Dear Jim: I have central air-conditioning, but I thought about also using a portable, roll-about air conditioner in various rooms that need extra cooling at times. Does this make sense and are they efficient? – Kate T.

Dear Kate: It makes a lot of sense from both money-saving and comfort standpoints. Even though I have a new super-efficient central air conditioner at my own home, I also use a portable AC unit. I run it in my study during the daytime and then roll it to my bedroom at night.

Nearly every house has a room, or even an entire floor, which never gets cool enough while the rest of the house is comfortable. Instead of setting the central AC thermostat lower to cool those problem rooms, use a portable AC unit in them. Overall, this will save a lot of electricity.

For example, a portable AC unit is often used in the kitchen while cooking. After dinner, roll it into the living room while you’re watching TV. At bedtime, move it to the bedroom that is always a little too warm. I set mine on the low, quiet speed at night so it dehumidifies the air better.

Portable ACs are small, lightweight units (as low as 36 pounds) mounted on castors for rolling from room to room. Most models also have built-in handles that allow for easy carrying up stairs. They all operate on standard 110-volt house power, so you can plug them into any electrical wall outlet.

Many units are tall (about 25 inches) and narrow (about 11 inches) so they fit nicely in a corner. You can direct the cooled air to blow in one direction, if you are sitting in a nearby chair, or in several directions. For the best air distribution, pick a model that has oscillating louvers.

Portable AC models work somewhat like a super-efficient window AC, except the entire unit is indoors. The heat that is drawn out of the indoor air is exhausted outdoors through a flexible clothes dryer-type of duct.

All of the models come with an adjustable duct adapter plate that fits in any window. You just open the window, move out the adjustable slides, and close the window on the plate’s foam seals. It takes only about one minute for me to move mine from my study and have it running in the bedroom.

The newer models have

automatic digital controls for

the fan speed, cooling output

and temperature. As it is cooling the air, moisture is condensed out to reduce the humidity level for better comfort. This water is collected in a small water drawer that you slip out and empty once a day.

The most efficient, powerful models have a separate condenser unit that you place outside the window. It is connected to the quiet indoor unit with a long, thin hose.

Write for (instantly download – www.dulley.com) Update Bulletin No. 704 – buyer’s guide of nine manufacturers (25 models) of portable air conditioners listing efficiencies, cooling outputs, sizes, features, prices and a cost-to-operate chart. Send $3 and a business-size SASE to James Dulley, 6906 Royalgreen Drive, Cincinnati, OH 45244.