Assuming your appliances will need replaced every 15 years is great for budgeting, but actually setting that money aside in a low-earning account for theirr eventual replacement seems suboptimal to me.
HA. I don't think you're wrong but in the last 2 years I've replaced: Patio door, HVAC, hot water heater, washer/dryer, and a roof! Now as I replace things I don't buy the cheapest and hopefully better installed than original. Still coming up though are 6 windows, front door frame apparently has warped, chimney needs repairs, oven doesn't heat to right temp anymore, and the range top has a massive crack in it. All this on a house that was built in 2006.
Basically, even though I bought a "nice" house, it seems the people who build them do so like they're throwing together a cheap starter home. *Example: So far I've found 2 windows that were only held in with caulking.
Anyhow, thanks for the other responses. I of course am not expecting to make a bunch of money, just wasn't sure if anyone had a way that's worked for them. Really I already set aside money in Betterment for escrow and "emergency" but usually just pay from my bonus (comes right at tax time) or checking account. So it really just becomes indirect savings of my surplus income at this point.