I am hoping someone with professional or personal experience in this area might have some insight. We are in contract on a triplex investment property and are planning on using HELOC funds from our former primary residence (now a rental) for the down payment. Will the interest we pay on the HELOC be deductible as a rental expense against the new investment property rental business? To possibly make matters more complicated, one of the units in the triplex will be a long-term rental, and two of the units will be for a short-term rental business (Airbnb). Thanks!

Originally posted by @Liza Henty-Clark :

I am hoping someone with professional or personal experience in this area might have some insight. We are in contract on a triplex investment property and are planning on using HELOC funds from our former primary residence (now a rental) for the down payment. Will the interest we pay on the HELOC be deductible as a rental expense against the new investment property rental business? To possibly make matters more complicated, one of the units in the triplex will be a long-term rental, and two of the units will be for a short-term rental business (Airbnb). Thanks!

Yes, the interest will be deductible. You have to trace the interest to the activity you are using the funds for. You can search here for more detail.  

Where should I search?