Tankless Heater Temperature Swings in an RV
If your RV shower suddenly goes from hot to cold and back again, the water heater may not actually be broken.
I noticed this pretty quickly after switching from a traditional RV tank water heater to a tankless system. The tankless heater reacted much more to weak campground pressure, restrictive regulators, and low-flow shower settings than I expected.
Tankless RV heaters are more sensitive to water-flow changes than standard tank heaters. Small flow changes that a tank heater would ignore can cause a tankless system to cycle the burner on and off.
That creates the classic: hot… cold… hot… cold…
problem many RV owners run into.
First Thing I Would Check
One of the easiest tests is switching from campground city water to the RV water pump.
Disconnect city water, turn on the pump, and test the shower again.
If the hot water suddenly becomes more stable, the issue is often:
- weak campground pressure,
- a restrictive pressure regulator,
- a clogged filter,
- or reduced flow somewhere in the system.
I’ve seen situations where the RV pump actually gave more stable hot water than campground hookups.
Pressure Regulators Can Cause Problems
Some basic RV pressure regulators protect the plumbing but reduce flow more than people realize.
That may not matter much with a regular tank heater, but tankless heaters react differently because they depend on stable flow to keep the burner operating.
Common signs:
- shower pressure feels weak,
- temperature changes during the shower,
- the heater works better late at night,
- or works better on the RV pump.
Adjustable regulators usually perform much better than the small inexpensive inline versions.
Too Much Cold-Water Mixing
This catches a lot of RV owners.
Many people set the tankless heater very hot and then mix in a large amount of cold water at the shower handle.
Sometimes that reduces hot-side flow enough for the burner to shut off temporarily.
Instead, try setting the water heater closer to the actual shower temperature you want and use less cold-water mixing.
That alone can stabilize the temperature.
Check the Simple Restrictions
A partially clogged shower head or filter can affect tankless operation more than expected.
Things worth checking:
- shower head,
- faucet aerators,
- inline water filter,
- city-water inlet screen,
- hose kinks,
- restrictive fittings.
Even minor buildup can reduce flow enough to cause cycling problems.
Campground Pressure Changes More Than People Think
One campground may work perfectly while another gives constant problems.
Pressure can also change throughout the day depending on campground usage.
Morning and evening are usually the busiest times. If several RVs are showering or using water at once, flow may drop enough to affect tankless performance.
That’s one reason these issues can seem random.
Propane Flow Matters Too
Tankless heaters also need stable propane flow.
If propane flow is weak, you may see:
- ignition failures,
- inconsistent burner operation,
- intermittent hot water,
- or E1-type errors.
One RV-specific issue involves the Excess Flow Valve (EFV).
If the propane valve is opened too quickly, the EFV can partially restrict propane flow. The heater may still try to operate, but performance becomes inconsistent.
If that happens:
- shut off propane appliances,
- close the propane valve,
- wait briefly,
- reopen the valve slowly,
- then test the heater again.
Cold Weather Changes Performance
Incoming water temperature makes a big difference with RV tankless heaters.
Very cold incoming water requires more heating time and can reduce overall shower performance.
You may notice:
- lower flow,
- less stable temperatures,
- or shorter comfortable shower times during colder weather.
That’s fairly normal with smaller RV systems.
What I Would Check First
Instead of replacing parts immediately, I would start with:
- testing the RV pump,
- checking the regulator,
- inspecting the filter and inlet screen,
- cleaning the shower head,
- and confirming stable propane flow.
A lot of tankless complaints end up being flow-related rather than actual heater failure.