I can't think of a single system (which has water associated with it) that wouldn't require a legionella risk assessment. Its a process, not a document. I know what you're on about as it sounds like an extremely low risk system but, without trying to be too pedantic, you've made that decision based upon an assessment of risk have you not? ;) So - full on documented mega 30 page document? Probably not. A few lines indicating how you've reached your conclusion of low risk and how you'll keep an eye on that decision in the future - probably worthwhile.