The linked video is of the outboard end of the water heater. On the inboard end, are bypass valves. These allow you to isolate the water heater from your plumbing. Then you simply drain the tank for winterizing, then pump pink antifreeze through the rest of the plumbing. If those valves aren't set right, water bypasses the heater and isn't warmed. Also, if you have any "mixing" fixtures, like indoor and outdoor shower heads, they can't be simply turned off at the head. The Valves have to be off, or hot water will mix with cold and give you lukewarm.