You likely sussed out the problem thanks to a drip, drip, drip sound coming from the bottom of your RV. “Is that … my freshwater tank?” you wondered. “Why would my tank be full?” Sure enough, the water was leaking out of the tank overflow vent. You double-checked your water connection and bypass valve orientation. Yup, everything was correct. “So …. why is my tank filling up when I’m connected to city water?”

Good question. This is a somewhat common affliction for many full-time RVers. It can have multiple causes. Let’s round up the usual suspects and see which one might be culpable in your case.

P.S. Not all tank overflow tubes exit below the subfloor. Some overflow tubes exit through a sidewall vent. Take note of which one you have!

Suspect 1: The Water Pump

It has often been said that the RV water pump has a built-in check valve. This is actually only partially true, but you get the gist: A properly functioning water pump only allows water to flow in one direction.

Unfortunately, as a pump ages, it can get leaky (no old people jokes, please). Pressurized water can bypass the pump’s internals and leak backwards into the freshwater tank. Over a few hours or days, this small leak can completely fill up a 20, 30, or 50-gallon tank!

There are three fixes:

• Either replace the water pump ($150)
• Replace the pump internals with a rebuild kit ($50)
• Or add an inline check valve (i.e. $15 backflow preventer) to the outlet of the water pump.

Any option will get the job done. The by-the-book answer is to replace the water pump, but if the water pump is “pumping” just fine, it’s easier to stomach the $15 check valve fix. 

Some over-engineered types prefer to install a shut-off valve on the outlet side of the pump instead of a check valve, but this requires manually opening/closing when switching between city water and the dry camping (freshwater tank) modes. 

(I will say that sometimes the pump gets “stuck” in this position if you accidentally turn on the pump while connected to city water. If you suspect this problem, just disconnect from city water and run your pump a few times by turning a faucet on and off. This may dislodge any temporary obstructions and stop the leak.)

>>> READ MORE: Should I Leave My RV Water Turned On When I’m Away from the Campsite?

P.S. While you’re monkeying around with the water pump, install a strainer/filter on the inlet side of the pump if you don’t already have one. Debris and sediment stirred up from the water tank can get caught in the pump internals and cause the very situation you’re facing. 

Suspect 2: Operator Error

At the risk of insulting your intelligence, did you double-check your water bay valves orientations?

Some of you may not know what I’m talking about. You see, many modern RVs use a water bay system that looks something like this:

Most of these water bay systems are manufactured by B&B Molders under their Nautilus brand. These water bay systems combine many common operations – winterization, sanitation, tank filling, blank tank rinse, etc. – into a single manifold/control panel. If you were to peer behind the facade, you would see a labyrinth of PEX tubing and valve bodies.

If you have a water bay system, then you likely have color-coded valve handles with instructions as to which orientation combinations match which operating mode. So … did you double-check your valves? Are you sure you didn’t accidentally put the water bay in PowerFill mode to fill up your tank? 

If you have an older or simpler RV, then you may or may not have a dedicated tank fill valve. If so, check it. The same rules apply as above. 

Suspect 3: A Faulty Valve

If all your valves are turned to the correct orientation (based on the handle) but the freshwater tank is still filling up, then we must consider the possibility of a failed valve.

I’ve seen all sorts of valves fail. The most common are 1-way check valves, also known as backflow preventers. They have a simple spring-loaded mechanism inside which is easily dislodged or damaged. 

I’ve seen regular ball valves and gate valves also fail, although this is more rare, and it usually happens with ancient (30+ year-old) RVs and campers. It takes a lot to damage a brass ball valve. I usually see these leak externally rather than internally.

Selector (multi-position) valves are a bit more finicky. I’ve seen these fail as well. They are more common on older Class A coaches and 5th wheel trailers. 

Diagnosing and replacing a failed valve is beyond the scope of this article. It normally means disassembling some portion of your water supply system, and for that, you should be an amateur plumber. 

Suspect 4: High Water Pressure

Most RV water supply systems are designed to operate at 45-55 psi. I normally tell customers that anything above 65 psi is hazardous, and anything above 80 psi is begging for a leak (even though yes, RVs are supposed to be leak-tested to 80 psi minimum). 

However, the actual water pressure from a campground or house can be wildly dependent on location, time of day, your neighbors’ water consumption, etc. I’ve seen campground water pressure as high as 95 psi! That’s why I recommend a water pressure regulator to all RV owners.

If you’re allowing supercharged water pressure into your RV, it may be too powerful for the pump internals, any check valves, or any bypass valves. Water will follow the path of least resistance, and that path just may leak into your freshwater tank. You can pick one up for $25 – $50; that’s cheap insurance!

Is your RV freshwater tank filling up on city water? Now you know! If these culprits aren’t responsible for problem, leave a comment below with your solution.