With mortgage rates still at tempting four-decade lows, homeowners are doing the math and beating a path to their lenders to refinance.

As interest rates keep falling, some homeowners are on their second and third refis in as many years.

The current flood of business means it could take from 30 to 60 days to refinance your mortgage, from application to closing.

We asked mortgage experts and lenders for a refresher course in refinancing basics and for an update on trends and new products. Here are the questions we posed and a wrap-up of the answers we got:

Q: Is it still a good time to refinance? A: Rates crept up a hair last week, but they're still extremely low, so yes, it's still a good time.

Determine the difference between your mortgage loan rate and current market rates, then ask your lender for the difference refinancing at the lower rate will make in your monthly payment or do it yourself with the refinance calculator at www.quickenloans.quicken.com

Your lender will estimate the costs of refinancing. Find out how long it'll take you to recoup those costs by dividing your monthly savings into your estimated costs of refinancing. That's how many months it'll take you to break even.

Q: What can you hope to accomplish by refinancing?

A: That's the question to ask yourself as you choose a new loan.

If your goal is to increase your cash flow and lower your monthly payments, trade your higher-interest 30-year fixed loan for a new 30-year fixed at a lower rate. Going from 8 percent to 6.25 percent on a 0,000 loan will take your payment on principal and interest from ,100 to 3, a savings of nearly 0 a month.

Or, if you know you're not going to be in the house for longer than, say, seven years, save even more by converting to an adjustable rate mortgage. Your payment would drop another 0 at current adjustable rates of 5.125 percent. That's what you'd pay for seven years, then the interest rate could go up annually with market rates. But if you leave the house before then, you won't incur the risk of rising rates.

If your goal is to pay off your mortgage sooner, consider converting from a 30-year to a 15-year fixed mortgage. The payment may be a bit higher, but you'll save thousands on interest and the loan will be paid off in half the time.

One of Ron Rudy's customers at Portland (Ore.) Mortgage Co. last week traded her 30-year, 0,000 mortgage with a rate of 7.125 percent and payments of 8 for a 15-year at 5.5 percent and payments of 0. Her payments will increase 2.

"But she'll save ,000 in interest, and her house will be paid off in 15 years instead of 27," said Rudy, who is president of Portland Mortgage and a director on the board of the Mortgage Bankers Association of America.

Another way to shorten a mortgage lets you knock years off a 30-year loan by making a surprisingly smaller extra payment toward your principal in the early years. Or make a house payment every four weeks, which adds up to 13 a year.

But don't fall for the come-on many newly refinanced homeowners get in the mail soon after their deal goes through. It's an offer to set up a system to pay a little extra every month and pay down your principal, for a "one-time enrollment fee of 0." But why not just do it yourself for free?

Q: How do I get money out of my house?

A: Another motive for refinancing is the chance it gives a homeowner to convert the accumulated value in the home equity to cash, which can then be reinvested or spent.

Someone with a 0,000 loan on a 0,000 house, for example, could borrow as much as another ,000 and still qualify for the conventional 80 percent "loan-to-value" ratio. You can't borrow more than 80 percent of your home's value, in other words, and still get the lowest interest rates.

The wisdom of borrowing that much is another matter entirely if property values fall, borrowers can get caught owing more than their home is worth. Nevertheless, people are less resistant than they used to be about pulling equity out of their homes, and they're using the money for everything from remodeling projects to paying college tuition to paying off debts.

Need a quick ,000? Trade that 5,000, 30-year loan at 8.25 percent for a 0,000 loan at 6.25 percent. Your house payments go down from ,014 to 5 and you get ,000 to boot.

Q: Are "no-cost" refinances a good deal?

A: They could be for people who can't come up with the money to pay the fees upfront. But be assured that a no-cost or low-cost refinance doesn't mean you're paying less in the long run. It means you're paying less upfront and either wrapping the cost of the transaction into the loan by borrowing more or paying a slightly higher interest rate to cover the costs.

Still, the low-fee, low-stress approach can be appealing to anyone who's been through the long division of the mortgage application process. All the paperwork, documentation and proving this and that can be arduous and, at times, maddening.

Lenders have tuned in to that and have come up with a variety of solutions. Wells Fargo, for example, advertises its Simple Refinance Plan with the siren song of "No points. No closing costs. No hassle."

Last week, that deal would get you a 15-year mortgage at 6 percent for only 0 upfront. Normally, closing costs on a loan of 0,000 or so could be ,500 or more. The reason costs are so low is that you're paying 0.5 percent more than the going market rate for your loan and that's more expensive for you in the long run.

Q: What's an "interest-only" loan?

A: According to Frank Sloan and Dennis McAuliffe at Wells Fargo Home Mortgage, "interest-only" loans are becoming a popular option, especially among affluent homeowners.

The borrower pays a below-prime rate on his mortgage balance to cover interest and makes principal payments whenever he wants on the occasion of a year-end bonus or stock option sale, for example.

The rate right now is 4.49 percent on loans of as much as million, and it's flexible like a home equity loan so you can tap your equity when you need it.

Q: Should borrowers lock in current rates? What happens if rates decline further?

A: Most mortgage lenders let borrowers lock in an interest rate for 30 to 60 days for free. If rates go up before the loan closes, the borrower still gets the lower rate.

But what about borrowers who lock and then watch the rates drop? Lenders will hold you to the higher rate you locked unless you planned ahead with a "float down" option or are able to negotiate a compromise rate with your lender.

Float-downs aren't offered universally, and they're not free the option could add a quarter-point to your initial rate but lets you float down if rates drop before you close.

Q: What's a yield-spread premium?

A: Assuming your credit is good and you haven't locked, you should get the best market rate for a mortgage. But if your broker doesn't tell you, for instance, that rates have dropped from 6.5 percent to 6.25 percent, he may be getting a premium payment from the lender to keep you in the 6.5 percent loan.

If that's the case, it will show up on the HUD-1 statement, a required list of loan expense disclosures you get before closing. Look for "YSP," and if you see it listed, ask more questions about your mortgage loan rate and loan origination fees.

Related links:

1. US mortgage market
2. Homeowners Taking Advantage of Low Rates to Refinance Loans
3. Mortgage Refinance Rush Greater than Last Fall's