While 1984 may be a presidential year to some and an Orwellian landmark to others, well-informed Americans will recognize it as the upcoming 50th anniversary year of Washington's steam grates.

The vents scattered among the city's sidewalks are most familiar as sources of wintertime warmth for some of Washington's homeless, but they are also the signposts of an underground labyrinth of tunnels and pipes that have carried heat to the offices of thousands of federal workers since 1934.

The General Services Administration, charged by Congress with the responsibility for federal hot air, presides over the government's central heating plant, its 7 1/2 miles of tunnels and five miles of buried pipe. And it's not as easy as you might think.

The National Gallery of Art, for example, likes its buildings warm and humid for the paintings; the Pentagon (warmed by a separate GSA steam plant across the Potomac) likes its cool and humid for the computers. The Bureau of Engraving and Printing uses the same steam that warms the grate sitters to dry newly-printed $100 bills.

In addition, "Some of the--ah--homeless occasionally use the grates as their--ah--sanitary facility, and we're a little touchy about that," said Charles Pollinger, 38, GSA's steam czar. "I mean, our men have to work down there. But on the whole they don't give us much trouble. One day I was headed into the tunnel with some equipment and a bunch of them were on the grate having a picnic. Tablecloth and everything.

"I said, 'Excuse me, gentlemen, I have to get in there.' They were very nice about it. They invited me to lunch."

The steam grate side of the business isn't all tablecloths. Back in the early '70s some militant protest groups were discovered plotting to bomb the tunnels in an effort to let some steam out of the Vietnam War.

Now locks and alarms secure the tunnels and all manholes are tack-welded during inaugural parades to seal out the crazies.

Pollinger, who bears up well under the title of "Manager, Heat, Maintenance, Operation and Transmission District" in GSA's Office of Public Buildings and Real Property, says the grates are actually a sort of cloudy monument to the government's foresight and clear thinking.

Back around 1913, when every federal building had its own coal furnace and stokers, Congress decided it would make more sense to have one furnace serving them all.

The result 20 years later (these things do take time, after all) was a Central Heating Plant at 13th and C streets SW, where four coal-fired boilers now crank out steam for the heating plants of 112 public buildings housing 35 million square feet of floor space and 110,000 federal workers.

Superheated to 406 degrees at a pressure of 250 pounds per square inch, the steam is piped through a labyrinth of seven-feet-high by seven-feet-wide tunnels via one 18-inch line and one 20-inch line, each insulated with (or being upgraded to) four inches of calcium silicate wrapped with aluminum sheeting.

The last stretch carries the steam to and from the auxiliary plant at 29th and K streets in Georgetown, which generally serves buildings west of the Ellipse.

Pollinger and the alarm system willing, one could wander the underground maze from the Food and Drug Administration southwest of the Capitol to the Treasury Building or the U.S. Court of Military Appeals. But one might have to make the trip in a loincloth: It gets a little warm in the tunnels: about 130 degrees.

Which is why the grates. That's not really wasted steam they exude--aside from the odd leaking valve--but the heated air from around the pipes, carrying with it the residual moisture of the tunnels. It simply steams when it hits the cold outside.

Technically, Pollinger says, it might be possible to save a few BTUs by keeping all that hot air in the tunnels.

"But then nobody could work down there. We need the grates both for access and ventilation. The key to more efficiency is not sealing off the tunnels, but better insulation of the pipes."

The GSA, which has not always been universally acclaimed for efficiency, is nonetheless proud of its heating operation. The centralization of the boiler plants, Pollinger says, permits better control over pollution abatement, fuel and manpower costs and heating flexibility for the varied needs of diverse agencies.

GSA sells its steam to each building or agency at the rate of $14 per thousand pounds (roughly 1 million BTU).

The "customers" then adapt it to their particular needs and send it back as 160-degree water through a 10-inch condensate pipe.

The water, in turn, is so softened, filtered and scrubbed of oxygen at the Central Heating Plant that Pollinger says the system's transmission pipes have remained virtually rust- and scale-free for half a century. Valves, however, with their moving parts, occasionally show wear.

In addition to heating such obvious federal buildings as the White House, the Smithsonian and the Department of Agriculture, GSA sells steam to such agencies as the District of Columbia courts, the Martin Luther King Library and various quasi-governmental buildings including the Organization of American States and the American Red Cross.

It definitely does not sell steam to the Capitol, where the separation of executive and legislative powers reaches all the way underground. Congress has its own independent heating plant next to the Southwest Freeway, serving 16 major buildings, including the Capitol, the various House and Senate office buildings and annexes, the Library of Congress and the Supreme Court, plus Union Station, the Folger Shakespeare Library and the Government Printing Office.

According to Eliot Carroll, executive assistant to the architect of the Capitol, this system also has its own 3 1/2-mile system of tunnels, but no steam grates.