Common Tankless Hot Water Heater Problems

Common tankless water heater problems caused by improper installation

Tankless hot water heaters, unlike standard storage water heaters, need far more attention to detail for a proper installation.  If installed improperly the warranty may be invalidated.  Most tankless manufacturers require the heaters to be installed by trained service people knowledgeable.
 

Gas Supply Problems

Tankless water heaters typically require at least 3/4" diameter supply piping as they require much more gas flow than storage heaters.  Both the gas flow rate and the gas pressure are important.  Tankless heaters modulate the gas valve to control the set temperature of the water exiting the unit. Tankless heaters measure the gas flow and will shut the heater off if the gas flow is insufficient for the flow of water through the heater.  This means it may work fine a lower flows but shut off at higher flows.
 

Improper Exhaust venting

Tankless hot water heaters have sensors to monitor the exhaust temperature to make sure the combustion is proper.  Exhaust from the heaters becomes hot very fast and needs to be quickly vented. The the venting isn't installed properly it can cause the water heater to malfunction and shut down.
 

Heat Exchanger Corrosion

Because tankless hot water heaters are highly efficient, condensation occurs from the exhaust, and the condensate is highly corrosive. If the condensate drain is not installed correctly to keep the condensation off of the heaters parts it can severely damage them. 

The heat exchanger is the heart of a tankless heater and it is quite expensive if you need to replace it.  Most heat exchangers are made of stainless steel or other special alloys that resist corrosion, but it can still occur.  Other parts of the heater are even more susceptible to corrosion.  Any damage resulting from corrosion will not generally be covered under the warranty.
 

Burner Corrosion Problems

The burners on tankless heaters are typically mounted underneath the heat exchanger, so any condensate dripping on the heat exchanger can screw up the burner. It plugs the little holes in the burner. The internal computer is monitoring the burners continuously and the computer will turn off the heater if the burner is not burning clean or is at the wrong temperature.  Again, the warranty will not cover this problem.
 

Problems With Freezing

Since tankless hot water heaters are often installed outdoors, most will have an internal heater to prevent the heat exchanger from freeze damage.  You should always insulate the pipes to and from the heater if the temperature ever drops below freezing at that location.  The internal freeze protection will only protect the internal parts like the heat exchanger and not the water pipes.
 

Buildup of Scale

In areas that have hard water with high mineral content it is important to either treat the water to make it softer or to occasionally flush out the unit as recommended by the manufacturer. Hard water causes scale to build up on the walls of the heat exchanger and will cause problems and even early failure of the heater, and is again, not covered by the warranty.
 

Water Pressure Problems

If the water pressure is very high it can create excess stress on the plumbing parts of the heater.  The result can be premature failure of the heater. Be sure the water pressure is within the specifications spelled out in the installation instructions. If the water pressure is too high you need to install a pressure regulator to bring the pressure down to the proper pressure.

Installed and sized and maintained correctly, a tankless water heater can last for 20 or more years and operate without problems its entire life.