Zone valves. More than once. Our system is a year or two older than yours, a Baxi. The plumber who recommended it and installed it didn't offer a maintenance plan and then vanished once he was done and paid, leaving a system that had been installed incorrectly and we had to get other plumbers in to correct his work. Then we had to hunt down a plumber who would work on Baxi units specifically, because that brand isn't in wide use in the U.S. So we got off to a bad start, and our issues aren't all down to the hard water, but as first-time owners in Cambridge, it presented an extra learning curve.