The Tax Cuts and Jobs Act of 2017, enacted Dec. 22, suspends from 2018 until 2026 the deduction for interest paid on home equity loans and lines of credit, unless they are used to buy, build or substantially improve the taxpayer’s home that secures the loan.
Since the rental property is not the property the secures the HELOC/Loan, the interest on the portion used to pay off the rental is not deductible.
https://www.irs.gov/newsroom/interest-on-home-equity-loans-often-still-deductible-under-new-law
