Question about maintenance plans. We've been getting mailers offering yearly service agreements and I can't tell if they're a genuine value or just a way to lock you into a subscription. Our system is about six years old and hasn't given us any trouble. Do these plans actually catch things early or is it mostly someone glancing at the unit for fifteen minutes and calling it a tune-up? Interested in real experience rather than the sales pitch. Our unit is a fairly standard setup, nothing exotic, and we do change filters on schedule. Mostly I want to know whether these plans pay for themselves or whether I'd be better off just banking the money for an eventual repair.