I am aware that I would need to disconnect and blank off the connections to the F&E tank, and install an expansion vessel, pressure relief valve and filling loop with pressure gauge. From what I can see, complete kits can be obtained for 70 or 80 quid.
The question is where in the system to install these items.The boiler and pumps are in an integral garage, surrounded by heavy workbenches, tool chests and shelves - making them very difficult to get to without moving lots of stuff. The F&E tank is in the attic above the airing cupboard, and connects into the flow and return of the HW circuit.
The airing cupboard would be by far the easiest place to install the kit. There is ready access to mains water for the filling loop. Because the secondary HW system is unvented, there is a tundish into which anything coming out of the hot cylinder's temperature and pressure relief valve would spill - so that could also accept the output from the primary circuit's PRV. There is an uninterrupted connection to the boiler - albeit in 15mm pipe, and probably 3 metres vertical and 3 metres horizontal away.
The boiler's installation manual says to put the the PRV on the flow pipe, close to the boiler and before the pump inlet. But that seems to assume a single pump on the flow side, prior to any zone valves, etc. But this system ain't like that! There are two pumps - one for CH and one for HW - both on the return side, feeding *into* the boiler.
Are there any technical - or even legal - reasons for not installing all the kit in the airing cupboard - some way from the boiler, but directly connected to it? If so, could I put the PRV near to the boiler, and the expansion vessel and filling loop somewhere else (preferably in the airing cupboard)?
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