The coal darkened movie theater is a refuge from summer's heat. It is also the favorite refuge of half of Washington. Too often the looping line never makes it to the box office. Sold out.

But there is another way to screen out summer. Libraries all around the Washington area have films which cardholdeers can check out. You can invite friends over to watch "Lawrence of Arabia" cope with heat much worse than Washington, or reverse the seasons with "White Christmas." Several couples recently gave a party where they showed an old Fred Astaire/Ginger Rogers movie and followed it with a late-night dinner dance.

In Prince George's County, holders of Maryland library cards pay from $1 for films under 11 minutes up to $4 for feature-length movies like "Yellow Submarine," "Arsenic and Old Lace," "Sound of Music" or "The Lady Vanishes." Titles must be reserved in advance through the main library headquarters, 6532 Adelphi Rd., Hyattsville, (699-3500), audio-visual section, though branch libraries have catalogues which list availble titles.

Fairfax, Arlington, Alexandria, Montgomer and Loudoun counties pool their films in a five-county listing. None of the conties charge cardholders for films, though each has slightly different procedures.

Fairfax County, 321-9166, requires that films be reserved in writing three weeks in advance at a branch library. Films may be kept out for 24 hours.

Residents with an Alexandria county library card will find catalogues of avialable films from the five-county listing and feature films may already be booked as far in advance as June, 1981. Films may be checked out for 24 hours, or over the weekend. The main library number is 750-6354.

Card holders in London County will find catalogues at the branch libraries in Leesburg, Purcellville and Sterling, where there are also projectors which may be checked out. Three weeks' notice is necessary to obtain films from the five-county listing. For information call the main library, 471-6050, and ask for the audio-visual department.

To take out films in Arlington Conty, you must first register at the central library, audio-visual office, 1015 N. Quincy St., Arlington, 527-4777. Films which must be returned after 24 hours, should be reserved two weeks or more in advance. The library also rents pojectors, $3 for overnights.

If Montgomery County residents paln to show the library film outside of the county, they are limited to borrowing films owned by Montgomery, not by any of the other counties. Films may be kept overnight or for the weekend. bOne week's notice is necessary to borrow Montgomery's films, three-weeks' for the five-county listing, though films often are booked up as much as a year in advance. Borrowers can fill out written request forms at any of the branch libraries, through the films can be picked up only at the Bethesda, Davis, Silver Spring, Wheaton or Rockville branches. Incorporated, non-profit organizations can rent projectors through the library for $5. The number of the audio-visual dpartment of the main library is 279-1944.

The District of Columbia no longer offers feature films since most of what they had has been stolen.

For District residents, there are commercail firms which rent films:

The Film Center, 938 K St. Nw., 393-1205, lists 5,000 films in its catalogue with prices ranging from $25 to $150. Rental is for 24-hours, or a weekend, and films should be reserved as far in advance as possible. The company also has a limited number of projectors for rent for $25.

Reruns, Unltd., 4907 Cordell Ave., Bethesda, 989-9440, currently has only an interim catalogue available but expects to put out a new, expanded one in the fall. Prices range from $20 up to $80 for films like "Hamlet," Henry V," or "Lawrence of Arabia." There are a limited number of projectors available for $15 to people who rent films.

The catalogue of New York City's Films, Inc., 800-223-6246, runs from new releases to old MGM musicals to foreign titles, etc, Prices range from $50 to $500 for a movie like "The Rocky Horror Show." They need two to three weeks' notice, confirmation in writing, and payment in advance on new accounts. You pay a $6 shipping cahrge and send the film back next day.

For places which rent projectors, see the Yellow Pages under Motion Picture Equipment and supplies.