The Fairfax County Board unanimously endorsed yesterday the conversion Commonwealth Hospital in Fairfax City into a 120-bed nursing home.

The plan for the 19-year-old hospital, which is being replaced this year by Fair Oaks Hospital west of the city, is one of six under consideration by State Health Commissioner James B. Kenley.

Northern Virginia faces a severe shortage of nursing home beds, and hundreds of elderly have been sent to homes more than 100 miles away, according to proponents of the conversion.

The Fairfax Hospital Association, a nonprofit organization that owns Commonwealth Hospital, says the conversion would cost $1.2 million. That would be among the least expensive options for adding nursing home beds in Northern Virginia, it said.

Other options include: a plan by Tri-State Associates to build a 120-bed home in Purcellville for $26.1 million; Manor Care's proposal to build a 120-bed home west of Fair Oaks for $10.9 million; Medical Facilities of America's proposal to build a 120-bed home in Burke for $13.9 million; Woodbine Valley Nursing Home's $9.1 million plan to build a 120-bed facility next to its existing nursing home, and Wessex's $1.2 million plan to convert Circle Terrace Hospital to a 114-bed home in Alexandria.

The association's plan for Commonwealth also calls for keeping the hospital's emergency room open and operating a day care center for the elderly.

The proposal to convert Commonwealth, county officials said in a memorandum to the board, has low capital costs, and could be completed in about a year.

The proposal already has the endorsement of the Fairfax County Health Care Advisory Board.

"It's right for the county," said Fairfax Supervisor Nancy K. Falck of the Dranesville district.