When claiming interest paid using my mortgage credit certificate, do I claim only my mortgage interest or can I also claim the HELOC loan interest?

 It depends. There’s a lot of controversy on the topic because the IRS changed the guidelines in 2018, limiting who can write off their home equity loan interest, as well as what types of home equity loan interest can be written off on income taxes

“Home equity loan interest. No matter when the indebtedness was incurred, you can no longer deduct the interest from a loan secured by your home to the extent the loan proceeds weren't used to buy, build, or substantially improve your home.”

“Home mortgage interest. You can deduct home mortgage interest on the first $750,000 ($375,000 if married filing separately) of indebtedness. However, higher limitations ($1 million ($500,000 if married filing separately)) apply if you are deducting mortgage interest from indebtedness incurred before December 16, 2017.”

IRS Pub 936 Home Mortgage Interest Deduction