NOTE re the last response (he said what I would have)
Had a mother/daughter team. they had spoken with the maker and no joy.. I explained just wahat Schlep said and just like that they were in Hot Water (A good thing in this case)
They were unable to remove the valve for replacement (That tight)
This is a major issue with the single bypass valve systems.
What is the purpose for this check valve?
When you have a single valve bypass system.. Well without the check valve water will "Back fill" the water heater. NOT a good thing in cold weather.
With it that does not happen.
With the 3 valve system I had. You don't need the check valve. Thus I had none.
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