Now comes the season when firewood gyps make unwary consumers hot under the collar.

A lot of grief can be avoided, say weights and measures inspectors in this area, if buyers insist on purchasing by the cord -- or by cord fractions expressed in arithmetical dimensions.

A cord measures 128 cubic feet, closely stacked with the pieces parallel. Generally it is stacked eight feet long, four feet wide and four feet high. But the important element is its 128 cubic foot total.

If a seller offers a "rack" of wood or a "load," tell him to get lost unless he specifies just what these amount measure arithmetically, consumers are advised.

Laws in this region accent the cord as the basic measurement. Maryland and Virginia also okay visual arrangements where the buyer is shown a load of wood and a price is quoted.

Earl Maxwell, chief of the District's Weights, Measures and Markets Division, says this is not precisely legal in the city, but he adds, "We don't know what we can do about it" if buyer and seller agree on the size and price.

James F. Lyles, supervisor of the Virginia Department of Weights and Measures, says that "most legitmate door-to-door salesman will be glad to tell you specifically just how much wood you are getting for you money, either in terms of a cord or parts of a cord or in cubic feet -- whichever is appropriate.

"But it you encounter a gypster who refuses to tell you exactly what you are getting for your money, report him to the weights and measures section of the Virginia Department of Agriculture and Commerce," Northern Virginia residents may write to the section, Room 32, 1 N. 14th St., Richmond 23219, or call (804) 770-2476.

A description of the seller's vehicle and its license number should be given, along with any other pertinent information, including names.

"More often than not," Lyles said, "if you take the salesman's name, address and vehicle license number and ask him to specify the exact amount of wood you received, in writing, you will scare off a potential gypster."

Consumer affairs officials in Northern Virginia and Maryland will also handle complaints. Phone numbers are: Fairfax County, 691-3214; Arlington, 558-2142; Alexandria, 750-6675. The Virginia Department of Consumer Affairs (703) 573-1286 and the Fairfax City police (703) 385-7952 handle complaints from Fairfax City residents. The city does not have a consumer office.

Prince George's County residents with complaints can call 952-4700; Montgomery County residents, 279-1776.

Authorities say consumers should avoid buying green, hard-to-burn wood. Seasoned wood generally feels lighter because it is drier. It is darker than green wood when split; it splinters more easily, and emits a sharp crack when pieces are struck together.

The heating value of wood varies by species, says the Cooperative Extension Service of Virginia Polytechnic Institute.

"Woods such as pine and cedar are easy to ignite and burn rapidly with a hot flame," the service advises. "However, a fire made entirely of these woods burns out quickly, For a long-lasting fire, it is best to use heavier hard wood, such as oak, maple, or beech. A winning combination is to start with pine or cedar kindling and switch to the heavier woods when the fire catches hold. Aroma can be obtained from some woods such as apple, cedar or maple."

The service said a fire is best laid on a bed of ashes to provide a reflecting surface and to protect the fireplace. In a fireplace, the ashes should be several inches deep, reaching almost to the andirons for better control of the draught underneath the bolts.

"However, do not let them over-accumulate and reach the grates," the service said. "The fire is most readily started with a few sheets of paper plus small sticks or twigs. Once a draught is started up the flue, larger sticks can be added gradually, and these in turn ignite the logs.

"Usually no more than four logs are needed to make a good fire. Adjust the logs and maintain the flame by pushing the ends into the fire from time to time. Add kindling and new logs as needed. Rake the coals toward the front of the grate before adding new logs. Add the new logs at the rear of the fireplace."