Really? Well That's news to me! Also, news to the Health and Safety Executive.
The definitive document on recognized good practice is the 'Legionnaires' Disease, The control of Legionella bacteria in water systems, Approved Code of Practice and Guidance, L8'. Nowhere in that document does it say the unvented systems are safe because the water is not exposed to air. In fact, on P 35 it mentions 3 types of water system, of which the mains pressurized system is one. The same precautions are required as for any other system.
Tell me where it says the water must be exposed to air. I have my copy before me. Tell me of any document which says legionella requires air exposure.
Well, blow me down. More news.
See 'Essential Information for Provider of Residential Accomodation'
Read this in conjunction with L8, Page 2; "Infection with legionella can be fatal in approximately 12% of reported cases.
You really are an ignorant and dangerous fool.