Troubleshooting Tankless Water Heater Problems

Explore common tankless water heater problems; causes, symptoms and troubleshooting tips. See how to repair a tankless heater when there is no hot water, water is too hot, there is not enough hot water, the pressure is low, cold water sandwich (the temperature of water fluctuates: hot-cold-hot), the gas burner doesn’t ignite, and what to do if there is an error code.

If you are the owner of a "smart" tankless water heater from Rinnai, Noritz, Paloma, Bosch, Takagi or similar, this troubleshooting guide is for you. The guide deals with the most common problems and explains some of the error codes.

Tankless water heaters are durable and long lasting units that do not require much maintenance. They are expensive, efficient, environment-friendly, have the advanced technology, could display an error code, but can also fail and cost you a lot to fix.

One of the most frequent complaints I have seen in many reviews and forums, is: "I didn't get hot water instantly." Alternatively, it takes too long to get the hot water at the fixture.

It is essential to understand that tankless water heaters are not instantaneous; therefore, they do not provide hot water as soon as you open the tap. Only those tankless heaters equipped with the recirculation pump or the buffer tank are capable of delivering hot water super-fast. If this is what you need, buy Rinnai RUR98 or Navien.

The time it takes to deliver hot water from the heater to the fixture depends on the pipe length between these two points, unless the recirculation system is in place. Due to the long water pipes, cold water that resides inside must be pushed away before hot water reaches the fixture, affecting the comfort. This is not a "problem" so do not spend hours trying to find the solution or calling the “expert.”

Common tankless water heater problems

"Hot water is not delivered instantly," and other common tankless water heater problems are listed below:

Some of these problems are easy to fix, some are not. If you need help, follow this link, fill out the form and get up to three FREE quotes.

The list of the error codes can be reached if following the provided links below.

Fixing tankless water heater problems

No hot water

  • No hot water is one of the most common problems in water heating systems. The first things you should check are the correct supply of electricity, water, and gas to the water heater. Make sure that the shut-off valve, is not closed.
  • Check the gas burner and does the flame rod generate sparks when the unit is turned on. If not, check this article that deals with the ignition failure problem.
  • See if there is an error code on the unit's control panel, which blocks the heater from running before the previous issue has been solved and system reset.
  • Is the minimum water flow rate achieved? Make sure that the tap is open enough to ensure the required flow rate, and there are no obstructions in the piping line.
  • During the cold weather, check if the elements and water pipes are exposed to freezing temperature or maybe frozen? Make sure to insulate all the exposed pipes, even covering the heater with the insulation jacket.

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Water is too hot

  • The temperature is set too high. Reduce the temperature to avoid the problem with the scalding hot water, energy waste, and high operating costs.
  • If the water flow is reduced due to the obstructions in the water filter or plumbing, only the small amount of water will be heated resulting in higher than normal temperatures. Clean the water passageways.
  • Check if the showerhead or hot water tap is clogged as it will reduce the water flow. Clean the fixtures.
  • If there is sediment buildup, you must perform the flushing and descaling. Check out this guide to learn how.
  • If the temperature sensor is broken, or it is not correctly positioned, try to either re-position it, so it is firmly attached to the pipe or replace it with a new one.
  • Check out the output temperature sensor; it might be broken. Replace it if required.

Water is not hot enough

  • Water temperature is set too low. Increase the temperature to approximately 125-130 F, or higher if needed.
  • Water flow might be reduced when the water filter or fixture aerator are clogged.
  • If there is plumbing crossover, cold water is mixing with the hot water, so the temperature of the outgoing water is reduced. One of the reasons for the plumbing crossover is if the single-lever mixing valve was installed, and it failed.
  • Gas valve does not provide enough pressure; either due to the low gas pressure or if it is not open fully. Is the gas line sized correctly? Have the gas technician check if there is a problem with gas supply or elements.
  • Sediment and limescale build-up inside the heat exchanger. You might want to proceed with descaling and flush all the deposits. The scale deposits act as an insulator preventing the heat exchanger to transfer the heat to water.
  • Is venting system clean of the debris and does it provide enough fresh air for proper combustion?

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Gas burner does not ignite, and no flame is present

  • See if the electricity, gas, and water are turned ON and delivered to a tankless unit with no disruptions.
  • Ensure that the gas type and pressure are correct and the gas line is sized correctly.
  • Bleed all the air from the gas line.
  • Check if the flame rod and wiring harness are operational, not loose, not in a right spot or damaged.
  • Does the gas control valve open at all or there is a short circuit?
  • Is the vent system installed correctly and has a proper length?
  • Is there an excessive condensation that can distinguish the flame?
  • Is the flow rate over its recommended minimum? Ensure that the flow sensor is working correctly.

The unit is too noisy

  • Check the fan for debris.
  • Check if the burner is burning the gas with the clear, light blue flame.
  • Leak in the sealed combustion results in irregular gas combustion.
  • Unstable burner flames due to the improper venting, lack of combustion air and gas pressure.

Low water pressure

  • Ensure that the water pressure from the utility company is as required.
  • Ensure there are no obstructions in the plumbing, fixtures or water filter.
  • The water draw is greater, and many applications are running at the same time.
  • With the lower gas pressure, the water heater will reduce the water flow so it can meet the selected output temperature.

Cold water sandwich or hot water temperature fluctuates

Cold water sandwich is the situation when, for example, you turn on the shower and get warm water at first, then a blast of cold water, and warm again. If the water heater was used before your shower, some hot water remained from previous use – and that is what you felt.

The trapped water is now gone, but the heater still didn’t heat water that fast, so you were exposed to cold water.

The longer the pipes, further the water has to travel, more you will feel the gap.

  • Fluctuation in the plumbing system.
  • Check if the gas line is adequately sized and for the maximum BTU.
  • Make sure there is no plumbing crossover when cold water mixes with hot.
  • Ensure that the length of the venting pipe is within limits.
  • A flow sensor might be damaged.
  • Is the water filter on the cold water inlet clean and free of debris?

Note: If the temperature of hot water fluctuates during the shower, for example, it is recommended to combine your tankless with the “buffer” mini tank. The mini tank will always have hot water ready and deliver it to the shower before the tankless warms up.

How to test for the plumbing crossover problem

  • Shut off the cold water to the heater.
  • Open all the hot water taps.
  • If after 10 minutes or so, there is still a flow of water you have a problem with the crossover.

Error codes

Most tankless water heaters are equipped with a diagnostic system that incorporate the microprocessor and LCD screen, which displays an error code if the problem occurs. An error code can appear due to many reasons, from a simple spider web and air/exhaust blockage to more serious gas or exhaust leak.

Use the following articles to get help on how to troubleshoot problems on Rinnai, AO Smith, Noritz and American.

Keep in mind that some unusual activities are normal; such as when the fan motor is still spinning after the water heating operation stopped. In this case, the fan motor runs for some time to force the exhaust gases out through the vent system.

Many tankless water heater problems are associated with the flow rate. For example; the water got cold during the shower because the flow from the tap was below the minimum. By keeping the flow rate above the minimum required, this and many other problems might be prevented or resolved. 

Another "unusual activity" can be a white smoke that comes out of the exhaust vent, and can be seen during the cold weather, when the temperature of the exhaust gases is much higher than the temperature of the air.

Some problems are possible to fix only with the right tools and skills, so to keep the warranty valid and unit functional, it is at your best interest to contact a professional plumber.

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