I have a walk through design and have water tank on one side and sink and hot water on the other side. So i have at least 3 drains locations for the water system. One is the drain for the tank it self, another is between the fill inlet and the tank. It is important to get this one drained in the winter, just using the tank drain will leave water there, and possibly cause problems. The third drain is on the sink water heater side. When I installed the water heater I replaced the small drain sportsmobile uses with
When trying to flush or clean the system, I generally add some bleach to a half full tank, I will then make sure I run water at the tank until I can tell that the bleached water has made it through the system, both hot and cold. Now it's time to run some errands or such, I like to drive around for about an hour total to let the solution slosh around in the system for a while. I will then drain, by running the water pump and having both hot and cold water open . I also have the tank drain and inlet drain open, since I added a larger drain when installing the water heater I generally leave that closed at this time. Make sure if you have a gray water tank that it is open (This step is important, other wise you may overflow the sink
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I will generally file the system with just fresh water and repeat 2 more times. I may also do a third rinse and this time I will open the drain by the water heater.
For winterizing I generally use air and one of the cheap adapters you can find in any RV shop. This is also the time I check the anode for the water heater, I generally replace that every two years. When that is all done I will turn the bypass valve on the hot water heater, turn off the connections for the outside faucet. If I choose to use the water in the van during the winter, I don't have top worry about these things which are know problem areas in the cold.
Just using the small drain cock for the tank I would think take awhile, it also doesn't flush the rest of your water system. My water pump is 15 years old and going strong, so I have no concerns using the pump.
-greg