RV Doctor – Basic RV Water System & Check Valve Placement

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July 16, 2009

Dear RV Doc,
I purchased a truck camper recently which does not have a hot water system which I am planning on installing. Could you provide me with a basic RV water system schematic? I have a good idea of the basic layout of water lines and components but I’m not sure where the check valves should be placed to prevent unwanted backflow. Any help would be appreciated. – Albert Lepore, (Rochester, NY)


Gary BunzerAlbert, I don’t have a diagram for you, but it is easily understood logic. At the back of the water heater, you’ll find a cold water inlet and a hot water outlet. Obviously, you’ll connect the existing cold water line to the cold inlet.

This is the point where you’ll also want to install a one-way check valve, a backflow preventer; directly to the cold inlet with the directional arrow pointing into the tank. This will prevent heated water from migrating out of the water heater and feeding back into the cold system. This usually isn’t an issue unless the cold water line tee’s off to another fixture very close to the back of the water heater. I have seen where the cold connection to the toilet was tee’d very close to the inlet of the water heater and every time the owner flushed the toilet, steam would rise out of the toilet bowl. Installing a check valve at the water heater inlet curtailed that immediately. Most RV manufacturers now do this at the factory. Likewise with the by-pass kit. It installs at the rear of the water heater between the cold inlet and hot outlet tubes. From the hot outlet on the water heater, simply route the water line to every faucet with a hot water valve.

Concerning check valves, typically you’ll end up with three fresh water check valves; one at the cold inlet to the water heater, one at the city water inlet and one located at the outlet of the water pump. When on city water, you’ll want to keep water from entering the pump and when operating the water pump, you want to keep the water from spewing out the city water inlet. I always recommend carrying a spare backflow check valve just in case.

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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  2. I have a 5th wheel when I hook up to city water my fresh water tank fills up how can I fix this

  3. Some RIVers leave the check valve out of the system – when the water heats and expands the extra pressure does not go out the pressure relief valve.

    Once the pressure relief valve seat sees small particles-this valve seeps forever.

    Happy Camping,
    Fred b.

  4. I have had a lot of RVs in my time and have never seen a check valve just before the water heater. I am wondering if this is right. If cold water is pumped in the heater and then heated the water pressure will increase quite a bit. If it goes too high, it will either relieve through the P&T relief or give a high pressure shot of hot water when the valve is first turned on. Hopefully the first little bit will be cool.
    Just a thought.

  5. Curt

    Generator Question: How can you tell if you have a presure or splash type of oil generator? Is it marked some where? In the the numbers? Or is just the absents of the oil filter?