Fresh-Water Tank Fills While on City Water: Check Valve and Selector Diagnosis

Canadian RV owner’s repair guide · Educational information • Updated October 23, 2026 • About 6 minute read

A fresh-water tank that rises while the RV is connected to city water may be receiving water through an intended tank-fill setting, a leaking selector or a reverse-flow path through the pump. The first step is to confirm the tank is actually gaining water. A changing gauge alone is not enough to identify a failed component.

This guide focuses on unintended tank filling during city-water use. It is different from a pump that will not prime, low pressure at fixtures or a gravity fill that spits water back. Water-system layouts vary considerably, so use the RV's plumbing diagram rather than an assumed universal valve arrangement.

Turn off the city-water source if the tank level is rising unexpectedly. Do not block the tank vent or overflow to stop water appearing outside. Stop for a swollen tank, loose supports, leakage inside the RV or uncertainty about a connection labelled for black-tank flushing. Never connect potable plumbing to a waste-flush connection as a test.

1. Confirm which inlet is connected

Read labels carefully and consult the manual. An RV may have separate gravity fill, city inlet, tank-fill selector and waste-flush connection. Some fresh tanks cannot be filled from the ordinary city connection; others are intentionally filled through a manifold ^[1]. Photograph the utility panel and selector labels before changing anything. If labels are missing or contradictory, identify the system with the manufacturer first.

2. Verify actual tank gain

Use only the level-check method provided by the RV maker, such as a safely visible tank or reliable documented indication. Note the RV's parked angle and starting level. Do not open the tank or disconnect a hose to confirm it. If water reaches an overflow, shut off the source; there is no need to keep filling for stronger proof. A useful observation is simply that the level rises with city pressure and stops when the source is off.

3. Set the normal operating configuration

Follow the utility-panel diagram for normal city-water use and record every selector position. Do not force a stiff valve or guess that a handle pointing along a hose always means the same thing. Check whether a winterization or fill arrangement was left in a storage setting. A valve whose handle moves does not necessarily have intact internal parts, but correct settings should be confirmed before internal failure is assumed.

4. Compare city-water and tank operation separately

With the city supply off and disconnected safely, observe the normal pump-fed system according to the RV manual. Record whether the pump maintains pressure, cycles unexpectedly or works normally. Then report the earlier city-water behaviour as a separate observation. Do not run the pump dry or repeatedly cycle it as an unverified attempt to reseat a valve. Pentair's troubleshooting chart lists both tank-fill selector faults and pump/check-valve faults among relevant possibilities ^[2].

5. Identify any approved isolation points

A technician may distinguish the selector and pump paths using the RV's specified isolation valves. Owners should operate only clearly identified normal controls. Do not clamp hoses, cap vents or install a temporary plug to create an isolation test. An improvised change can trap pressure or remove a necessary flow path. If the layout cannot be established from diagrams and visible plumbing, stop and request service.

A leaking internal pump valve may be serviceable with a model-matched kit, or the manufacturer may recommend pump replacement. A faulty city/tank selector needs its own repair. These are different purchases. Ask the technician to explain which path was confirmed and how, rather than accepting a new pump solely because the tank filled.

An additional inline check valve is sometimes proposed, but it is a plumbing-design change, not a universal cure. Its location, flow direction, pressure rating, winterization effect and service access must suit the RV. Have the RV or pump manufacturer approve the arrangement. Do not add a valve that conceals a damaged selector or makes the system difficult to drain.

Use the RV manufacturer's required pressure-control arrangement and suitable potable-water equipment. Do not increase pressure to force a suspected valve to seat. A regulator that reduces excessive source pressure does not establish that the reverse-flow path is repaired. If tank filling stops only at an unusually restricted setting, report that observation and continue diagnosis rather than accepting weak fixture flow as the solution.

Set the system to normal city-water operation, establish the starting tank level and observe for unintended gain over a suitable service test period. Check all repaired joints for leakage. Then test normal pump-fed use and any intended tank-fill function separately. Verify the tank vent and overflow remain unobstructed and all controls are returned to their documented positions.

Record the replaced part, the confirmed cause and photographs of the correct utility-panel settings. Keep a small labelled diagram with the RV records. That is more dependable than remembering a valve position from one camping season to the next, particularly if more than one person handles setup.

A problem first noticed after dewinterizing may involve a selector left in a different configuration or a component damaged during freezing. Restore water cautiously and inspect before leaving the RV unattended. In a remote campground, disconnect city water and use the tank system only if it is otherwise safe and the fault has been isolated appropriately. Do not keep an overflowing connection running merely to avoid changing the camping routine.

Before you book, send the component label, the RV year and model, the exact symptom and any displayed code, when it happens, photos taken from a safe position and the checks you have already completed. Mention recent repairs, freezing, impact or water exposure. Ask for a written quote in Canadian dollars that separates diagnosis, parts, labour, mobile travel charges and applicable taxes. The final cost depends on the confirmed fault, access and parts availability, so be wary of a firm price quoted before anyone has looked at the problem.

For this repair: Ask whether the shop will prove the backflow path before replacing parts. Include selector valves, inlet check assembly, pump service kits and sanitation where required.

These are original manufacturer or component-maker resources to help you find the instructions for your exact equipment. A manual for a named model is an example, not authority for every RV. Match the manufacturer, model and serial number before applying any procedure, and follow the matching manual where its details differ.

1. Jayco: 2017 Jay Flight Owner’s Manual, Plumbing SystemExample of separate gravity and city-water functions; not instructions for every RV.
2. Pentair Shurflo: Pump Troubleshooting GuideLists selector, pump-valve and check-valve possibilities for tank filling on city water.
3. Forest River owner manuals: actual RV plumbing layout
4. Forest River component manuals, including tank-flush and water-management equipment

Reference links were selected on October 23, 2026. Manufacturers move and rename documents; if a link has moved, search the maker’s support site for your model number.