More than 15,000 federal workers in half a dozen agencies face long-term furloughs, some beginning next week, because their agencies have run out of money to meet full summertime payroll costs.

Thousands slated to be furloughed, or put on half-day (and half-pay) schedules are employed here by the Merit Systems Protection Board, the Office of Inspector General at Health and Human Services, Treasury's Bureau of Alcohol, Tobacco and Firearms, the Department of Education, and three major Labor Department units -- the Employment Standards Administration, Bureau of Labor Statistics and Employment and Training Administration.

At worst the furloughs could range from 10 days at the Department of Education to an almost total summer-long shutdown for HHS's Inspector General's Office and at Treasury's BATF.

Money to operate all or portions of the agencies was carried in two special versions of emergency supplemental appropriations bills approved by Congress but vetoed by President Reagan.

The Senate has cleared a third measure, stripped of extra spending proposals opposed by the president. But the Senate and House are on vacation until July 12.

When and if Congress votes funds, furloughs can be canceled in most cases. This is the outlook as of now:

* An almost complete shutdown of the IG office at Health and Human Services, the largest waste- and fraud-fighting unit outside of Defense, beginning July 30. Staff members of the Federal Government Service Task Force headed by Rep. Mike Barnes (D-Md.) says HHS might call some workers back for two or three days for caretaker chores, but otherwise shut down until Sept. 30 or until the supplemental budget is approved.

* Twenty-two days of furloughs, beginning next Tuesday for the Merit Systems Protection Board. The furloughs would mean MSPB could not process appeals from fired federal workers and whistle blowers, including about 11,000 cases filed by fired air traffic controllers.

The MSPB is in a Catch-22 situation, because a federal judge in California recently ruled that the agency must handle certain controller appeal cases before August, or find itself in contempt of court.

* Staffers at the ETA, BLS and ESA agencies of the Labor Department are due to be furloughed for 22 days beginning July 19 unless Congress has approved the special supplemental.

* Treasury's alcohol, tobacco and firearms unit, with 1,900 employes, is scheduled to begin 73 days of furloughs starting July 25 and running through Sept. 30 unless it gets extra funds.

* More than 450 workers at the Economic Development Administration (Commerce Department) could be furloughed later on unless the supplemental is approved by mid-July.

Democratic and Republican House leaders have indicated they will not rush to approve the latest version of the supplemental money package when they return July 12.

The Senate has approved a new compromise worked out with its GOP leadership and the White House. But House leaders are irked by the two previous vetoes and what they believe is White House failure to advise them of what would be acceptable to the President.