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Ask Dave: Why does the fresh water tank overfill when hooked to city water?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses an RV’s fresh water tank

Fresh water tank overfills on city water hookup

Hi Dave,
I have a 2020 Fox Mountain from Northwood. Strangest thing – When hooked up to city water in the summer I sometimes get water filling up my fresh water tank, to the point where it actually has so much pressure it pushes back out where you put the hose in to fill it up. I actually wouldn’t have noticed if it had not come trickling back out. I have noticed though that it only happens when my outside shower nozzle is hooked up. In this rig the outside shower nozzle is like a compressed air hookup. Any ideas? Thanks so much. —Andrew

Hi Andrew,
It could be one of two things. First, I would check the water pump check valve. On the pump, there is a valve that should allow flow through the pump from the fresh water tank and not allow flow from the city water line. It is common for this valve to get plugged with plastic shavings and other building materials and not allow the valve to close completely. It can also get scaled with lime and sediment from hard water. I would suggest taking the pump out and cleaning all the screens and valves connected to it.

It’s good to use a pressure regulator

Another issue could be high pressure from your campground source. It’s always good to use a pressure regulator such as this 40 psi inline version. There are some that you can adjust for higher pressure. However, I would not suggest going over 50 psi as the check valve might not be able to handle the higher pressure.

The next place I would look is at the city water/fresh water tank fill valve. Most rigs like yours have a valve that you can turn from city water going through the plumbing system to filling your fresh water tank with pressurized city water without the need to disconnect and use the gravity fill.

If this valve does not thoroughly shut, water will flow to the city water plumbing and the fresh water tank. The only way I know of testing this is to disconnect the water line either at the valve or inline to the tank, hook up the city water, and see if it leaks when the valve is directed to city water or, in this case, “normal flow.” As before, high pressure could cause this to leak, so make sure you use a pressure regulator.

I do not know why this only happens with the shower hose connected. Maybe the shower head has a leak that allows air to vent and high pressure water flows through a check valve. Seems odd, and hopefully someone else has run into this situation and can explain it.

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Dick and Sandy near Buffalo, NY
1 month ago

Our 2015 HR Ambassador 38DB DP has a 4 way water valve. Sounds good on paper. City Water Draw, Fresh Tank Draw, Fresh Tank Fill and Winterization. After 90 days it started to fill the Fresh Water Tank (very slowly) while in the City Water position. Under warranty I had the Four Position Valve replaced. About 3 months later, same thing under warranty. Then not only did I have them replace the water valve again, I had them install a 1/4 turn hand valve in the fresh water tank fill and draw line at their cost and labor. Sure enough the third water valve did the same as the other two but now I can isolate the fresh water tank from the rest of the system using that 1/4 turn hand valve so that the tank does not over fill while in the City Water position. Stay safe, Stay well

Paul
29 days ago

Nice brute force fix. Have you checked to see if the check valve on the pump is leaking? That check valve is there to keep water from going through the pump to the fresh water tank. My tank started filling and I immediately replaced the pump, problem solved.

Dick and Sandy near Buffalo, NY
29 days ago
Reply to  Paul

Paul, it’s the o ring seals on the four position valve and the cheap plastic four position valve housing. In the City Water position, water is leaking by the seals and the first thing the water then goes past those seals it gets to the fresh water fill line. The four position valve works and seals (in theory) in only the selected position and the other three positions are open. NOT SUBMARINE CERTIFIED. It is a very slow leak but will over flow the fresh water tank if not addressed to. The 1/4 turn hand valve solved the problem. Have replaced worn out and leaking water pumps a few times and it still tries to fill the fresh water tank in the city water position mostly because the city water even with a 40 psi restrictor, is the highest pressure going into our coach. The techs knew of the problem and the four position valve is the problem. Thanks for your reply. Stay safe, Stay well