A growing number of mortgage applicants, especially those refinancing older loans, are opting for 15-year mortgages instead of the 30-year variety, according to the Mortgage Bankers Association.

While the benefits of lower interest charges on 15-year deals seem obvious on the surface, the choice isn’t always as simple as it looks. A 15-year deal may not appeal at all to someone buying a home, but could offer real savings to someone refinancing an older mortgage.

About 20% of mortgage applicants opted for 15-year, fixed-rate loans in October, up from 9.6% a year earlier and 8.8% two years earlier, the MBA says, noting that most 15-year applications involve refinancings rather than home purchases.

The initial appeal lies in the low rates. The BankingMyWay.com Survey shows the average 15-year loan charging just 4.581%, versus 5.132% on 30-year fixed-rate loans.

A lower rate means lower interest charges. Despite that, most homebuyers shy away from 15-year loans because payments must be higher to retire the debt in 15 years instead of 30.

For a $300,000 loan, monthly payments at the average rates would be $2,307 for a 15-year loan, $1,635 for a 30-year one. The 15-year loan would charge $115,335 in interest over its life, compared to $288,513 for the 30-year deal.

But the 15-year deal can be much more appealing for a homeowner who has already had the loan for some time. Consider a homeowner who took out a $300,000 30-year loan 15 years ago, charging 9%, about the going rate at the time. The monthly payment would be $2,414., according to the Mortgage Loan Calculator.

Today, the homeowner would still owe $237,991 and would pay about $196,000 in interest over the next 15 years. Refinancing to a 15-year loan at 4.581% would cut the monthly payment to $1,830 and reduce total interest costs to about $91,000. With a 30-year loan at 5.132%, the payment would be even lower, at $1,297 a month. But the borrower would pay much more in interest, $229,000, if the loan were kept for the full 30 years.

The borrower who can switch to a 15-year loan without taking on an overly burdensome payment can benefit substantially from today’s extraordinarily low rates. Use the Refinance Interest Savings Calculator to see if this will work for you.

Be sure to use the BankingMyWay.com Shopping Tool to track down a market-beating deal. Wells Fargo (Stock Quote: WFC) and Bank of America (Stock Quote: BAC) for example, offer 15-year loans at 4.375%.

—For the best rates on loans, bank accounts and credit cards, enter your ZIP code at BankingMyWay.com.