Troubleshooting Tankless Water Heater Problems


Explore the most common tankless water heater problems; the cause, symptoms and troubleshooting. If you own a "smart" heater from Rinnai, Noritz, Paloma, Bosch, or any other manufacturer, this troubleshooting guide will present and explain the diagnostic error codes.


One of the most frequent complaints I have seen in reviews and forums, is: " I don't get the hot water instantly." Usually, tankless water heaters do not provide hot water straight away, unless it uses the recirculation pump or the buffer tank. The time it takes to deliver hot water from the heater to the fixture depends on the length between these two points. And that is the reason why some users will experience shorter or longer wait for hot water. This is not a "problem" so do not spend hours in trying to find the solution and repair.

Common tankless water heater problems

"Hot water is not delivered instantly" and other common tankless water heater problems are listed below, while the list of the error codes will be presented in other articles:
  • No hot water

  • Water is not hot enough

  • Water is too hot

  • Burner does not ignite

  • Burner is too noisy

  • Low water pressure

  • Cold water sandwiches or hot water fluctuates

Fixing the tankless water heater problems

No hot water

  • No hot water is one of the most common tankless water heater problems. The first things you should check are the electric power, water and gas supply to the water heater. Check the shut-off valves, they might be in the closed position.

  • Check the burner and does the flame rod generate the sparks.

  • See is there an error code on the unit's control panel, which blocks the heater from running before the previous problem has been solved and system reset.

  • Is the minimum flow rate achieved? Make sure that the tap is open enough and there are no obstructions in the piping line.

  • Are the unit or water pipes frozen?

Water is too hot

  • The temperature is set too high. Reduce the temperature to avoid the problem with the scalding.

  • If water flow is reduced due to the obstructions in the water filter or plumbing, only small amount of water will be heated. Clean the water passageways.

  • Check are the showerhead or hot water tap clogged and reduce the flow of hot water. Clean the fixtures.

  • If there is a sediment buildup, you should perform flushing and descaling.

  • 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 the output temperature sensor, it might be broken.


emergency plumber

Water is not hot enough

  • Water temperature is set too low. Increase the temperature to approximately 125-130 F.

  • Reduced the water flow when the water filter or fixture aerator are clogged.

  • If there is a plumbing crossover, cold water is mixing with the hot water. 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?

  • Sediment and limescale build-up inside the heat exchanger. You might want to use the discaling procedure and flush all the deposits. The scale deposits acts as an insulator and it prevents heat exchanger to transfer heat to water.

  • Is venting system clean of the debris and does it provide enough fresh air for proper combustion?

Burner does not ignite and no flame is present

  • See are the electricity, gas and water turned ON and supply a tankless unit with no problems.

  • Ensure the gas type and pressure are correct and the gas line is properly sized.

  • Bleed all the air from the gas line.

  • Check are the flame rod and wiring harness operational, not loose, out of position 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 the flow sensor is working properly.

The unit is too noisy

  • Check the fan for debris.

  • Check the burner is it 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 OK.
  • Ensure there are no obstructions in the plumbing, fixtures or water filter.

  • Greater draw with many applications running at the same time.
  • With the lower gas pressure water heater will lower water flow to it can meet selected output temperature.

Cold water sandwiches or hot water fluctuates

  • Fluctuation in the plumbing system.

  • Check is the gas line sized properly 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 the limits.

  • Flow sensor might be damaged.

  • Is the water filter on the cold water inlet clean and free of debris?

How to test for plumbing crossover problem

  • Shut off the cold water to the heater.

  • Open all the hot water taps.

  • After a 10 minute or so, there should be no water flow, at all. Otherwise the crossover is present.

Keep also 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 you have the reduced flow from the tap below the minimum. By keeping the flow rate above the minimum required, this and many other problems might be resolved. Another "unusual activity" can be a white smoke that comes out of the exhaust vent,and can be seen during the colder weather, when the exhaust temperature is much higher than the temperature of the air.

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