the IRS has reg. 1.163-8T and 10T that basically say you trace the use of loan proceeds to initially determine if interest may be deductible. Then there are other rules that determine if the interest is fact deductible.
Generally interest on HELOC is no longer deductible (as mortgage interest) for years after 2018 EXCEPT when its used to buy, build or substantially improve the taxpayer's home that secures the loan.. if it does, the loan becomes what the IRS calls acquisition indebtedness. Then as long as acquisition indebtedness doesn't exceed $750,000 (1/2 that if MFS) the interest is deductible.
so if you had a $750,000 mortgage on your home before the HELOC, the interest on the HELOC would not be deductible
if you were to refinance the mortgage/ heloc there are other rules that would apply to determine deductibility.
