My place is only 1100 square feet. The kitchen, bath, & master bath are all next to each other, so 12' is my longest run. That's why I can get away with 3/8. But in a 2 story home with the water heater in the garage, I'm sure it would be a mistake. When i get ready to plumb the whole thing, I'll hook up 50' of PEX, run it out side and see how much flow is lost.