Does a Low Point Drain Empty an RV Water Heater?

Why Low Point Drains Leave Water Trapped in Your Water Heater

Opening the low point drain lines (drain lines hanging beneath the RV frame) is usually the first step people take when prepping their rig for cold weather. You crawl under the camper, unscrew the little plastic caps from the red and blue plastic pipes, and watch several quarts of water pour out onto the gravel. It feels like the entire plumbing system is emptying itself. But inside your water heater, almost every single drop of water stays right where it started.

To understand why this happens, picture how water behaves inside a sealed container. When you submerge a drinking straw in a glass of water, cap your thumb over the top opening, and lift it out, the liquid stays trapped inside the straw. Atmospheric pressure beneath the open end holds the water up because air cannot enter from the top to break the vacuum. Your RV water heater works the exact same way. When the tank is sealed and your faucets are shut, water cannot freely flow backward through the cold water line because air cannot enter the top of the tank to displace it.

There is also a physical mechanical barrier inside modern RV plumbing. Most travel trailers and fifth wheels built in the last two decades have a check valve (a one-way valve that stops water from flowing backward) screwed directly into the hot water outlet at the back of the tank. This small brass or plastic valve has an internal spring-loaded flap. It allows hot water to push outward toward your sinks and shower whenever you run the water pump. But the moment water tries to travel backward from the tank toward the low point drains, the internal flap snaps shut against its seat. The plumbing lines between the tank and the low point drains will empty, but the check valve seals the water heater tank shut.

The height of the plumbing lines matters as well. Low point drains are designed to clear the fresh water pipes running through the floor channels and cabinet bases. The water heater tank sits higher up on the floor deck or inside an exterior compartment. Its cold inlet sits an inch or two above the actual steel bottom of the tank. Even if your rig does not have a functional check valve, gravity can only pull water down to the level of that inlet pipe. Several inches of water, amounting to several gallons, remain trapped at the base of the tank. Relying on underbelly drain caps leaves your water heater nearly full every single time.

The Costly Freeze Risk of Trapped Water Heater Moisture

Leaving water trapped in an RV water heater over the winter is one of the most common and expensive mistakes an owner can make. Water has a unique physical property that sets it apart from almost every other liquid on earth. As water drops below 32 degrees Fahrenheit and transitions into solid ice, it expands by roughly 9 percent in volume. In an open bucket, ice simply rises higher toward the rim. Inside a rigid, closed metal tank, that expanding ice exerts thousands of pounds of pressure per square inch against the walls.

RV water heater tanks are made from either welded aluminum or heavy steel lined with porcelain enamel. Neither material can stretch to accommodate expanding ice. As the trapped water freezes into a solid block, it bulges the metal tank heads outward. It tears the welded seams apart along the base and fractures the metal around the heating element socket. In glass-lined steel tanks, the internal porcelain lining cracks and flakes away in large sheets, exposing raw steel to rapid rust and corrosion.

A cracked water heater tank cannot be patched with epoxy, and welding an aluminum tank rarely holds up against household water pressure. Once the tank body splits or deforms, the entire water heater must be removed and replaced. Buying a replacement water heater typically costs between $600 and $1,200 for the unit alone. If you hire a mobile technician or visit an RV repair shop, labor to disconnect gas lines, wire electrical controls, and seal the exterior wall flange easily pushes the total repair bill past $1,500.

The damage often extends beyond the heater itself. Many owners do not realize their water heater cracked during the winter until they hook up a garden hose at a campground the following spring. When you pressurize the system, dozens of gallons of water spray out of the split tank seam inside the RV cabin. Water soaks through wood paneling, rots subflooring, and ruins electrical wiring behind the cabinets before anyone notices the puddle on the floor. If you camp late in the season, running a reliable heater to stay warm off-grid keeps your living quarters comfortable, but cabin heat does not protect exterior plumbing components. The water heater tank sits against the outer sidewall behind an uninsulated metal hatch, meaning freezing overnight air will freeze the tank regardless of how warm you keep the camper interior.

How to Drain an RV Water Heater Completely: Step-by-Step

Draining an RV water heater is a simple job that requires only five to ten minutes and a standard socket wrench. You do not need plumbing experience, but you must follow the correct order of operations to avoid scalding yourself or getting soaked by high-pressure water. Follow these steps to empty the tank safely and completely.

1. Shut off all power to the water heater. Turn off the 120-volt electric element switch on your interior control monitor or on the face of the water heater itself. Switch off the LP gas water heater toggle switch inside the cabin. Never drain a tank while the heating elements are active. If an electric heating element runs without water surrounding it, the metal burns out in less than thirty seconds.
2. Allow the water to cool completely. RV water heaters heat water to between 130 and 140 degrees Fahrenheit, which is hot enough to cause severe burns in seconds. Wait several hours after turning off the switches, or turn on the kitchen hot water faucet and let it run until the water flows cool to the touch.
3. Relieve the plumbing system pressure. Turn off your 12-volt fresh water pump. Disconnect your city water hose from the exterior inlet of the RV. Open any hot water faucet inside the camper to let residual line pressure escape, then turn the faucet back off.
4. Open the exterior water heater access panel. Walk around the outside of your RV and find the vented metal door that covers the water heater. Twist the latch at the top, swing the door downward, and set it aside or let it hang safely.
5. Open the pressure relief valve. Near the top of the tank, locate the pressure relief valve (a safety lever that lets trapped air and pressure escape). Pull the small metal lever straight outward until it snaps into the open position. A small burst of water may spit out, which is completely normal. Leaving this valve flipped open breaks the vacuum seal so air can enter the top of the tank as water drains out the bottom.
6. Identify your drain plug or anode rod. Look at the bottom center of the heater compartment. If you have an Atwood or Dometic water heater, you will see a white plastic nylon drain plug that usually takes a 15/16-inch socket. If you have a Suburban water heater, you will see a heavy steel hex head that takes a 1-1/16-inch socket. That steel plug is attached to an anode rod (a metal rod that prevents tank rust and doubles as a drain plug).
7. Unscrew the plug and let the water flow. Fit your socket wrench onto the plug and turn it counterclockwise. As the threads reach the end, stand to the side rather than kneeling directly in front of the opening. Pull the plug free and let the water gush straight out onto the ground. Leave the compartment open until water stops dripping from the drain port.

Setting the Bypass Valves and Flushing Tank Sediment

Once the water stops draining from the tank, your water heater is safe from freezing, but your plumbing chores are not quite finished. If you plan to pump non-toxic pink RV antifreeze through your pipes for winter storage, you must isolate the empty tank first. You also have an ideal opportunity to clean out the mineral grit that settles inside the tank over months of road trips.

Isolating the tank requires setting your bypass valve (a turn valve that routes water around the tank). Water heater bypass systems prevent you from accidentally pumping six to ten gallons of expensive antifreeze into an empty water heater tank. RV antifreeze cannot harm the tank metal, but filling the vessel wastes gallons of fluid and leaves an unpleasant chemical smell that takes hours to rinse out in the spring. You can access these valves inside your camper, usually located directly behind the water heater tank beneath a false cabinet floor, under a dinette seat, or behind a removable plywood panel held by screws.

Most campers feature one of three bypass valve setups. A three-valve system has an inlet valve, an outlet valve, and a center crossover valve. To bypass the tank, turn the inlet and outlet handles so they point across the pipes (closed), and turn the center crossover valve so its handle runs parallel with the pipe (open). A two-valve system uses three-way valves on the inlet and outlet; turn both valve handles perpendicular to the lines entering the tank. A modern single-valve system uses a single turn handle located on the cold inlet line; turn the handle 90 degrees to route incoming water up through the bypass loop instead of into the heater.

Before closing the exterior hatch for the winter season, take five minutes to flush out mineral sludge. Natural minerals like calcium and lime settle out of heated water and form a layer of white, chalky flakes at the bottom of the vessel. If you leave this debris alone, it hardens into a dense scale that coats the electric element and clogs your sink aerators next year. Follow this short maintenance checklist before putting your camper away:

• Attach a plastic RV tank cleaning wand with an angled spray nozzle to your standard garden hose.
• Slide the nozzle through the open drain plug hole and push it all the way to the back wall of the water heater tank.
• Turn on the garden hose water and move the wand back and forth in a sweeping motion to blast mineral buildup loose.
• Watch the drain hole until the white chalky slurry stops flowing and the rinse water runs crystal clear.
• Check the condition of your anode rod if your heater uses one. If the sacrificial magnesium or aluminum metal is chewed down by more than 75 percent, or if the thin center steel wire is exposed, discard it and purchase a fresh rod before 2026 camping begins.
• Leave the drain plug removed or thread it in loosely by two turns so air can circulate through the empty tank during winter storage.

FAQs

Can I use compressed air through the city water inlet to blow out the water heater tank?

Blowing compressed air through the city water connection clears the small water pipes, but it will not empty your water heater tank. The incoming air stays at the very top of the tank and escapes directly through the hot water outlet pipe, leaving several gallons of water sitting undisturbed at the bottom. You must remove the drain plug or anode rod on the exterior face of the heater to drain the vessel completely.

Should I put RV antifreeze directly into my water heater?

You should never pump pink RV antifreeze into your water heater tank. Antifreeze leaves behind a stubborn chemical taste and foaming residue that requires multiple tank rinses to clear out before you can use the hot water system again. Always turn the bypass valves behind the heater to route plumbing lines around the tank, then leave the empty water heater dry all winter.

Is it safe to leave the water heater drain plug out all winter?

Leaving the drain plug or anode rod out through the winter is safe and helps residual moisture evaporate completely from the tank. Place the removed plug in a small zip-top bag and store it inside the exterior water heater compartment so you do not misplace it. You can also place a small piece of wire mesh over the open drain hole to keep spiders and wasps from building nests inside the threads.