Follow this strategy far enough and a question appears on its own: if parking income against a line works this well, why keep the mortgage at all? Why not make the line of credit the mortgage? That product exists. It's called a first-lien HELOC, and it replaces your mortgage entirely, putting it in first position with your whole housing balance inside it.
The appeal is real, so let's state it fairly. Every dollar of income parks directly against your largest debt from the day it arrives. There's no separate mortgage payment, no chunk transfers between accounts, no second lien. One instrument, maximum contact between your cash flow and your biggest balance. For a household with strong surplus and iron habits, it's the purest expression of everything this site teaches.
Now the bill for that purity. Your entire housing debt floats. A fixed mortgage is protection you already own, often at a rate you'll miss the day it's gone. The first-lien HELOC trades all of that for rate risk on six figures. Run the stress test at three points higher on your full balance and see how it feels. The interest-only minimum is harmless on a small parked balance. On a mortgage-sized one it's an invitation to drift. These products are also harder to find, and they often carry higher rates than a traditional mortgage. Closing one means giving up a fixed loan you can't just get back at the old terms.
The first-lien HELOC punishes spending creep harder than any setup we cover. The room is enormous, and the required payment never forces the balance down. The same discipline gap that costs a second-lien parker a few hundred dollars can cost a first-lien holder the whole strategy, with their house wrapped around the lesson.
Here's my take, hedged the way it should be. Most households get most of the benefit from the ordinary setup. A fixed mortgage, plus a modest second-lien line of credit for parking and chunking, which we compared in HELOC or Personal Line of Credit. The first-lien version isn't wrong, it's advanced, and advanced tools belong in hands that have already proven boring and consistent with the basic ones. Prove it first. The mortgage will still be replaceable later if the math and the habits both say go.