Debt or furnace fund?

So, we’ve been trying to start a sinking fund for a new furnace, for whenever our 50 year old one finally bites the dust. But every time we start to get a little money in it, Murphy and another household issue comes up.

So, I just got a little raise that after taxes will amount to another $100 a month or so. Should that go towards our snowball, a furnace fund, or a little of both? We still have a long way to go on step 2 🙂