When John Velez, co-owner of Sutton Cleaners, arrives at work at 7 a.m. on Manhattan's East Side, he opens a steam valve in the back of his shop. "When I come into the shop in the morning, it's one, two, three," he says, "and you're up and running in less than a minute."

Nearly every piece of equipment that Velez will operate for the rest of the day uses steam. The spotter shoots out steam to remove stains while a puffer sends out steam to remove wrinkles. The dry cleaning machine and the pressing machine also use steam.

So do Rockefeller Center, the Metropolitan Museum of Art and the United Nations -- which use it for heating and cooling - along with some 2,000 other customers and 100,000 buildings, from residential low-rises to commercial skyscrapers. All are in Manhattan, primarily because steam is most efficient and cost effective for high-rise buildings.

The City of New York is one of the largest consumers of steam. The Roman Catholic Church of St. Peter in lower Manhattan began using steam to warm its sanctuary in 1882, the year the first steam generation plant went into operation in New York. The church has used steam ever since.

NOT JUST A GIANT STEAM KETTLE

Some 30 billion pounds of steam every year flow beneath the streets of Manhattan from the Battery to 96th Street. While it is unknown to most New Yorkers, Con Edison's subterranean steam system is the biggest steam district in the world, larger than the next four largest U.S. steam systems combined and boasting an annual steam production more than double that of Paris, Europe's largest system.

If steam is an easier concept to grasp than, say, fiber optics, producing it on this scale is nowhere as simple as turning on a giant teakettle. Some of the steam comes from the Con Edison steam generating plant on 14th Street and the East River. Inside, two massive boilers -- one 95 feet high -- burn natural gas or fuel oil and air. The resulting heat sends the temperature of water inside the boiler's pipes to a blistering 1,000 degrees Fahrenheit, converting it to steam. Workers wear ear protectors to keep out some of the roar of millions of gallons of flowing water and the sound of spinning turbine blades. Little, though, can shield them from the warmth radiating from pipes filled with superheated vapor.

"When we used to bring people into the plant for tours, their first impression, when we have them look inside of the furnace, is that it's huge. People are taken back by the sheer size of it - eight stories high," says Wilton Cedeno, former manager of Con Edison's Hudson Avenue steam plant. "There are intricate control systems -- systems for treating the water, electrical systems, pressure systems. It's all very complex."

On average, the plant converts a gallon of water into eight pounds of steam -- every hour, approximately 125,000 gallons of water are turned into more than one million pounds of steam. "We use city water, but the water has to be processed and cleaned," says Cedeno. "If it's not pure, you get build up on pipes and can damage the turbine blades."

The East River plant is one of seven Con Edison plants -- five in Manhattan and one each in Queens and Brooklyn. Three of these plants, including the one on the East Side, produce both steam and electricity through a process called co-generation. In these plants, the steam leaves the boiler and then goes through pipes into a turbine generator. The resulting spinning of the turbine blades produces electricity. The remaining steam then goes into the steam system.

From the plants, the steam goes into Con Edison's underground pipes. On a cold winter day, nearly 10 million pounds of steam at 350 degrees Fahrenheit flow each hour through 105 miles of underground mains. The pipes coming out of the plant can be several feet in diameter, but the steam travels through progressively smaller pipes, ending at ones that may be only a couple of inches wide.

STERILIZING, HEATING, CLEANING -- NOT SEEPING FROM MANHOLE COVERS

The steam coursing under Manhattan is not to be confused with what is popularly called steam rising from many city manholes. The steam in the pipes is invisible, while the so-called steam wafting up from the streets is often vapor produced when underground water hits hot equipment and escapes from beneath the streets. It can also be condensed steam leaking from the Con Ed system.

Some of the real steam winds up at St. Vincent's Hospital on 12th Street, where the forceps in the operating room are sterilized using steam. Every day nearly 200 cleaning trays of surgical instruments pass through the Central Sterilization unit at St. Vincent's. A cleaning tray can contain anywhere from six to 130 pieces of surgical steel, depending on the size of the instruments. First, the trays are placed in a washer-disinfector, similar to a regular dishwasher. Then the trays go into the sterilizer where steam circulates inside. The intensity of the heat from the steam kills any viral or bacterial pathogens.

Most of the steam, though, is used for heating and cooling. Steam used for heating flows from Con Edison's underground mains into a building's internal heating pipes and then into a radiator where it heats a room. For cooling, steam flows from the main into a building's steam air conditioning unit.

The use of steam eliminates the need for boilers in individual buildings. Because steam from Con Edison's centralized plants is mass-produced, it is generally more economical, efficient and environmentally friendly than individual oil or gas boilers.

Joe Petta of Con Edison compares it to mass transit. "Which is better for the environment, 50 people riding to the city on a bus or 50 people riding 50 different vehicles? The emissions from one bus will be less than the emissions for 50 cars," says Petta. And he adds, the plants that produce steam are subject to "more stringent environmental and emissions controls than individual buildings generating their own heat either by burning gas or oil."

A NEGLECTED POWER SOURCE

Last year, Clean Air Communities -- a project of Con Edison, the Natural Resources Defense Council, the New York State Department of Environmental Conservation and the Northeast States Clean Air Foundations -- installed a new steam energy station in an underground vault at the Seward Park Housing cooperative, four apartment buildings with 1,728 apartments in lower Manhattan. The station, which ties into Con Edison's steam system, replaced four onsite oil boilers that burned approximately one million gallons of heavy fuel oil a year. This has eliminated traffic from fuel trucks and markedly cut air pollutants and reduced carbon dioxide emissions.

But many New Yorkers cannot follow Seward Park's example. Converting to steam often requires taking out an old boiler and putting in new piping and other equipment to link up with the steam distribution system. This can prove difficult and costly.

Partly as a result, Petta says, the number of steam customers has not increased in the past few years. "A lot of people don't know about it or don't know it's an option," he says. "Or building owners don't want to go through the conversion process and don't want to spend the money to convert."

And so, for now at least, steam remains New York's neglected power source. "The steam system is a great asset to the city and delivers clean energy," says Ashok Gupta, director of air and energy programs at the National Resources Defense Council, an environmental group. "We can clearly be doing more with it."