Nearly every HELOC floats. The rate is built as an index, usually prime, plus a fixed margin, and when the index moves, your rate moves with it, often the very next statement. Any strategy built on a line of credit is a strategy built on a rate you don't control.
So before you commit, run the plan twice more. Once at your current rate plus two points, once at plus three. If the math still clears at plus three, you have a plan. If it only clears at today's rate, you have a bet on rates staying put, and the people who price these things for a living wouldn't take that bet with your confidence.
Here's the good news the stress test usually reveals: for genuine paycheck parking, the rate matters less than you'd fear. When your line balance stays modest and your surplus keeps grinding it down, a couple of extra points on a small average daily balance costs you coffee money, not the strategy. We showed the same effect from the other direction in A Cheaper Line Changes Less Than You'd Think. Rate changes hurt in proportion to the balance you carry, which is one more argument for carrying less of one.
Where a rising rate does real damage is the big, slow chunk. If you've moved a large sum onto the line and your surplus only nibbles at it, you've effectively traded fixed-rate mortgage debt for floating-rate debt in size. At plus three points on a balance like that, the strategy can flip from saving interest to losing it. That's the scenario to test before your first chunk, and Run the Numbers Before Your First Chunk walks the arithmetic.
Two practical notes. Some HELOCs offer a fixed-rate lock on a portion of the balance for a fee, useful when a chunk will take a while to clear, ask your lender if it's on the menu. And yes, rates fall too, sometimes for years at a stretch. If they do, the stress test cost you nothing but a few minutes. Plans that survive the bad case get to enjoy the good one. Plans that require the good case don't always make it there.