It is a dynamic number that is reduced with each payment as the unpaid balance goes down. The only way to increase acquisition debt is to borrow money to make capital improvements.
Prior to the new law, homeowners could additionally borrow up to $100,000 of home equity debt for any purpose and deduct the interest when itemizing deductions. Mortgage interest on home equity debt is no longer deductible unless it is for capital improvements.
Acquisition debt cannot be increased by refinancing. Some confusion occurs because mortgage lenders are concerned in making home loans that will be repaid according the terms of the note and using the home as collateral. That does not include making a tax-deductible mortgage.
Another thing that adds confusion to the issue is that the lenders will annually report how much interest was paid in a year but only the amount that is attributable to acquisition debt is deductible.
Even if the interest on the cash-out refinance is not deductible, it may be advantageous to pay off higher interest debt such as credit card debt and replacing it with lower mortgage debt.
It is the responsibility of the taxpayer to know what part of their mortgage debt is deductible. The challenge becomes more difficult after a cash-out refinance. Homeowners should keep records of all financing and capital improvements and consult with their tax professional.