Common Problems with Tankless Water Heaters

Hard water, too much demand and electronics failure top the list.

Posted by Steve Graham | Feb 07, 2011
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Photo: Radical.librarian/Flickr.On-Demand (also known as tankless) water heaters are more efficient than standard heaters that continually use energy to keep a large tank of water hot. Instead, they quickly heat water as needed when you turn on the hot tap.

However, upgrading to an on-demand water heater is much like replacing an old car. New models are likely more energy-efficient and have plenty of new bells and whistles, but both new cars and new on-demand water heaters also use advanced computer sensors and other electronics. This means they have more visible and audible indicators of potential problems. However, they may also be more complicated to repair and trouble-shoot than older, simpler water heaters (or cars).

A high-quality on-demand (or tankless) water heater should last 20 years, but it must be properly maintained. Hard water, corrosion, excessive demand and improper installation can all cause problems with on-demand water heaters. Here are the best ways to address the most common issues.

Mineral Build-Up

Consumer Reports magazine questions the economic benefits of on-demand water heaters in most homes. Its reviewers estimate that an average homeowner will spend 22 years paying off a new water heater that will only last 20 years.

The magazine also warns that maintenance can be expensive. It tested a model that had scaly calcium buildup thanks to mineral-heavy hard water. An indicator light identified the problem. This is a common problem that can decrease efficiency and prematurely damage or break the water heater. Such a breakdown likely won’t be covered by a warranty.

If you have hard water, you will need to use a water softener, install a scale filter or flush the unit regularly with a vinegar solution, according to the owners’ manual. This may require professional service for some models.


On-demand water heaters are less subject to corrosion than tank-type heaters. However, if corrosion does occur, it can lead to serious problems. Condensation may drip onto on the gas burner and cause corrosion that blocks the gas supply. Though unlikely, even a small restriction could cause the sensitive computer in modern on-demand water heaters to shut down the system.

A poorly installed heat exchanger could also be damaged by corrosion. If installed properly, this should not be an issue for the lifespan of the heater. However, in the unlikely event of a corroded heat exchanger, replacement can be very costly.

Demand Overload

If you demand a large amount of hot water at once, such as taking a shower and running the dishwasher at the same time, the heater may not be able to meet the demand. The heater’s computer could simply shut down if it cannot get enough natural gas to heat enough water to meet your demands.

You may need to expand or upgrade your natural gas line to properly install an on-demand water heater. This can add significantly to the installation costs, but may be necessary to make sure the water heater is getting enough gas to function properly.

Combustion Shutdown

On-demand water heaters also must be vented carefully to avoid other computer-generated shutdowns. The heaters have very hot exhaust which must escape quickly and efficiently. If venting is restricted, the computer may shut down the heater. Venting tubes that are too long or convoluted typically cause this type of problem. 

Hard water, corrosion, excessive demand and improper installation all cause problems with on-demand water heaters.

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