@John Semioli, I like @Nathan Gesner's approach as well. I actually just did the same for my primary. I replaced the old HELOC with a new and larger one, and then used some of the new HELOC for a down payment on an STR. Planning to pay it back quickly.
Why didn't I do a cashout refinance instead? Working for a large lender, I surely could have obtained a good rate. I opted for a HELOC because of the following:
(1) I could go up to 100% LTV for the HELOC, unlike a conventional cashout where the limit is 80%,
(2) my mortgage rate was already very low, so I didn't want to increase my rate for the bulk of my debt just to get a comparatively small amount of cash,
(3) I only pay interest for HELOC money that I'm using, unlike a cashout where I'm paying interest for all of the cash even if some of it isn't yet being utilized,
(4) It seemed likely that I'd be shopping for several months before landing a deal (which proved true), meaning if I took the cashout approach instead of HELOC I might pay interest on the cash for many months before putting it to use,
(5) HELOCs often don't require a full appraisal, which makes life easier (hard to keep the primary presentable with kids!), and
(6) once I payoff the HELOC my overall monthly nut for the home goes down, unlike the cashout approach where principal reduction doesn't help me lower the payment.
Hope this helps. If you end up looking to buy an STR in the Asheville area, or any other market in the southeast, I'm happy to help you think on markets, connect to an STR savvy agent, help with mortgage planning, etc.