Originally Posted by Flybob
The back flow prevent valve is used for several reasons in Rvs. One is winterizing. By using it, you can eliminate one valve in the normal three valve winterizing set up. It also prevents the crud in the bottom of the HW tank from being sucked into the cold water line should you open the low point drains with water in the WH. It also keeps antifreeze from entering the WH. If you add the third valve( so you can isolate WH from hot water line between bypass valve and hot water outlet and drain WH before opening low point drains, you can get away without it.
My TT was pre-owned, l replaced the water heater a few months ago, the back flow preventer was on the top( hot water) outlet, made me wonder but there are many things different when it comes to TT's. It has the 2 bypass valves for hot and cold. I want be winterizing my water system, live in it full time, but when it's below 25 or so degree's l do drain the lines at the low point drains. I did take the back flow out and the spring assembly was damaged so l removed it and replaced the fitting so we could have hot water.