@Tony Sessa
I'm actually using it for repairs/upgrades on the property. I didn't explicitly use it for the down payment because I went a little more creative for basically no money down (I still am making these repairs, so I'm putting money in). But I created a win-win-win-win situation that let me do it that way and this is the wrong post to get into that.
However, in your situation, once you get the HELOC on your personal residence, you can use that money just like a checking account. Some places actually even have checks. But no matter where you use the down payment money from, any bank is going to want to know where you got the money, so just be sure to be honest and disclose that info to them. The HELOC does tend to be higher interest, and it's a variable rate loan. So even if you get a low rate now, it will most definitely go up at some point, so you need to calculate your cash flows based on worst case scenario. That worst case scenario is the max interest rate that you can pay and should be spelled out in your HELOC agreement. I would suggest being careful with getting as low of a down payment as you can find if you're using this method. Your overall payment is lower based on interest, but still check the difference in your cash flows because you may have to factor in PMI. For example, I'm looking at a deal right now where if I pay a bank loan at 4% and finance the down payment at 6%, the difference between putting 20% down and 15% down is only like $1/month in cash flow, but the PMI on the 15% down loan would cut into cash flows significantly. It could cut a $100 cash flow in half or less pretty easily.