I will echo others, since I am currently in the process of closing on my first property. I found a local bank that would allow me to use my HELOC to assist with the down payment on my property. It is important that you explain your plan to your lender prior to shopping for a property. The last thing you want is for the bank to deny your funds late in the underwriting period because your DTI ratio changed without warning.
Because my HELOC's introductory rate is 1.9% interest, I drew as much as I knew I could comfortably pay off within that year's time frame based on my existing budget. Using all unspent dollars from my budget, along with the dollars I am already committing to using as investment money, I am projected to pay off the loan before it hits it's normal market rates in October. Once that happens, I should be able to repeat the process for my second property in the same year.
With that said, I intend to use my HELOC as short term funding for BRRRR deals in the future once I have enough equity position on my primary residence to cover a property in cash (this shouldn't take long in Kansas City). You can only get so many conventional loans before the banks start telling you "no".