1:21 am : I was reading this thread (now three years long!) and as Peter says, one has to look at EXACTLY how each plan's interest is calculated. The typical mortgage Amortization schedule is calculated once per month. If you add principal to a payment to reduce your loan balance, most, but not all, loans will recalculate an amortization schedule for the lower balance and the same number of payments. The HELOCs have quite a few ways of calculating interest, usually based on the prior months' average daily balance and applies the daily rate to each day's balance -- not unlike a credit card. So, if you were to transfer a sum of money from the HELOC to your mortgage at a favorable time of the month -- the 1st day of the mortgage month, and you delay as long as possible the paying of bills (changing their due dates, for example) but taking care to account for processing days so you don't earn late fees, the average daily balance can be held to as much as 60% below what the mortgage period balance is. You have to be very attentive for about a year until you get used to this system. One mistake will offset any gains you make for a month between the two interest calculation systems. The additional benefit, regardless of whether you gain inside the HELOC, is that those larger transfers early in a mortgage's life REALLY knock out the number of years the mortgage is