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Weird plumbing story... surprise ending


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Soooo, we're learning our new-to-us 2013 Keystone Fuzion 315 toyhauler. We've had it on the road for a few weeks shaking it down, things are good. We'll we have unlabeled factory 12v switches in the garage, but that's not plumbing.

The rig has a wash hookup. We scored one of those apartment top-loader units that connect to the sink faucet. Price was right...free. And we tested it out at the house before hitting the road. Workshttps://www.rvnetwork.com/forum/6-technical-tips-and-tricks/?do=add great. But these units only have one water input which is fine if you hook it to a faucet where you control the water temp via the faucet. Simple fix. Found a 'Y' hose on eBay and it fit/worked perfectly.

So on the road we decide to do laundry. Same as the test load, wash washed fine. Cool.

The next morning went to take a shower and the shower only put out luke warm water. Hmmmm. The sink in the bathroom put out full hot water (it's right over the hot water tank) for about 30 seconds then the water went luke warm. Kitchen sink... luke warm water.

Now I jump into trouble shooting mode:

- check water heater breaker.... good

- Check power to water heater element...check

Water in tank is warm.

Now it gets interesting.

Turn on the bathroom sink faucet on hot, then turn on the shower at the same time. The sink faucet goes luke warm, the shower goes full hot.

Turn on the kitchen sink and the shower, kitchen goes luke warm, shower goes full hot.

Now I am fully puzzled.

On my way out the back door of the garage where the washer is located, I notice some water seeping from the hose connector to the washer. I wipe if off, and then turn off the hot and cold water to the washer.

I head outside to further futz with the water heater. Then I hear the wife say "hay, I have have hot water in the kitchen..... and the bathroom.... it's back to normal."

So what happened to fix it?

Apparently the 'Y' hose adapter became a flow-through to the lower pressure side of the system. If you turned on the hot water anywhere in the trailer, that side of the system is now lower in pressure than the other and the cold water 'back flowed' through the 'Y' on the washing machine, allowing the cold water to replace and mix with the hot. And by shutting off the water supply valves to the washer, back flow issue solved. Guess I could put back flow preventers in line with the hoses, but simply turning off the water spigots is simple/cheap fix.

Go figure.

Interesting RV plumbing problem, consuming a few hours, solved.


Edited by trimster
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That's actually a somewhat common situation when someone leaves the shower or outside shower shut off at the shower head with the faucets still on. The open connection between the hot and cold becomes a mixing point that warms up the cold side or cools down the hot side.

Edited by Dutch_12078
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10 hours ago, trimster said:

Apparently the 'Y' hose adapter became a flow-through to the lower pressure side of the system.

9 hours ago, Dutch_12078 said:

That's actually a somewhat common situation when someone leaves the shower or outside shower shut off at the shower head with the faucets still on.

What happens in such situations is that both the hot and the cold water share a common source from either the city connection or the water pump, but the hot water must be pushed through the water heater before it reaches the faucets and in doing so it has less pressure at the faucet than does the hot water. Since the outside shower faucet or your Y connection tie them together, the higher pressure in the cold supply causes it to mix into the hot supply to other sources. In most RVs if you open a faucet completely first to hot, then shut it and open cold there is a quite noticeably higher water pressure with the cold. 

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    I will add that the water heater probably has a check valve on the out going hot water fitting. That can reduce the pressure in the hot water line.


   You can experience the same problem with a out door shower cold and hot water valves on, but the actual shower head closed. This is mostly seen on rv’s that have a Aquahot unit in them. 

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11 hours ago, trimster said:

Just thought I'd share my experience. As I found it both weird & frustrating. 

Even though I was aware of the problem happening with the outside shower or even occasionally with the one inside, I can't say for sure that I would not have made that mistake with a washer. The time that it happened to us from the outside shower it was because the wife had used it to bathe our dog and she left it with the shower off and both valves open, then went to take a shower several hours later. Even though I had known of the issue, it still took me a while to think of what was causing the lack of how water.

If you were to ad a check valve to the hot side of your Y connection, it should prevent it ever happening again. That way there would never be backflow from the cold into the hot side. 

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