A fire early yesterday caused an estimated $500,000 damage to Fairfax County's George Mason Regional Library in Annandale, officials said. It was the second blaze at the building in five months.

Fairfax fire department spokeswoman Stephanie Hoover said the fire began on the second floor of the H-shaped building at 4:49 a.m. and destroyed the facility's heating and air conditioning equipment, along with a small computer that controlled them.

Firefighters said they feared many of the branch's 150,000 books on the first floor of the building suffered extensive water damage, but a library official disputed their account, saying that a survey yesterday indicated that the book damage may be minimal.

Although the first floor of the building has a sprinkler system, the second floor does not. Hoover said as a result, the fire spread until noticed by passers-by.

William L. Whitesides, director of the county's library system, said a building inspector estimated the damage to the structure, located at 7001 Little River Turnpike, at $500,000.

Fifty firefighters with 16 pieces of equipment took an hour to contain the blaze, Hoover said. Their efforts were hampered by the building's metal roof, which was too hot for the firefighters and had to be hosed down, Hoover added.

She said the blaze started near heating equipment, the area where the first fire broke out Oct. 27, causing $3,000 damage.

Hoover said the cause of yesterday's fire was still under investigation. Whitesides said a malfunction in the heating equipment caused the first fire, and the malfunctioning unit "was fully corrected. This is not the same unit." a