Biweekly mortgages are expected to become more widely available in the Washington area after the Federal National Mortgage Association, one of the largest investors in home mortgages, begins buying them Monday. But many lenders said this week that they remain skeptical about the ultimate appeal of the loans to borrowers.

"While some lenders find it useful, many lenders are not intrigued," said Warren Lasko, executive vice president of the Mortgage Bankers Association of America. "They view it as a gimmick."

With a biweekly mortgage, the borrower pays the equivalent of half the normal monthly payment every two weeks. Typically, payments are automatically drawn from the borrower's checking or savings account.

The 26 or 27 biweekly payments add up to about one extra monthly payment each year.

Combined with biweekly amortization, the extra payment reduces the principal faster than with conventional monthly payments, boosting the borrower's equity, shortening the overall term of the loan and significantly reducing interest costs.

Interest rates for the loans are generally comparable to those on 30-year fixed-rate mortgages.

Long a staple of the Canadian mortgage market, biweekly loans have found pockets of popularity around the United States, particularly in Connecticut and California, but they have been offered by only a few lenders in the Washington area.

Lenders said they have been discouraged by the dearth of secondary market investors willing to buy the mortgages and the cost of modifying their computer systems to handle the nontraditional payment scheme.

The Federal National Mortgage Association (Fannie Mae), which announced its decision to enter the secondary market earlier this month, predicted a large potential demand for the loans, saying they could appeal especially to the 50 percent of American workers who are paid on a biweekly basis.

More than 80 percent of workers in the Washington area, including federal employees, are paid on a biweekly basis.

Financial experts agree that the effects of biweekly payments can be dramatic, and the statistics speak for themselves. Making biweekly payments on a $100,000 mortgage at 10 1/4 percent interest, a borrower would pare nine years and six months off the traditional 30-year fixed-rate term, according to calculations by Fannie Mae.

In the process, the borrower would pay an additional $672 annually and reduce total interest costs from $222,596 to $141,333, a savings of 36.5 percent.

On a $75,000 loan, the biweekly borrower would reduce total interest by $60,947, and on a $125,000 loan, savings would total $101,579. For much of the term of the biweekly loan, the borrower would have an average of about 70 percent more equity in his or her home, according to bankers' estimates, compared with a traditional 30-year loan.

Although some Washington area lenders said they plan to introduce biweeklies in the coming months, many played down the advantages of the product and said they perceived no significant demand.

Biweekly loans merely discipline borrowers to reap advantages they can already achieve through a variety of methods, lenders and financial advisers said.

Only lack of funds or will power prevent a homeowner with a conventional 30-year mortgage from paying back the loan faster, they said.

Fannie Mae's computations illustrate some frequently mentioned possibilities. By adding $25 to the monthly principal payment, for example, the borrower can pay back a 30-year, $100,000 mortgage in 25 years and nine months and save $38,659 in interest payments.

Similarly, with a 15-year mortgage, borrowers can save $129,167 in interest by paying $12,895 annually, spread over 12 monthly payments, compared to $10,753 for a standard 30-year mortgage.

Lenders cautioned that making mandatory biweekly payments could become a strain for borrowers whose incomes or budgets become unstable.

While some said they were concerned about exposing customers to an increased risk of default, the Fannie Mae biweekly plan allows a measure of leniency for borrowers who fall behind by three payments.

Their loans would be converted to monthly payment schedules, although they could incur penalties.

Some lenders pointed to the automatic payment arrangement as a convenience factor for the borrower, but others said it could be a disadvantage if it required borrowers to maintain accounts at the lending institutions that gave them the mortgages.

"I don't see any real advantage to {the biweekly mortgage} other than forcing a person to make that extra payment," said A. William Blake Jr., senior vice president of the Maryland State Savings and Loan Association, which is preparing to offer biweeklies.

Other lenders said they would refrain from introducing the product.

"From a bookkeeping standpoint, it's more hassle than it's worth," said Benjamin Delaney, president of First Citizens Mortgage Co. "It's just a sales tool."

"Right now, people aren't asking for it," said John F. Elder, executive vice president of Perpetual Mortgage Co. "It's hard to come out with a product that nobody seems to want."

Thomas Edmunds, vice president and regional manager of Crestar Mortgage Co., one of the few lenders already offering biweekly mortgages in the Washington area, said these account for 1,200, or 20 percent to 30 percent, of Crestar's fixed-rate loans.

United Virginia Mortgage, now part of Crestar, introduced the product in April 1985. Unlike the Fannie Mae program, Crestar's amortizes on a monthly basis.

Crestar allows dissatisfied biweekly customers to switch to a monthly payment schedule, but almost all borrowers have stayed with the plan, Edmunds said.

Crestar plans to begin buying biweekly mortgages from lenders nationwide and projects that they could eventually account for 6 percent of the home loan market, Edmunds said.