WARMING up a chilly room inside a house usually involves little more than plugging in an electric heater to supplement the heat that is already there.

Heating an otherwise unheated garage, workshop, patio or greenhouse, though, typically means hauling out the high-B.T.U.-producing heavy artillery. But choosing among the scores of heavy-duty heaters available requires an understanding of how different types of heaters work, what kind of heat they produce and just how large a heater is required.

And for heaters that are fueled by kerosene or propane and generate heat by combustion, care must be taken to use them safely. ''Any time you have a fuel-burning appliance, you're going to produce carbon monoxide,'' said Ken Giles, a spokesman for the Consumer Product Safety Commission in Washington. ''And carbon monoxide can kill you.''

Mr. Giles noted that the first thing consumers should do when thinking about purchasing a gas- or kerosene-fueled portable space heater is to determine whether local fire and building codes permit such devices. ''Some jurisdictions ban kerosene heaters entirely,'' he said. ''But then you step across the border into the town next door, and you can use a kerosene appliance.'' New York City bans kerosene and propane heaters inside a dwelling.

A visit to a home improvement center, however, reveals kerosene and propane heaters displayed prominently for sale.

It is critical, Mr. Giles said, to use such heaters safely. He noted that unsafe levels of carbon monoxide can be produced by a portable space heater, either because of improper venting or incomplete combustion. So, he said, the most important thing to remember is to provide proper and consistent ventilation. ''Never completely seal off the room you are heating,'' he said. Rather, leave a window or door open a little to allow fresh air into the room.

It is also important to be confident that the heater is burning fuel efficiently and completely. ''Choose a space heater that has been tested and certified by a nationally recognized testing laboratory,'' Mr. Giles said. ''These heaters meet specific safety standards.''

Charlie Smolko, customer service representative for Mr. Heater, a space heater manufacturer in Cleveland, said there are three basic types of space heaters: forced air, convection and radiant.

Forced-air heaters, he said, are typically torpedo-shaped devices that use either kerosene or propane for fuel. Mr. Smolko noted that Mr. Heater's forced-air heaters' output ranges from 35,000 B.T.U.'s, which would heat about 800 square feet, to 170,000 B.T.U.'s, which would heat up to 4,000 square feet. With such heaters, he said, combustion takes place inside the heater and the hot air is pushed by a fan. (In fact, because of the fan, such heaters typically can be used only in places that have a source of electricity.)

Another type of heater that heats the air, Mr. Smolko said, is a convection heater. Such heaters -- which generally use natural gas or propane -- usually do not need a source of electric power and are good for heating large areas. Mr. Heater's propane-fueled convection heater, for example, produces 30,000 to 250,000 B.T.U.'s, depending on the setting, and will heat up to 5,000 square feet.

Forced air and convection heaters, he said, are typically used outdoors and in large, unheated spaces like detached garages, barns and greenhouses.

For smaller areas and for more concentrated ''spot heating,'' it may be better to use a radiant heater. ''These provide nice, clean and efficient heat for a workshop or garage,'' Mr. Smolko said, adding that radiant heaters can either be ''tank top'' devices that mount directly on top of a 20-pound propane tank (the type used for a barbecue grill) or stand-alone units that are attached to the fuel source by a hose.

Radiant heaters, Mr. Smolko said, heat objects -- including people -- instead of air. ''It's like walking out into the sun,'' he said.

Propane-fueled radiant heaters made by Mr. Heater range from 4,000 to 150,000 B.T.U.'s.

The most popular radiant heater made by Mr. Heater is the Portable Buddy, a small propane model that uses either a 1-pound screw-on disposable propane tank or 20-pound tanks connected to the heater with a hose sold separately.

The unit, he said, has an oxygen depletion sensor, which shuts the unit off if the oxygen level in the area falls below 18 percent. (The normal oxygen level in the air is 21 percent.) It also has a switch that will turn the unit off if it is tipped over. The Portable Buddy, which will heat areas up to 200 square feet, sells for about $89.

Another type of radiant heater is the patio heater -- either tabletop or stand-alone -- which looks like a torchier lamp. ''The ones we see are mostly run on propane,'' said Charles Mattingly, general manager for Amerimerc.com, an online distributor of heating products, based in Wichita, Kan. ''They're excellent for outdoor areas because they project heat at you, like a heat lamp, and are less susceptible to wind.''

Patio lamps, he said, range in price from about $150 for a tabletop unit that uses small disposable propane tanks to $500 for a large stand-alone heater that uses a 20-pound tank.

While it is possible to buy electric heaters that produce up to about 17,000 B.T.U.'s, another heater that will produce the same amount of heat without electricity is the cylinder-shaped kerosene heater sold in many hardware stores. Mr. Giles of the Consumer Product Safety Commission said that newer models incorporate safety devices like wick-stop mechanisms, which prevent incomplete combustion, and switches that extinguish the flame if a heater is tipped over.

It is critical to keep combustibles -- like furniture, drapes and boxes -- a safe distance from any heater. ''Space heaters need space,'' said John Drengenberg, consumer affairs manager for Underwriters Laboratories, a nonprofit independent testing laboratory based in Northbrook, Ill. ''They should be placed at least three feet away from anything that can burn.''

Mr. Drengenberg added that space heaters have only one function -- and a temporary one, at that -- to provide supplemental heat. ''Don't use them to warm bedding, cook food, thaw pipes or dry clothing,'' he said.

When using liquid-fueled heaters -- like kerosene heaters -- it is important to use only the fuel recommended by the manufacturer. ''If the heater runs on K1 kerosene, then use only K1 kerosene. Don't use K2 or kerosene mixed with oil or gas.'' Mr. Drengenberg explained that using the wrong fuel could cause the heater to burn hotter than the unit's design limits and cause a fire.

Finally, he said, it is wise to look for heaters that have automatic safety devices like tip-over switches, flame guards and oxygen sensors.

Mr. Giles said that it is generally not a great idea to buy used space heaters at a garage sale or flea market. ''A 20-year-old heater is not going to meet current safety standards,'' he said.

Additional information about space heaters can be found on the Internet site of Underwriters Laboratories (www.ul.com) and the site of the Consumer Product Safety Commission (www.cpsc.gov).

Drawing (Drawing by Tom Bloom)