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(Photo courtesy of the Go-RV catalog, Page 350)
They are threaded directly into the back of the water heater, one on the inlet side and the other on the outlet side. Their sole purpose in life, other than to allow me to hone my plumbing skills, is to prevent water from going the wrong direction. Why, you ask. Good question. My take on it is, the one on the inlet side serves the same purpose as the one on our home water heaters; to prevent water from draining out of the water heater if there is a loss of pressure on the inlet side, and causing the heating element to fry (which is not a good thing). The one on the outlet side is to prevent antifreeze from entering the water heater when the bypass valve is engaged during winterization, when antifreeze is being force-fed into the system.
Replacement of the water heater valves is fairly simple. First, turn off the city water feed, or the pump. To prevent a big mess, it is best to drain the water out of the system, though not necessary for the outlet side, since it is on the high side of the tank. A few towels thrown in the floor should catch the drips, especially if you are quick. Remove the plastic water fitting from the back of the valve, and then using wrench, unscrew the valve from the tank. For an emergency fix, simply remove the plunger from the valve and discard ceremoniously in the campfire. The valve will simply work as a pipe nipple (I know, that sounds a litte risque), but will get you though the camping trip. If you happen to carry a spare valve, then you really came prepared, and install that one. When replacing the valve, sans plunger, or with a new valve, a little teflon tape or plumbers dope would be a good idea.
As noted in another post, when the spring fails in these cheap valves, they slam shut, preventing any hot water flow. Both of my failures have been on the outlet side. It may have something to do with the fact they are exposed to the hot water flow, but the inlet side only sees cold. Still, the spring shouldn't corrode that fast. Since Atwood tanks of my vintage do not have sacrificial anode rods, maybe the spring has become the anode rod.
Who knows.
