Tankless Water Heater Suddenly Shuts Off During a Shower

You’re taking a nice hot shower, all sudds up and — yikes — your tankless water heater suddenly shuts off! Icy cold water splashes your noggin and down your back.

Then, just as suddenly, the water may turn hot again.

What gives?


Sudsing man in shower feels tankless water heater shutting off and on

If You’re the Second Person Taking a Shower

If you’re the second person to enter the shower soon after the first’s shower, it’s likely not a problem with the tankless unit at all! It’s simply that you initially get the hot water remaining in the pipes from the first person’s shower.

Then when that residual warm water is depleted, cold water pushes through until the hot water that YOU turned on has time to reach the shower. The length of time that takes depends on the length of pipe the hot water has to travel from the tankless water heater to the bathroom.

Tankless “on demand” Does NOT Mean “Instant” Hot Water in a Large Home

In other words, don’t confuse “on demand” tankless water heaters with “instant” hot water. Yes, the tankless is designed to turn on whenever anyone turns on a hot water spigot. But the hot water still has to pass through the water pipes to reach its final destination. In a large home where a shower is located a long distance from the garage (where most water heaters are located), that can take some time.

If the hot-cold-hot phenomena is related strictly to the second person immediately entering the shower, there are a few remedies.

Simplest Remedy for Hot-Cold-Hot in Second Person’s Shower Works but Wastes Water

The simplest remedy, of course, is for the second person to wait after turning on the hot water for the residual warm water and cold water to push through the pipes turn hot again. Yes, that wastes water. So it’s not ideal, especially in our area where water is so precious and expensive.

Option #2 Adding Recirculation Pump System to Tankless Saves Water and Provides Instant Hot Water

Another option is to connect a hot water recirculator (also known as a circulator) to your tankless water heater. You’ll also need to connect it to a timer (if your circulator doesn’t include one). That’s because tankless water heaters are not designed to run continuously. (Circulator pumps are better suited to conventional tank-style water heaters.)

Also, the main benefit of tankless is to save energy so it would defeat the purpose to have it run continuously. (Most newer tankless systems will issue an error code if it continuously runs an hour or more.)

And if your home was not originally built with return pipes for a hot water circulation system, you’ll either need to invest in new piping or use your cold water line as a return path.

How a Hot Water Re-circulator System Works

The circulator system works by pushing hot water from the water heater out through pipes to each hot water fixture and circulate back to the water heater. But it needs a path back to the water heater to re-circulate the hot water. (You can read additional information on hot water circulation systems here.)

Installing new pipes for the return loop back to the water heater works best, but it’s more costly and sometimes not possible.

Re-circulator Retrofitting Requirements for Hot Water Return Loop Using Cold Water Line

Most folks go with using the cold water line as a return path since it costs less than adding new return pipes. To do that, we attach a thermostat and valve to the cold water pipe on the furthest fixture (usually a bathroom under the sink) from the water heater to serve as the starting point for the return loop, ending at the cold water inlet to the water heater. And we also need to install buttons or switches at each hot water outlet to remotely start the timer to turn on the circulator pump.

A circulation pump system with a tankless water heater usually alleviates the hot-cold-hot shower “sandwich”. The second person just needs to activate the circulator’s timer by pushing a remote button or switch near the hot tap a few minutes before turning on the shower. That minute or two after pushing the button or switch allows enough time for the residual warm water from the first shower and the cold water to push through the pipe back to the water heater before the newly demanded hot water arrives to the shower.

But be aware of a few exceptions and issues that may arise after retrofitting to the cold water line.

Cold Water When Turning on Hot Tap With a Retrofitted Cold Line Return Loop When The Circulator Pump is Off

When you turn on a hot water tap or shower without activating the circulation pump, you might get cold water with a retrofitted cold line return. That’s because water travels through the least resistance. And of course there is more resistance for water to go through the water heater when the pump is off and not able to push it through the tankless heat exchanger.

What to do?

  • So if you’re getting cold water when you turn on a hot tap, check to make sure your circulator pump is turned on.
  • Most circulator pumps have a check valve to prevent water from going backwards. Sometimes the check valve can become stuck in the on position. Use a screwdriver or small tool to tap on the pump to break it free, freeing the stuck check valve to the off position.

Other Potential Surprises With Cold Line Return Loop When Circulator Pump is Off

If you have a shower with a mixer valve and separate volume controllers, you may also get an occasional cold-water surprise. Something similar may happen if you have the old style bidet with volume control and a diverter.

Cold water can travel through the cold side into the hot side thus feeding a shower in another bathroom a mixture of hot and cold into the hot tap. Any two handle valve in your home that has been capped with the hot and cold water on gives hot water the ability to travel through the valve and into the cold side of your plumbing system. I have seen this happen in showers, sinks, mop sinks, bidets. 

One of my customers had removed a washing machine with supplies that would not shut off. So they attached the hot and the cold together with a washing machine supply hose so they could use their water in their house until they received their new washer. You guessed it the cold water fed into the hot side and prevented them from getting hot water in the shower and sinks. No matter how high they turned up the water heater all they would get is warm water.

Tankless Requires at Least Half Gallon of Water to Turn On

Since a tankless needs at least a half gallon of water to turn on, don’t be surprised if it doesn’t ignite during times when you barely turn on the hot water.

Other Reasons Why a Tankless Water Heater Suddenly Shuts Off

Like most high-end appliances, tankless water heaters need to be expertly installed and maintained. When they’re not, they will eventually develop problems. Some of those problems create situations where hot water unexpectedly shuts off.

We’ll dive into a list of other reasons why a tankless water heater may unexpectedly shut off. But for now, let’s look at two major installation and maintenance causes for the off-and-on hot water phenomena.

Mineral Deposits Inside Tankless Water Heaters Need Flushing Out

Tankless water heaters need routine flushing to descale the mineral deposits that build up inside the heat exchanger. Descaling is the critical part of routine maintenance. Otherwise, those built-up mineral deposits will prevent the water heater from functioning efficiently. Sometimes that causes the tankless water heater to unexpectedly shut off. Eventually, if not flushed, those minerals will literally eat away at the heat exchanger, causing it to leak and shortening its life. Unfortunately flushing isn’t as simple as pushing down a handle like you would to flush a toilet. Read more about flushing tankless water heaters here.

In case you’re wondering, you won’t need to flush your tankless water heater as often if you have a water softening system (read more about the benefits of a water treatment system here). But you still should not neglect flushing it altogether. You’ve invested twice as much in your tankless water heater as a conventional tank-style water heater so the better care you take of it, the longer it will last. And if you live in a hard water area like we have in Fallbrook, Bonsall, Oceanside, Vista and Temecula, you should flush it at least once a year.

When properly maintained, a tankless water heater should last about 20 years (according to National Labs). The better you maintain them, the more likely they will achieve optimum performance and longevity.

heat exchanger in tankless water heater corroded never flushed

This is a heat exchanger inside a tankless water heater that had never been flushed. Mineral deposits eventually corroded it. That’s the turquoise colors. This one actually corroded to the point it started leaking. It had to be replaced.

Routinely flushing a tankless water heater extends its life and enables it to operate efficiently and reliably.

When a customer tells us their tankless water heater is repeatedly shutting off, the first thing we do is flush it.

A Recent Example of Another Tankless Water Heater That Was Never Flushed

Here’s another example from a recent customer call. The customer said their tankless water heater kept shutting off in the middle of a sower. We asked “when was the last time it was flushed? The response: 10 years – the same age as the water heater! Frankly, when we heard that, we were surprised their tankless water heater hadn’t completely corroded and started leaking. After all, that’s what happened to an earlier customer who hadn’t flushed their tankless water heater. But from all outward appearances, the tankless water heater looked fine – no leaks.

So we hooked up the pump to flush it. But no water would flow out.

That told us water was not getting through the fittings. So we disconnected the tankless water heater from its fittings. And that’s when we saw the problem.

Tankless Water Heater Not Installed Properly – Used Dielectric Unions

Often, mistakes take a while to show up. This is an example. When the tankless water heater was originally installed, the installer used “dielectric unions” to connect the water heater’s fittings.

Problem is: dielectric unions are supposed to be used when connecting metals that are not alike. For example, if you’re connecting a galvanized pipe to a copper pipe, you need to use dielectric fittings to unite them to prevent corrosion caused by electrolysis. But in this case, dielectric unions should NOT have been used. In fact, these dielectric unions actually created the corrosion. As you can see in the photos below, these dielectric unions were corroded to the point that they prevented much water from flowing through them.

dielectric unions corroded because should not be used on tankless water heaters
It pays to hire water heater experts to install your water heater. Then you won’t need to worry about improper installation becoming a problem in the future.

Instead of the dielectric unions, brass fittings should have been used. That’s because tankless water heaters have brass connections.

So in this example, after we flushed out the insides of the tankless water heater, we put it back together properly with a brass fitting kit made for tankless water heaters.

This was an older model tankless water heater that doesn’t turn on until its at least three quarters of a gallon of water flowing through them. That’s why it was turning off and on during a shower. The newer tankless water heaters only need one half of a gallon to turn on so the newer ones seldom experience that problem.

A Few Other Things That Can Cause a Tankless Water Heater to Turn Off

Here are a few examples of other things that might cause a tankless water heater to unexpectedly turn off:

  • using a water-saving shower head
  • the temperature on water heater is set too high
  • a dirty cold water inlet filter
  • flow-sensor damage or problem
  • improperly installed or too-short venting pipes
  • wrong size gas lines installed (on gas tankless water heater)
  • gas pressure problem
  • out of propane (if propane used)
  • loose electrical wire
  • condensation dripping on gas burner
  • improperly installed heat exchanger
  • damaged ignitor
  • dirty burners
  • dirty air filter (usually pet hair) needs cleaning

When the simple troubleshooting solutions don’t work, call us! We’re the area’s leading water heater experts and we’re here to help.

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