Dear Jim: I like open windows, but sometimes I need some cooling in one room or another. Instead of switching the central air conditioner on, would it be better to roll a small portable model from room to room? – Ellie N.

Dear Ellie: Using a portable air conditioner would be an excellent and money-saving choice for you. Being portable, you have to purchase only one unit and you can roll it from room to room depending on where you need instant cooling. They are mounted on castors and some weigh only 36 pounds.

The energy efficiency of most portable air conditioners is in the 8 to 10 range, but some new models are as high as 15, about 50 percent better than most older central air conditioners. The greatest savings are realized because only one room, instead of the entire house, is being kept extra cool.

Most portable air conditioners are about the size of room dehumidifiers, so they easily fit in a corner or against a wall. In fact, if you do not vent the warm exhaust air out through a window, they basically function as an effective dehumidifier. Even the small ones remove 35 pints of moisture per day from the room to make you comfortable and reduce allergens.

Portable air conditioners don’t consume a lot of electricity, so they can be plugged into a standard wall electric outlet. A collapsible duct, similar in size to a clothes-dryer duct, carries the exhaust heat energy outdoors. The duct attaches to a window adapter with adjustable width and rubber edge seals. You place the adapter in a slider or double-hung window, adjust the width and close the window sash on it, which takes about a minute to set up. You will have to make a special adapter for casement windows. Extra tall adapters (up to 96 inches) are available to fit sliding glass doors.

They operate somewhat similar to a window air conditioner. Inside the unit, room air circulates over cold evaporator coils to cool and dehumidify. Room air also flows over the hot condenser and is exhausted outdoor. The newest designs use two ducts so no already-cooled indoor air is drawn outdoors.

The most convenient models have electronic controls to set the desired room temperature, blower speed, start and stop timer, and operation mode (cool, dehumidify, heat). With a hand-held remote control, you can change all of the settings without leaving your easy chair or bed at night. I use one in my own bedroom. It is quiet and I use the remote or timer to switch it off.

A built-in air cleaner with an optional charcoal filter improves indoor air quality. Some units have oscillating louvers to distribute the cool air throughout the room. When you are alone, switch the louvers off so the cool air flows out directly toward you for maximum quick comfort.

Write for, or instantly download at www.dulley.com, Update Bulletin No. 463 – buyer’s guide of 10 manufacturers (36 models) of portable air conditioners listing cooling outputs, efficiencies, size, weights, features, prices and a cost-to-operate chart. Include $3 and a business-size SASE, and send to James Dulley, 6906 Royalgreen Drive, Cincinnati, OH 45244.