In addition to flushing it out answer this question: How many valves to bypass? One, Two or Three?
IF the answer is ONE: THEN you defiantly have a check valve on the hot water outlet. (if the answer is two or three you may still have one but it is not needed)
THIS valve gets gummed up with mineral sediment during normal operation and sticks closed.. TWO solutions, depending on how easy it is to remove.
1: remove it and replace with a quarter turn valve (or replace it straight up)
2: Gut the thing and ADD a quarter turn valve..
NOTE: the valve is NOT needed if you winterize by the dry method so gutting and forget the quarter turn works in that case, but if you do that you should "Tag" the unit some way to make sure a future owner knows there is no check valve.
Home is where I park it!