What started as a $200 million emergency measure to keep elderly Americans from freezing to death has, in the space of three years, become a $2-billion-a-year thread in an elaborate income security net provided by the huge Department of Health and Human Services.

The goal of the program (called HEAP for Home Energy Assistance Program by some and LIEAP for Low-Income Energy Assistance Program by others) is to soften the blows of rising energy costs on all low-income families, to keep them warm in winter and, to a lesser extent, cool in the summer.

But HEAP, political handmaiden of the oil profits tax, has become a government-created market for welfare that is fast becoming prohibitively expensive to supply. And if the Reagan administration succeeds in deregulating the price of natural gas by the end of September, as it hopes to do, the price tag will get even higher.

"The frightening thing on the horizon is [President] Reagan's desire to accelerate natural gas deregulation," said Carol Werner of the National Consumer Law Center. "That will bring horrendous economic dislocation," especially in the southwest.

More low-income families depend on natural gas than any other fuel. Unless Reagan simultaneously increases the program's budget -- considered unlikely in this year of budget-slashing -- fewer people will be helped.

"It's an old problem," said Otis Brent of the Dallas County Community Services Agency. "How far do you try to spread a few dollars? Do you give a few pennies to all and have some of them still frustrated, or try to meet all the needs of some poor people and none of the needs of others? This program is going to frustrate quite a few people."

Federal officials in Washington describe HEAP as neat and tidy. By the time you reach the local level, it is a snarl of paperwork and ad hoc decisions by people who act first and ask Washington for approval later. "You just shuffle the money around and tell them," said James Adams of Dallas. Ultimately, many recipients have no understanding of why or how the money comes.

Some citizens who have come to depend on the money already receive less under today's 900 percent larger program than they did a few years ago. In Illinois, for example, the average benefit this year is about $200 compared to $296 last year. The maximum payment for some Massachusetts residents is down slightly from last year. In Texas, families that once got up to $400 may get as little as $100 this winter.

On the other hand, millions more families now get something. California expects to double the number of households that will receive benefits. Massachusetts may do even better. If Texas officials are successful in their recruitment campaign, more people will receive funds from this program than the combined roster of recipients of food stamps, aid to families with dependent children, and supplemental security income.

"We have a moral requirement as well as a legal one to go out and find people who are eligible," said Charlie Jennings of the state Department of Human Resources.

That philosophy is a far cry from the early days of the program, when citizens sought help if they could not pay utility bills. The program was born in the mid-1970s, when bitter cold and the sheiks of the Organization of Petroleum Exporting Countries conspired against poor people struggling to stay warm. In 1977, the federal government responded with a one-time appropriation of $200 million, which the Community Services Administration used to prevent utilities from shutting off the heat in homes of poor people who had not paid their bills.

The program has changed annually since then. One year it was called Special Crisis Intervention Program (SCIP). The next year it was Emergency Energy Assistance Program (EEAP). One year it was administered by the states. The following year, CSA gave money to community action agencies. One year money went automatically to all recipients of supplemental security income, even those in nursing homes who paid no utility bills. In other years, federal officials had difficulty tracking where the money went. And often there were problems getting the aid out promptly. One year it arrived in July.

"I'd like to see them maintain a couple of years of stability," said Wayne Curtis, program manager in Illinois.

In fiscal 1980 the program underwent a transformation, growing from $200 million to $1.6 billion. Skyrocketing oil costs and former president Carter's decision to decontrol prices forced people to take a different look at the once-modest program.

"The question was, should it be an income transfer or energy assistance program?" said Tony Maggiore of Milwaukee's Social Development Commission. "That was never fully resolved in the debate, but in practice it was when it was shifted to HHS."

Many supporters opposed the changes, arguing that there should be an integrated program to help poor people fix up their leaky houses. "Once checks go directly in the mail, the character of the program changes. The bureaucrats in HHS won."

The rationale sounded logical enough. "If you only pay for those people who have a crisis, then it's inequitable," said Ira Goldstein, acting associate director of Social Security for family assistance. "You're rewarding people who haven't paid their bills and not people who have paid their bills but who can't buy shoes or food. Part of the change was a movement to assess who really needs the money and give them a fair share of it."

But the price of that equity has been mind-boggling complexity.

"HHS took it over this year and the model they used was a welfare approach, which is replete with regulations, rules and Catch 22s," said Robert Coard, executive director of Action for Boston Community Development.

"There's a new rule about SMSAs [standard metropolitan statistical areas] being a determinant of income," Coard said. "The city of Worcester, Mass., is part of four SMSAs. People living across the street from one another are eligible for different amounts of money."

Rules vary enormously from state to state, because states design their own programs. Some people automatically qualify for the money and get checks in the mail. Others must apply to get any money. Some people who get automatic checks are urged to apply for additional aid. Some states send all the money directly to recipients. Others pay it to utility companies, apartment building owners or fuel oil distributors. In Massachusetts, some of the money is used to pay for cheap oil provided through an arrangement initiated by Joseph Kennedy.

In spite of the annual changes, the program is still far from equitable, if its real goal is to cushion poor people from rising energy prices. The current program provides money for cooling only where medically necessary. Some states require a doctor's certification, others may give it to anyone with an infant or elderly person in the house. Even so, the amount of money the states set aside for summer cooling is minimal.

Here in Texas, with memories of last summer still searingly fresh, the state has reserved only $3 million of a total grant of $39 million for this summer. But in Dallas alone, local officials spent almost $400,000 during last summer's devastating heat wave, helping citizens pay their electric bills and distributing 4,484 fans -- and even that was poorly received.

"With the temperatures we had, the fans basically just threw out hot air," said Otis Brent. "The public was pretty hostile to that program."

Such is the price of good intentions.

The Home Energy Assistance Program has hardly made a dent in the extra billions of dollars that low-income Americans are paying to heat and cool their homes. But as a salve to politicians opposed to energy price decontrol, it has succeeded admirably.

The program undoubtedly has helped some people survive cold winters, allowed others to buy food, clothing and pay their utility bills. But the question now is whether it has reached the point of no return.