No hot water flow, stopped up ?

The ONLY reason a W/H would require a check valve is if it is part of the Bypass system, that uses a handle and not the 3 valves behind the Water Heater. YEARS ago, RV's built in California required the check valve due to state plumbing codes. Once California dropped this requirement for RV's they stopped installing them. The problem with Check Valves that are NOT part of the Bypass system is, the usually had a squealing/humming noise when water flowed thru them. National Motor homes(built in California) were the brands we sold and serviced. We would remove the check valves when customers complained about the noise. National were the ones to inform us about the California code that required that check valve. But since our customers were not in California, we could remove the check valve. I always use and tell people to use the BRASS CHECK VALVE when replacing a faulty check valve. In this case, if you HAVE a single lever Bypass valve, Odds are you need to have that valve installed to make the Bypass system work correctly. Doug