Here's another way to look at it...
Let's say you have a HELOC and a 30-year fixed 1st mortgage.
Take the entire available balance of your HELOC and apply it to your first. Then, pay down the HELOC while keeping up your payments on the 1st.
Everytime you get a big chunk available in your HELOC, throw it at the 1st.
Why it works:
The 1st mortgage is amortized. That means the early payments are mostly interest while the principal balance is high. The payment is fixed and not dependent on the balance owing - the payment always stays the same unless the interest rate adjusts. (Yes, this works for ARM's, also.)
The HELOC, on the other hand, is like any other revolving credit. The payment is determined by the balance owing or the average daily balance - check the terms of your HELOC to determine what it is in your specific case - plus the interest on that balance for the payment period.
If you can manage your HELOC payments so you're throwing big chunks at your 1st on a regular schedule you'll payoff your 1st much faster than making double payments or any of that.
I don't have the numbers or the charts right at hand, but yes - using this technique, you CAN payoff a 30-year fixed in as little as seven years ... nine to ten is more likely, however.
Note, however, that every time you take a big chunk out on your HELOC your credit will take a hit: the spike in utilization will lower your score and impact your credit profile. Sorry - no free rides.
David J Dachtera