Setting your RV water heater bypass valves

By Steve Savage, Mobility RV Service

One of the common service calls I receive begins with,”The water coming out of the hot water faucet is only hot for a few seconds, then it goes cold.” Only rarely does a call like this result in a service call. I want to explain what’s happening and why you don’t need to spend your money for someone like me — at least until you check a few things on your own.

First, to get any hot water at a faucet, it means you’ll need a full tank of hot water. You won’t get hot water unless the water in the water heater is hot. That means the controls need to be working. Do not replace the module board or any other parts of the water heater. The problem is not a lack of hot water in the water heater. The task is getting it through the faucet.

Most often the problem is simply that the bypass valve or the cold water and hot water valves on the back side of the tank are set incorrectly. Here is a common example: First you’ll need to look at the rear of your water heater. That is often done by removing a panel inside of your RV, but you have to be able to see the backside of your water heater somehow. It is never done by simply dropping the outside door on the water heater.

Let’s suppose you can see the back of the water heater. There is a cold water line connected at the bottom right-hand corner of the heater, and a hot water line at the top right-hand corner. Water goes in at the bottom and out at the top. Attached to those water lines you will see one of three possible combinations of water valves. They have to be set correctly or even with a full tank of hot water you will not get more than a teacupful of hot water at the faucet.

If you see three valves, turn the handle on the top and bottom valves so they’re in line with the water line. Doing that opens the valve and water will flow through the water heater. At the same time, the valve that connects the water line running between the top and bottom water lines, the “bypass” valve, must be closed. It is closed when the handle is turned across the water line to which it is attached. The only time that valve is ever open is when you are winterizing your RV. If you leave it open, you create a path for water to flow without going through the water heater, thus no hot water or lukewarm water at the faucet.

The next case is one is which you have a valve on the cold water line and a bypass valve in the middle, but there is no valve on the hot water line. What you will see instead of a valve is a brass fitting at the hot water outlet on the water heater. This is called a “back check” valve. It allows water to flow out of the top of the water heater but will not allow water to flow in at the top. The rule here for the valve positions is the same. Make sure the bottom valve is open and the bypass valve in the middle is closed.

In the last case, you have a back check valve on the top water heater outlet and a single tee-valve connecting the cold water line and bypass line. The single valve combines the functions of the cold water valve and the bypass valve. Turn the handle in line with the cold water line and the bypass is closed. Turn the handle so it points up and the bypass line is opened and the cold water line is closed. Since in this scenario the bypass valve can be located remotely, you may not find it directly on the back of the water heater, although that is far and away the most common location. Fleetwood, for example, often located it inside the bathroom vanity on some models.

Now here are some things to remember. Give your water heater about two hours before you start checking for hot water. Recovery is slower on electric than gas and you can run both at the same time without hurting anything. If you start out heating the water with one system (electric or propane), once the water is hot and the thermostat opens, you cannot check the other system. For example, if the water is hot on electric, the propane will often not fire because the thermostat has opened. The water has to cool off first, so the thermostat calls for heat before the propane system will fire.

Let’s suppose the valves are correctly open. Are there other possibilities? Yes, but provided you have hot water, anything else is much less likely. Just in case you are wondering how to tell if the water in the water heater is hot, you can do one of two things. The simplest thing to do is just put your hand on the hot water outlet on the back of the hot water heater. If the water is hot, you will have no trouble telling it as the outlet will be hot to the touch. You can also open the temperature/pressure relief valve on the outside, but be careful as that water can be very hot. If you have hot water, you have a water circulation problem, not a water heater problem. That means either a valve closed or, if you have a back check valve, the valve may be broken internally.

Lastly, you might be wondering, if the valves are incorrectly set, why do you get any hot water at the faucet, even though it is brief? Although the water is not circulating through the water heater, there is enough convected heat on the water line to heat the water in the line allowing for a brief rush at the faucet before that supply is exhausted.