A lot of HELOC agreements carry a conversion feature. You pick a portion of the outstanding balance, the lender locks it at a fixed rate, and that portion gets its own amortized payment over a set term, often anywhere from five to twenty years. The rest of the line of credit keeps working the way it did. Some lenders charge a small fee per lock, and most limit how many you can have open at once.
Interest on the revolving side is figured on the average daily balance, so every paycheck you park lowers tomorrow's interest. The locked slice doesn't see any of that. Its balance drops only by the scheduled principal in each fixed payment, and money you deposit into the line of credit lands on the revolving side, never on the lock. So the sweep stops touching that piece of the debt. You've built a small second mortgage inside the line of credit, on purpose.
That's not automatically wrong. There's a version where it's the right call: you moved a large chunk to the mortgage, you know the payback will take three or four years instead of one, and a couple of rate increases would push the interest-only floor past what the budget can hold. Locking that chunk trades daily-interest efficiency for a payment that can't move. If the alternative is an income that can't absorb a rate jump, certainty wins.
The place it goes wrong is locking money you'd have cleared in a year anyway. A $12,000 chunk you'll pay back in ten months gets no benefit from a fixed rate, and it loses the sweep, so you pay more interest for less flexibility. I wouldn't lock anything with a payback under eighteen months.
Read the terms for undoing a lock before you use one. Most let you prepay the locked portion without a penalty, and the paid-off amount usually goes back into your available credit, but check whether prepayment closes the lock entirely or shortens it. And the locked rate is often set a little above the variable rate on the day you lock, so the certainty has a price from day one.
One more use: the end of the draw period. If your line of credit is a couple of years from converting to repayment and you're still carrying a big balance, locking part of it while you still control the terms can beat whatever the lender's default repayment schedule turns out to be. That transition is laid out in draw period vs. repayment period.
Before you lock anything, run the two payments side by side: the interest-only floor on the revolving side at a rate two points higher than today, against the fixed payment on the lock. Lock only the amount that makes the first number unsafe.