Wednesday, December 7, 2022


Ask Dave: Why does water I add to RV’s fresh water tank drain into black tank?


Dear Dave,
My 2005 Pace Arrow motorhome with a Tecma toilet has been sitting for about 3 months. I added water to the fresh water tank and it all drained into the black water tank. Any ideas as to why? —Del, 2005 Fleetwood Pace Arrow 36D

Dear Del,
Somewhere you have a faucet or toilet that is leaking and allowing water to drain into the black water tank. Your Tecma toilet is made by Thetford and is a pretty fancy design versus the typical push-the-foot-pedal-style toilet.

Notice there is no flush pedal to add water to the bowl or open the spade valve. It is done electronically from the touch pad. It also has a tank sensor in the lower right corner. The add water button will provide about one half gallon of water to the bowl when pressed. The flush button opens the valve, adds water, and starts the macerating process. Then a small amount of water is added at the end to reduce odors.

This toilet also has two flush modes: residential and marine. How fancy is that? It is shipped in the residential mode, which leaves a greater amount of water in the bowl. There are several other features such as a bowl level indicator. That is set about 5” below the rim and will not allow you to add water so the sewage doesn’t overflow. Also there is a tank sensor so the toilet does all the thinking for you… Wow!

What could be the cause

So what could be causing the water to continue to fill the black water tank? The first thing I would check is how the toilet is operating with the add water and flush switch. Then check to see if any water continues to flow into the tank after the initial half gallon. If the water switch is not shutting off, it could be filling the bowl. Then the discharge hose for the macerating pump could be pulled down and should be positioned up to the level line of the toilet nozzle. The discharge hose is labeled E.8 in this diagram.

If the discharge hose is not the issue, you can see there are several areas inside the toilet that can be a source of leaks. They would most likely follow down the drain hose, which is E.10 on the diagram, and go to the tank. However, you should be able to hear the water pump running in these cases. This is typically a smaller pump provided by Thetford and would be much quieter.

According to the service manual, the system requires a vented loop, which is labeled E.7. If this loop is not installed properly, it could draw outside water by siphoning and fill the toilet.

If the toilet is not the cause of water going from the fresh water tank into the black water tank, I would start looking at what sinks go into the tank, as well. Depending on the location of the sink in proximity to the tanks, some could drain into the black water tank and a leaky faucet would be the culprit in this case. However, this would cycle the onboard water pump, which you should hear unless you are connected to a city water source.

 You might also enjoy this from Dave 

When connected to city water, why does gray water tank start to fill?

Dear Dave,
We are in our sixth year of full-timing in this coach. I’m completely baffled by something that’s come up. When connected to city water, 3-5 gallons of water is bleeding into the gray tank over an eight-hour period. We have three sinks and a shower with no leaking faucets. I don’t see any other connections to the gray tank. I saw something online that mentioned the fresh water fill valve bleeding, but I don’t see how this could be connected to the gray tank. —Blane, 2016 Holiday Rambler Endeavor 40DP

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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1 month ago

He could also have been hooked up to the wrong water inlet, if he has a black tank flush fitting. This would put the water directly into the black tank.

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