I need to put 15mm isolating valves in the feed and return to the upstairs part of our central heating system (which has a combi boiler). The reason for doing this is to isolate the upstairs radiators/pipework to narrow down the search area for a very small leak under the floor boards (most of which are covered with laminate). Assuming I find and fix the leak, I am unlikely to need to open/close the valves in the long term but it is particularly important that they (a) don't leak for the next few years (b) don't restrict the flow. Any advice as to which type/make I should go for?
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