyl skin on the outside. Wrap the insulation arpound the duct, and secure the seam where the insulation meets, using a stapler that staples to each piece of insulation, forming a neat standing seam. Wrap duct tape around all seams. Doing the return duct will keep that return air from cooling off excessively, thus taking more fuel to reheat it. While you're at it, insulate all exposed ductwork in the basement.

If the ductwork is snug against the ceiling or ceiling joists, you cannot wrap insualtion around it, but you can lower the ducts a bit in order to do the wrap properly.

Q. The hot-air ducts in my floor are of an irregular size, and I am having trouble finding new registers for them. The openings are 2 by 13 3/4 inches and 2 1/2 by 13 7/8 inches. I've looked all over, without success. Do you know of any place where I can find them?

DEBORAH ELLINGTON, Holliston

A. Admittedly, they certainly are odd-size openings. But there is hope. Call the Reggio Register Co. in Ayer. This company makes cast-iron, brass, and aluminum grilles, even wood, for all kinds of openings. The nearest size that I found in the catalog has an inside measurement of 14 by 2 1/2 inches, obviously too big for your openings, but you could enlarge the openings enough for the grilles to fit. The grilles are handsome, even elegant, and will dress up those vents. You might even want to get them for your other vent openings. The company also sells movable registers if you want to use an adjustable cover. Call 508-772-3493. The handyman has installed many of them in his house, and they really look good.

Q. I saw an offer to have my air ducts cleaned. There are different prices, though, some involving the main ducts. What are the main ducts, and can the cleaning be restricted to them?

E.R., Braintree

A. The main ducts of the heating system are more correctly called trunks, and are large rectangular ducts running along the ceiling (usually) of the basement. The smaller ducts running off the trunk to serve each room are round. The handyman feels that all ducts should be cleaned. Doing the mains or auxiliaries, but not both is only a lick and a promise, not worth much. Do all or nothing.

Q. I am getting what seems like a lot of dust in the house. I have hot air heat. Can I put a filter over the vents in the living area of the house? Is there anything else I can do?

TOM ACKERMAN, Lynnfield

A. While there are filters designed to go over outlets and registers in the living areas, they can block heat flow as well as dust, so I don't think they're a good idea. You could have the ducts cleaned; if they have not been cleaned in 10, 20, or 30 years, you can only imagine what amount of dirt has built up in them. While this is always a good idea, some studies have shown that cleaning the ducts does not always keep the dust down.

However, if your ducts are not insulated and/or sealed at their joints, the system may be picking up dust from the basement as it recirculates the air. So, seal all the joints with duct tape. Change the furnace filter once a month. And, install an electronic air cleaner or filter in the furnace to take the place of the fiberglass filters there now. You can find such electronic air claners at dealers who sell hot air equipment. A humidifier, either on the furnace or a console type on each floor of the house, will also help keep the dust down.