i am no expert but i had a HW system which was not vented i.e. it had presure releif vales with plastic caps etc. what i found out was after about a year the valves started to let water go into the overflow pipe and into the back garden, the flow gradually increased over say 4 months until i had to replace the valves at Â£40 each. what i found out was these unvented cylinders rely on a air pocked in the top of the cylinder to allow for the expansion of the water. over time this air pocket bleads away until the cushion action is gone and the valves leak. what you need to do is drain the hot water tank one a year to enable air to get in and form the bubble! if the valves are leaking already they will need to be replaces as the water cuts a path in the valves seat.
hope this helps
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!