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On an UVC always, but, you can get away without one on primary circuits as it’s just the volume of expansion to get rid of. BCO’s never chase on that but are all over D1 & D2 on potable as it’s drinking quality water being ‘lost’. There are several waterless traps ( sealed ) which are transparent, so you can use one of those if a visible discharge is required internally. Where it runs to an outside wall you can rely on that as means of visible discharge.

I liberally smear compression fittings with Jet Blue Plus. They do up better, smoother and with none of that nasty brass on brass squeak. I don't use PTFE anymore. No weeping and going green and crusty after a few years either. Just my humble opinion.

It was sadly such an incident that resulted in a child in bed, getting drenched with scalding water when the tank did burst, that led to the introduction of the safety cut outs.