In order to deduct Mortgage Interest on a HELOC, starting in 2018, one of the conditions is that the loan must have been used to 'buy, build or substantially improve your home'.
If you can estimate that a certain % of the loan was used for this purpose (and would be able to document if requested by the IRS) you can claim that % of Mortgage Interest as an Itemized Deduction.
Here's more info:
https://ttlc.intuit.com/replies/5759569