Tankless water heaters require less maintenance than tank units — but the maintenance they do require is different and more specific. The primary task is descaling the heat exchanger, which must be done annually (or more frequently in hard water areas) to maintain performance, efficiency, and warranty validity.
The heat exchanger in a tankless water heater — typically stainless steel in premium units like the Navien NPE — has narrow water passages that are highly susceptible to mineral scale buildup. Unlike a tank unit where sediment settles on a large flat surface, scale in a tankless unit accumulates inside small-diameter channels. Even a thin layer of scale in these passages restricts flow, reduces efficiency, and creates hot spots that can cause the heat exchanger to fail prematurely. This is why annual descaling is not optional — it is the primary maintenance task that protects your investment.
Use a descaling kit that includes a small submersible pump, two hoses, and a 5-gallon bucket. Connect the hoses to the hot and cold service ports on the unit (most modern tankless units have dedicated service isolation valves for this). Fill the bucket with food-grade white vinegar or a commercial citric acid descaler and circulate it through the heat exchanger for 45 to 60 minutes. Follow with a 5-minute fresh water flush to remove all descaling solution residue.
In hard water areas (Phoenix, Las Vegas, Dallas, Los Angeles, Indianapolis), this step should be performed every 6 months rather than annually.
Most tankless units have a small filter screen on the cold water inlet that catches sediment before it enters the heat exchanger. Remove and rinse this screen annually. It is typically held in place by a hex plug that unscrews with a wrench. Rinse under running water and reinstall.
For condensing tankless units (which use a sealed direct vent), inspect both the air intake pipe and the exhaust vent pipe. Check for blockages (insect nests, debris), check that all joints are properly sealed, and verify that the exterior vent terminations are unobstructed. Condensing units produce an acidic condensate that drains through a small tube — verify this tube is clear and draining properly.
Briefly test the T&P valve by lifting the lever — water should flow freely and the valve should reseat when released. See our T&P valve guide for full testing instructions.
Most modern tankless units have a maintenance notification system that tracks flow history and alerts you when service is due. Check the display for any stored error codes and clear maintenance alerts after completing the descaling service. On Navien units, hold the buttons indicated in the manual for 3 seconds to reset the maintenance reminder.
Professional descaling service: If you're not comfortable performing the descaling procedure yourself, WaterHeaterMan offers annual maintenance service for tankless units. A factory-trained technician performs the full maintenance checklist and provides documentation for your warranty records.
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