Understanding how your RV's plumbing system works makes maintenance easier, troubleshooting faster, and repairs less mysterious. Here's a complete overview of the three water systems in your RV and how they interact.

The Three Water Systems

Every RV has three separate water systems: fresh water, gray water, and black water. They're completely separate — water only flows one direction through each system.

Fresh Water System

The fresh water system delivers clean water to your faucets, shower, toilet, and appliances. It has two operating modes:

City water mode: You connect a hose from a campground pedestal to your RV's city water inlet. The campground's pressure (typically 40–80 PSI) pushes water through your lines. Your water pump is bypassed.

Tank mode: Water stored in your fresh water tank (typically 30–100 gallons) is pressurized by your 12V water pump and delivered to your fixtures. The pump cycles on and off to maintain pressure.

A pressure regulator at the city water inlet is essential — campground pressure can exceed 100 PSI, which can burst RV water lines rated for 45–60 PSI.

Gray Water System

Gray water is the wastewater from your sinks and shower. It drains by gravity into the gray water holding tank (typically 30–50 gallons). The gray tank is emptied at a dump station by opening the gray valve on your sewer connection.

Always dump the black tank before the gray tank — the gray water helps flush the sewer hose.

Black Water System

Black water is toilet waste. It drains into the black water holding tank (typically 30–40 gallons). Black tanks require more careful management than gray tanks:

• Always keep some water in the tank — a dry tank allows waste to pyramid and cake on the walls
• Use plenty of water when flushing
• Use a tank treatment to control odors and break down solids
• Never leave the black tank valve open at a full-hookup site — this causes the liquid to drain while solids accumulate

The Water Pump

The 12V water pump is typically located under a cabinet near the fresh water tank. It's a diaphragm pump that cycles on and off to maintain system pressure (usually 35–45 PSI).

The pump should only run when a faucet is open or the system pressure drops. If your pump runs continuously or short-cycles (runs for a second, stops, runs again) with no faucets open, you have a leak somewhere in the system.

The Water Heater

Most RV water heaters are 6-gallon or 10-gallon tank-style units that run on propane, electric, or both. The water heater is plumbed in series with the fresh water system — cold water enters, hot water exits to the hot side of your faucets.

A bypass valve (or set of valves) allows you to isolate the water heater during winterization so you don't pump antifreeze through the tank.

Common Plumbing Components

Pressure regulator: Installed at the city water inlet to limit incoming pressure. Essential for protecting your lines and fittings.

Water filter: Many RVers install an inline filter at the city water inlet to improve taste and remove sediment.

Check valve: Prevents water from flowing backward in the system.

Accumulator tank: A small pressurized tank that reduces pump cycling frequency and smooths out pressure fluctuations.

Mobile Plumbing Service in North Texas

Done Right RV Repair handles all aspects of RV plumbing — from leak repair and pump replacement to tank sensor issues and winterization. Our mobile service covers North Texas and the DFW Metroplex.

Call us or request service online.