There are 3 different water heater winterization systems
1 2 and 3 valve
1 always has a check valve on the outlet. This is because the Bypass valve is a 3-way it either sends cold into the tank at the bottom or it sends it up to a "T" in the hot line and you need a check valve to keep it from flowing in the out port on the water heater.
2 and 3 valve systems do not need a check vavle.. but... well... if we install a 2 dollar valve we can add 10 bucks to the selling price so....
And that valve can "lime up" Good luck removing it if that's the problem
note you do not need it if you do not have water in your lines in the winter.
Or you can install a quarter turn valve and make your one valve into a two valve.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times