@Joe Splitrock You are misunderstanding me. In my scenario you are not making extra 10k / month overpayments. This is in stark contrast to what @Brian Cardwell has been saying, in his arguments you do make overpayments and the loan gets paid off ninja fast due to those overpayments and not because of the heloc.
In my scenario, in the FIRST month you do make a 10k payment towards the mortgage (in the amount of your monthly paycheck), but this is because you will eventually pull that same money right back out via your heloc. So it is a 10k payment, and a 10k withdrawl, which means there is a net of no additional payment as you still have a 100k total loan (90 on the mortgage, 10 on the heloc). All months after the first you make the mortgage payment completely as you normally would, and your 10k monthly paycheck goes towards paying off your heloc balance in it's entirety. If done properly you should only have a balance on your heloc for a few days at the end of the month, and will be paid of in full on the first of the next month with your paycheck. This prevents that 10k paycheck from sitting in your bank account earning no interest until it is time to pay bills, and instead immediately goes to pay down the mortgage, and then only costs you interest when you finally decide to withdraw the money via your heloc for a few days at the end of the month to pay those same bills.
I will attempt to show this in the attached spreadsheet. The first tab is a standard 100k loan at the stated 4.75% interest rate with no overpayments.
The second tab is the same 100k loan using the heloc method. On day 1 of the first month, you make a 10k overpayment, and will then take that money right back out via your heloc to pay your bills. This translates into a 90k balance on your mortgage, and 10k heloc so there is no actual overpayment since all 100k is still accounted for, it is just split into 2 different balances instead of one singular 100k loan. You will pay interest on your heloc balance, which if you can push your bills back far enough into the month to only pay 1 week worth of interest, that turns a 5.9% heloc into an effective 1.37% loan. This means each month you will pay roughly $11.47 in interest, and an extra $8.33 in heloc fees if your lender charges $100/year to keep the account open. Over the life of the loan this means the heloc will cost you a little over $5700. This means that you will pay roughly an extra $19.81 / month to your heloc using this method. Under this scenario your loan is paid in full after 24.25 years.
Tab 3 contains a regular 100k loan with monthly overpayments in the amount of $19.81 per month to simulate what would happen if you made an overpayment in the amount that your heloc costs you each month. Under this scenario your loan pays off in 27.75 years, or 3.5 years SLOWER.
Tab 4 is my feeble attempt to show the flow of money into and out of the heloc. The basic premise is that your paycheck gets direct deposited into your heloc account, and you use your heloc to pay all of your monthly bills. You want to push all of your bills back as far into the month as possible before paying them in order to pay as little interest as possible. Each month when you get paid, you want your paycheck to pay off the entire heloc balance such that nothing stays on your heloc for more than a handful of days.. If done properly this allows you to keep a very low balance on your heloc for the majority of the month, and a very high balance for only a few days at the end of the month. In my scenario I said that ALL bills were due on the 23rd of each month. Obviously real life scenarios will be more messy since some bills will have different due dates, but this gives you the basic idea of how the system should work.
Summary: Everyone who advocates doing very large chunking on a heloc that takes many months to pay off is incorrect, because if a balance remains on a heloc for the full 30 days, then simple math says that a 5.9% loan is worse than a 4.75 loan. However, if done properly and in small enough amounts such that you do not carry forward a balance from month to month on the heloc, and instead only keep a balance on the heloc for a handful of days then it can be very beneficial as it takes a portion of that 4.75 loan, and turns it into an effective 1.37% loan. The method is obviously dependent on how far back into the month you can push the majority of your bills, as this lowers the actual amount of interest that you will pay on your heloc balance. Also, the higher your household monthly earning, the better this technique works. As a family that earns 10k / month will be able to shift more onto their heloc than a family earning 7k / month. Also it goes without saying that it depends on the interest rate of your heloc versus your regular mortgage, and how much fees your lender charges. This method isn't magically going to eliminate decades from your loan, but for some people, under the right set of circumstances, it can be beneficial.
Heloc Loan spreadsheets