Plumbing Systems Compatibility Guide Standard System with stored water (15mm pipework): No Use WS355 Combination Boiler: Yes Unvented/Pressurised system / Megaflow: Yes This new release from BWT replaces the MC350 Water Softener. BWT WS355 Water Softener - Delivering Irresistibly soft silky luxury water. Now Includes Free Commissioning What does the Free Commissioning Involve This new free service will see a commissioning engineer taking on the customer service element of the new installation. The engineer's visit can be booked with the BWT commissioning team by either the installer or the customer directly. During the visit, the engineer will check the softener is set up in line with the homeowner's requirements, that it is operating efficiently, take the householder through.