HELOC to buy a STR property

Hello all, 

I currently have a question that I have been trying to wrap my brain around. Currently, My fiancé and I have our primary residence which we just started to rent out. We would like to take a HELOC out on the property to continue investing and purchase an STR. However, our conundrum is paying down the HELOC after using it for a down payment on the STR. If we went this route, in paying down the HELOC wouldn't it just eat most of the cash flow of the STR basically making it an equity and appreciation play? (which we don't really want). PLEASE let me know your thoughts or any other creative financing tips and tricks you may have.

Thanks in advance!