Contaminated fuel oil shut down the furnaces of at least 20 Fairfax County families during the cold wave this week.

The Exxon Co., supplier of most of the contaminated oil, has pumped the fuel from the families' storage tanks and replaced it with fresh, uncontaminated supplies.

An Exxon spokesman at regional headquarters in Baltimore said the problem apparently originated when the company drew oil from the bottom of its own storage tanks as supplies dwindled during the persistent cold water.

The residue oil apparently contained sludge and water that stopped up residential fuel lines, shutting oil furnaces.

According to some residents, the problem was not immediately identified, and their furnaces shut down several times between Tuesday and Thursday.

Judith N. Gebara of 6338 Villa La. said an Exxon technician told her on her first visit she needed a new burner. Before the source of the problem was identified, Mrs. Gebara said she signed a contract to buy a new burner costing $285.

To get the replacement immediately, she said she also signed a waiver under which she wouldn't be able to void the contract within three days.

"I still have this contract," she said. "But I'm not going to pay for it." The new burner was never installed.

Yesterday, three days after the Gebaras' furnace began shutting down intermittenly, Exxon pumped out their fuel tank.

Kathlynn D. Miller, who lives at 4416 Hillbrook Dr., Annadale, said Exxon replaced two motors in her furnace and told her husband, Grant, how to get the equipment running again before replacing the fuel supply of their tank.

While the Exxon spokesman in Baltimore said the problem affected about 20 customers, Mrs. Miller said the company deliveryman who replaced her oil said 100 homes were involved.

Mrs. Gebara said her deliveryman told her six tanks were cleaned out Thursday and there were 40 other jobs "still on the board" Friday.

The Exxon spokesman said it was "Absolutely not true" that more than "20 possibly 23" customers were affected.

He said the company "assumed all responsibility" for repairs and fuel replacement. The company, he said, has [WORD ILLEGIBLE] customers in the metropolitan area.