He’s gonna have kittens no matter how i word this so …
NEVER block off a dripping valve on a hot water system. Please get a plumber to check it out. Yes I know he said “Drain valve” but with out seeing it to be sure that is exactly the part we are talking about.
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or will be taken. And yes, I am a licensed Plumber and Drainer with my own business in Brisbane Australia