I am getting to install all of my plumbing on my new build and i bought 20'' Synergy External Overflow box which will be controlled by a Spears 1.5inch gate valve but is it necessary to have a 1.5inch ball valve on the secondary drain?
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The second and third drains do not require gate valves, but you should put unions on all your drain plumbing to allow for easier disassembly for cleaning. You adjust the first drain to match the output of your return pump with the tiniest trickle into the secondary drain to keep your overflow silent. The second and third are run wide open so that if your first drain (and for that matter second drain) is clogged they can (noisily) handle the flow from your return pump. This alerts you that something is wrong with your first drain (via drain noise) while preventing a soggy floor.