RV Black Tank Flush Not Working - Water Wont Go In

The Quick Answer

Your black tank flush system likely has a stuck check valve or debris blockage at the flush connection. The most common culprit is a spring-loaded check valve that's either stuck closed from mineral deposits or has debris preventing it from opening. This is a very common issue on RVs that sit unused for extended periods, especially in areas with hard water.

What's Causing This

Your black tank flush system uses a one-way check valve to prevent sewage from backing up into the fresh water supply - a critical safety feature. Always ensure your RV's fresh water system has proper backflow prevention devices, and never connect the black tank flush directly to municipal water supplies or potable water sources without appropriate backflow preventers. When you connect a hose and turn on the water, the pressure should open this valve automatically. However, several things can prevent this from happening.

The most likely cause is mineral buildup from hard water that has caused the valve spring mechanism to stick in the closed position. Calcium and lime deposits are notorious for jamming these valves, especially if the RV has been sitting unused. Another possibility is that toilet paper, waste debris, or even ice (if you've had freezing temperatures) has lodged against the valve seat, preventing it from opening properly.

This problem often develops gradually, which points to buildup or debris rather than mechanical failure of the valve itself.

Tools You'll Need

• Potable water hose (not regular garden hose) with adjustable spray nozzle - dedicate this hose exclusively to black tank use after first contact
• Needle-nose pliers or small pick tool
• White vinegar (2-3 cups)
• Small funnel
• Flashlight or headlamp
• Rubber gloves
• Safety glasses or goggles
• Face mask or respirator
• CLR (Calcium, Lime & Rust remover) or similar descaling product
• Small wire brush or old toothbrush

Step-by-Step Fix

1. Empty the black tank: Before beginning any work, ensure your black tank is completely empty and flushed. This is critical for safety when working with sewage system components.
2. Inspect the flush port: Disconnect your hose and examine the flush port connection. Look inside the threaded fitting with a flashlight - you should see the check valve mechanism, which typically looks like a small spring-loaded disc or ball.
3. Test the valve manually: Try gently pressing on the visible valve mechanism with a small pick or needle-nose pliers to check if it moves freely. If it appears unstuck, reconnect your hose and test with water pressure. If water starts flowing, the valve was simply stuck and this may solve your problem immediately.
4. Remove visible debris: If step 2 doesn't work, remove any visible debris around the valve seat using your pick tool. Be careful not to damage the rubber sealing surfaces.
5. Vinegar treatment: Pour about 2 cups of white vinegar through the flush port opening, allowing it to sit for 30 minutes to dissolve mineral buildup. The vinegar will pool at the valve and work on any calcium deposits.
6. Retry with water pressure: After the vinegar treatment, reconnect your hose and try the flush again. If it's still blocked, try applying steady water pressure while gently working the valve mechanism with your tool.
7. Use descaling product: For stubborn cases, use CLR or a commercial descaling product following the manufacturer's instructions. These are more aggressive than vinegar and often break through heavy mineral buildup.
8. Flush thoroughly: Once water flow is restored, run the flush for several minutes to clear out loosened debris and ensure the valve is operating smoothly.

When to Call a Pro

If these steps don't restore water flow to your black tank flush, you may be dealing with a completely failed check valve that needs replacement. This typically requires removing the external flush fitting and possibly accessing plumbing connections inside your RV's underbelly or bathroom area.

Call a professional if you discover that the valve mechanism is physically broken, if there are signs of water damage around the connection suggesting internal leaks, or if you're not comfortable working with the sewage system components. A mobile RV repair technician can typically replace a black tank flush valve for $150-250 including parts and labor.

Also seek professional help if you notice sewage odors coming from the flush port area, which could indicate a compromised valve that's allowing tank gases to escape through the water connection - a serious health and safety concern that needs immediate attention.