Brian, that was actually how it was here when we moved in back in late 1999. The whole house was one zone. When two months later there was a leak requiring totally gutting the powder room off the kitchen, we had a manifold put in to separate the kitchen and then-two baths into separate zones so each could be cut off independently. The addition in 2008 became a fourth zone, although it was impractical to provide easy-access cut-offs for the two baths and laundry on that side of the house due to distance and it all being crawl space.
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The most expensive tool is the one you buy "cheaply" and often...