Written by 2:02 pm Guides

RV Fresh Water System Diagram | Plumbing Schematic

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As you might be able to tell from the RV Plumbing Diagram below, an RV’s plumbing system has the same basic purpose as a residential plumbing system. They just have very different ways of performing their functions. 

Additionally, in a brick-and-mortar home, maintenance is minimal. You might have to fix a broken pipe. However, when you flush, you typically don’t need to think about what happens. Things aren’t so simple for an RV owner. 

RV Plumbing Diagram

Water Pump – The Heart of the Plumbing System

The water pump pressurizes the water lines, much like a heart to the circulatory system. It is located just outside of the fresh water tank, where it pulls the water through itself and into the water main.

The 12-volt water pump should be turned off when you aren’t going to be near the RV. If something goes wrong and you spring a leak, you will want to be there to limit the damage. If you are at the RV, don’t worry about it. Water pumps are on-demand devices. If your water pump doesn’t sense a lack of pressure in the pipes, it won’t try to pump the water.

An RV isn’t tied directly into the city plumbing, so a few holding tanks are required. There are three tanks in an RV; four if you count the water heater. One is for fresh water. The other two tanks store contaminated waters until you can release them into the city sewer system. The dirty water tanks are called grey water and black water, and we will talk about those further below.

Accumulator Tank – Assistant Water Pump

Some water pumps need a bit of extra help. In some systems, the water pressure stays a bit too low. To combat this, the pump rapidly cycles on and off to remedy that lack of pressure. 

Accumulator tanks are the solution to this problem. Using a pre-pressurized air bladder, these devices add the needed pressure to the lines, so the water pump will not need to cycle on again. This added pressure eases the burden on the pump and extends its life.

Accumulator Tanks can be installed anywhere within the pressurized parts of the pipelines. If you look at the RV plumbing diagram, it can be placed anywhere on the hot (orange) or cold (blue) lines. 

Water Heater

RV water heaters are powered by propane, electricity, or both. As water is pushed through the water lines, some of that water is forced into the water heater’s tank. As the water warms, it rises to the top of the tank. When a faucet or shower needs hot water, that water is pulled from the top of that tank and into the hot water lines. The emptying tank reduces the pressure of the plumbing system, which triggers the pump to refill the pipes and hot water tank with more water.

Only Run When Water Lines Are Pressurized

The water heater should never be turned on unless the plumbing is pressurized. The heating element will burn itself out without water to conduct the heat away from itself. It can take less than a minute before its copper components begin to melt.

Turn Off When Not In Use

An RV’s water heater tank is only 6 to 10 gallons, so it only takes about 5 to 10 minutes for the heating element to warm up the water. For this reason, most recommend that you turn off the water heater when it isn’t needed. It’ll save your battery power and potentially extend a boondocking trip.

Fresh Water System

As the name implies, the fresh water tank holds the drinkable (potable) water. There are typically two ways to fill it and three ways to drain it.

Ways to Fill

    • City Fill: This inlet requires a hose connected to the city water supply. The pressurized connection will pump the water through the plumbing system. Because of this continual pressurization, the RV’s water pump will not be needed.
    • Gravity Fill: These aren’t included on every RV, but they are common. You can pour the water through the inlet, and the RV’s water pump will circulate the water through the system.

Ways to Drain

    • Fresh Water Overflow: If you overfill the tank, the water will pour out of the overflow pipe at the top of the tank.
    • Water Drains: When you need to winterize the pipes, you will use these to drain the water from the tanks. The fresh, grey, and black tanks are all shaped to funnel the water to their lowest points. Those lowest points are where the drains will be located.
    • Fresh Water Main: This is the potable water that you drink from the taps


Grey and Black Tanks- The Dirty Water Tanks

When you clean the dishes, wash your hands, or take a shower, the grey tank is where the dirty water goes. The black tank holds all of the toilet water and its accompanying human waste. The grey and black tanks come equipped with vents that extend out of the roof of the RV. These vents prevent the unpleasant smells from building up and eventually entering back into the RV.

Black Tank Flush

Most of the modern RVs have an inlet to the black tank called a black tank flush. It is a port designed to flush out any filth that is refusing to slide out of the tank. Just hold a hose to the port and let the water do its work.

If your RV doesn’t have a black tank flush, it might be worth considering ordering one. They are an inexpensive purchase, easy to install, and they can make a day at the dump station a bit easier.

While you’re at it, you might want to consider purchasing a Camco product called a Swivel Stik. Rather than just pumping water into the tank, this hose attachment’s sprayer will rapidly spin 360 degrees and shoot water wherever it’s looking. The high-pressure stream can help to dislodge any cemented fecal matter.

Water Tank Monitors

There are two main types of tank monitors that you will commonly come across. 

Type One – Probe Sensors

This is the sensor type that is illustrated on the RV plumbing diagram. 

The probe-sensor type uses three or four sensors that are mounted onto the sides of a tank. Each sensor is wired to a light on a monitor, and each of those lights signifies a separate fill percentage: 25%, 50%, 75%, and Full. The sensors mounted on the tank have probes that pierce through the tank at heights that correspond with the percentage that they signify. If the sensor is connected to the “25%” light, the probe will be inserted into the tank at point 25% fill level. Those probes contain incomplete circuitry. When water levels rise and make contact with the circuitry, it completes the connection and turns on the light on the monitor.

Although clever, this is a highly inaccurate system. Many full-timers decide to replace it with the second type. It isn’t a necessary purchase, but its a great convenience to know your near-exact water tank levels at a glance.

Type Two – Ultrasonic Sensors

This monitoring system places a strip of ultrasonic sensors on the outside of the tank. It takes a bit of cutting and customization to install these sensors, but it isn’t a task too difficult for the average layman. The downside is that at $200 to $300 it can be a relatively spendy convenience upgrade. You will probably appreciate it more if you spend a lot of time boondocking in your RV.

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