RV on-demand water heaters work very differently from traditional tank-style water heaters.

Instead of heating and storing several gallons of hot water, an on-demand—or tankless—water heater heats the water as you use it.

Open a hot-water faucet, the heater detects water flow, the burner starts, and water is heated as it passes through the unit.

That sounds simple, but there’s an important difference I’ve found catches a lot of RV owners:

Water flow is part of what tells the heater to operate.

If the water pressure or flow is too low, an on-demand heater may not start at all—or it may start and stop while you’re trying to use it.

There are also a few tricks to using these heaters that can make them work considerably better, especially when showering.

Quick Summary: An RV on-demand water heater heats water only when there’s sufficient water flow. If yours isn’t producing hot water, don’t immediately assume the heater has failed. Check your water supply and flow first. And when showering, try setting the heater itself to your preferred shower temperature instead of mixing large amounts of cold water at the faucet.

How an RV On-Demand Water Heater Works

A traditional RV water heater usually has a tank containing approximately 6–12 gallons of water.

The burner or electric heating element heats that stored water and maintains it at a predetermined temperature.

An on-demand system eliminates the storage tank.

When you open a hot-water faucet:

Water begins flowing through the heater.
A flow sensor detects the movement of water.
The control board determines that hot water is being requested.
The burner ignites.
Water passes through the heat exchanger.
Heated water travels to the faucet.
When you shut off the water, the burner shuts down.

That means the heater isn’t constantly keeping a tank of water hot.

It heats only what you’re using.

The Biggest Difference: Water Flow Matters

This is one of the first things I consider when an RV on-demand water heater isn’t working properly.

The heater needs enough water flow to recognize that you’re asking for hot water.

If the flow falls below the heater’s minimum requirement, the burner may not ignite.

You might experience:

No hot water
Water that starts hot and turns cold
A burner that repeatedly starts and stops
Fluctuating water temperature
An error code
A heater that works at one campground but not another

That last one is particularly important.

If your water heater worked perfectly at your previous campsite but suddenly doesn’t work properly at the next one, don’t immediately blame the heater.

Check the water supply.

Poor Campground Water Pressure Can Cause Problems

I’ve run into situations where the problem wasn’t actually the water heater.

There simply wasn’t enough water pressure or flow from the campground supply to reliably activate it.

You can check for obvious restrictions such as:

A partially closed campground spigot
A kinked drinking-water hose
A plugged inlet screen
A restricted water filter
An incorrectly adjusted or restrictive pressure regulator
Other fixtures using water at the same time

But sometimes the campground simply has poor water pressure.

Technician Tip: Use Your Fresh-Water Tank

Here’s a workaround that can be extremely useful.

If the campground has poor water pressure:

Fill your RV’s fresh-water tank and use the RV water pump when you need hot water.

The pump can provide the more consistent water pressure and flow the on-demand heater needs to operate.

If the heater suddenly works properly while running from the fresh-water tank and pump, that’s a pretty good clue that the campground water supply—not necessarily the heater—is the problem.

One of My Favourite On-Demand Water Heater Tricks

There’s another feature of on-demand systems that many RV owners don’t take full advantage of.

On many systems, you can set the desired hot-water temperature through the heater’s control or monitor panel.

That opens up a much better way to take a shower.

With a traditional water heater, you normally have very hot water coming from the hot side of the faucet. You then add cold water until the shower feels comfortable.

With an on-demand heater, try something different.

Find Your Preferred Shower Temperature

Figure out approximately what water temperature feels comfortable to you when showering.

Then, before taking your shower, set the on-demand water heater to approximately that temperature.

Now open the hot side of the shower rather than heavily mixing hot and cold water.

The heater does the temperature regulation for you.

For example, if you discover that you like showering with water around 105°F (41°C), try setting the heater close to that temperature before getting into the shower.

You may only need minor adjustment afterward.

Why This Can Work Better

There’s another advantage besides convenience.

Remember that an on-demand heater needs sufficient water flowing through it.

If you set the heater very hot and then mix a large amount of cold water at the shower valve, you’re reducing how much water needs to travel through the hot-water side.

On some systems, that can contribute to unstable operation if hot-water flow becomes too low.

Setting the heater closer to your actual desired shower temperature lets you run more water through the heater instead.

That can mean:

More consistent flow through the heater
Less fiddling with the shower controls
More stable water temperature
Less chance of dropping below the heater’s minimum flow requirement

It takes a little experimentation to find your preferred temperature, but once you know it, it’s easy to set it before showering.

Advantages of an RV On-Demand Water Heater

You Don’t Run Out of a Tank of Hot Water

A conventional RV water heater has a limited amount of heated water available.

Once you’ve used it, you have to wait for the tank to recover.

An on-demand heater can continue producing hot water as long as it has adequate:

Water
Propane or other required energy source
Electrical power for its controls
Water flow

Of course, in an RV your fresh-water and waste-tank capacities still put practical limits on those wonderfully long showers.

No Waiting for a Tank to Heat

You don’t have to heat several gallons of stored water before using it.

Once adequate flow is detected and the heater fires, it begins heating water.

There will still be a delay while the cold water already sitting in the plumbing travels to the faucet.

Adjustable Water Temperature

The ability to select a target temperature is one of the features I particularly like.

Instead of producing extremely hot water and then diluting it with cold water, you can often tell the heater approximately what temperature you actually want.

No Large Water Storage Tank

Eliminating the traditional tank can reduce the amount of space dedicated to storing hot water and removes the need to continually maintain the temperature of a tank full of water.

Disadvantages of RV On-Demand Water Heaters

They’re useful systems, but they’re not perfect.

They Depend on Water Flow

This is probably the biggest operational difference.

Too little flow can prevent the heater from firing or cause unstable temperatures.

There Can Still Be a Delay

“On demand” doesn’t mean hot water instantly appears at the faucet.

The cold water already sitting between the heater and faucet still has to leave the plumbing first.

The farther the fixture is from the heater, the more noticeable this can be.

Temperature Can Fluctuate

Changes in water flow can affect heater operation.

You may notice temperature changes when:

Someone opens another faucet
The water pump cycles
Campground pressure changes
You reduce shower flow too much
A fixture or filter restricts water flow

They Can Be More Sensitive to Scale

The heat exchanger contains relatively small water passages.

Mineral buildup can restrict those passages and reduce heat-transfer efficiency.

If you frequently camp in areas with hard water, periodic maintenance becomes especially important.

Common Reasons an RV On-Demand Water Heater Doesn’t Work

If you’re getting no hot water, start with the basics before assuming an expensive component has failed.

1. Not Enough Water Flow

Open the hot-water faucet farther.

If the heater suddenly starts, insufficient flow may have been the problem.

2. Poor Campground Water Pressure

Try using your fresh-water tank and RV water pump.

If the heater works normally afterward, investigate the external water supply.

3. Restricted Water Flow

Look for:

Dirty inlet screens
Clogged filters
Kinked hoses
Restricted faucets or showerheads
Partially closed valves
Plumbing restrictions

4. Propane Supply Problems

If your heater uses propane, make sure propane is available and the supply valves are open.

If other propane appliances are also having trouble, the problem may not be specific to the water heater.

5. Electrical Power Problems

Even propane-fired on-demand heaters generally require 12-volt power for their controls, sensors, ignition system, and gas valve.

A propane tank full of fuel won’t help if the control board doesn’t have proper electrical power.

6. Flow Sensor Problems

If there’s clearly adequate water flow but the heater doesn’t recognize the demand, the flow sensor or related components may need to be investigated.

7. Scale or Mineral Buildup

Scale can restrict water passages and interfere with heat transfer.

Follow the manufacturer’s recommended maintenance and descaling procedure for your particular heater.

What If the Water Goes Hot-Cold-Hot?

Temperature cycling is a common complaint with on-demand systems.

Before assuming there’s a control-board or burner problem, look at water flow.

Ask yourself:

Is campground pressure stable?
Is the showerhead heavily restricted?
Am I mixing a lot of cold water with a small amount of hot?
Does the problem disappear when I open the hot faucet farther?
Does it work better using the RV water pump?

If increasing hot-water flow makes the temperature stabilize, that’s an important diagnostic clue.

Don’t Immediately Start Replacing Parts

On-demand heaters have sensors, control boards, igniters, gas valves, thermostats, and other components that can fail.

But don’t start there.

When I’m troubleshooting one, I want to know whether the heater has what it needs to operate first:

Water flow → 12V power → fuel → ignition → heat

If one of those first requirements is missing, replacing components inside the heater isn’t going to solve the problem.

Start simple and work your way down the list.

Basic Maintenance

Maintenance requirements vary by manufacturer, but some general things worth watching include:

Inspecting and cleaning water inlet screens
Checking for plumbing leaks
Keeping exterior vents unobstructed
Watching for mineral buildup
Descaling when recommended
Inspecting electrical connections where accessible and safe
Checking propane-system operation
Following the manufacturer’s winterization procedure

And if you live or store your RV somewhere that freezes, don’t forget that an on-demand heater still contains water passages that need to be properly winterized.

Tankless doesn’t mean waterless.

When to Call an RV Technician

Basic water-flow checks are reasonable for most RV owners.

Professional diagnosis becomes more appropriate when:

The burner won’t ignite despite adequate water flow.
The heater repeatedly displays an error code.
You smell propane.
You suspect a gas leak.
Electrical testing is required.
The heater continually overheats.
Water is leaking from inside the heater.
The problem persists despite good water flow, power, and propane supply.

If you smell propane, shut off the propane supply, avoid ignition sources, ventilate the area when safe to do so, and have the system inspected.

Frequently Asked Questions

Why won’t my RV on-demand water heater turn on?

Start by checking water flow. On-demand heaters need a minimum amount of flow before they’ll activate. Poor campground pressure, clogged screens, filters, restrictive showerheads, or plumbing restrictions can prevent operation.

Can low water pressure stop an RV tankless water heater from working?

Yes. If water flow through the heater is too low, it may not recognize a demand for hot water. If campground pressure is poor, filling your fresh-water tank and using the RV water pump can sometimes solve the problem.

Why does my RV tankless water heater keep going hot and cold?

Unstable or insufficient water flow is one possibility. Try increasing hot-water flow and check whether campground pressure, filters, hoses, or fixtures are restricting the system.

What temperature should I set my RV on-demand water heater to?

There’s no single perfect shower temperature for everyone. Find a comfortable temperature for you and set the heater close to it. On systems designed for this type of operation, you can then use primarily hot water at the shower instead of heating the water much hotter and mixing in a large amount of cold.

Does an RV on-demand water heater provide unlimited hot water?

It isn’t limited by the capacity of a hot-water tank, but it isn’t truly unlimited. You’ll still be limited by your water supply, propane supply, electrical power, and—when you’re not connected to sewer—your waste-tank capacity.

Do RV on-demand water heaters need electricity?

Most require 12-volt DC power to operate their electronics, ignition system, sensors, and controls even when propane provides the actual heat.

The Bottom Line

RV on-demand water heaters can be excellent systems once you understand one important difference:

They need water flow to work properly.

When one isn’t heating, don’t immediately assume something inside the heater has failed. Check the simple things first—especially the water supply and flow.

If campground pressure is poor, try filling your fresh-water tank and running the RV water pump.

And once you’ve got everything working, experiment with the temperature setting.

Find the shower temperature you actually like, set the heater close to it, and let the heater produce water at approximately the temperature you want rather than constantly playing the hot…cold…too hot…cold…hot game with the shower faucet.

Sometimes the best way to troubleshoot an on-demand water heater is simply understanding how it wants to be used.

Written by Dan Harvickson (Founder of Last Mountain Tech) RV Specialist

RV On-Demand Water Heaters: How They Work and What Owners Should Know