Tankless water heaters are marketed as "maintenance-free." That's a dangerous oversimplification. They have fewer parts than tank units, yes. But the parts they do have are precision components that scale buildup destroys.
If you have hard water (and most of the Philadelphia region does), your tankless heat exchanger is accumulating mineral scale every day. Spring is when you deal with it, before summer's increased water usage makes things worse.
What Scale Does to a Tankless Unit
Inside your tankless water heater is a heat exchanger: a series of small copper or stainless steel passages that transfer heat from the burner to flowing water. These passages are narrow by design, which is how the unit heats water so quickly.
Scale (calcium carbonate) coats the inside of these passages. As the coating thickens:
- Flow rate drops. The unit can't push enough water through, so your hot water output decreases.
- The heat exchanger overheats. Scale insulates the passages, so the burner has to run hotter to compensate. This triggers high-temperature limit switches and error codes.
- Efficiency drops. Your gas bill goes up 10-20% as the unit works harder to produce the same hot water.
- The heat exchanger cracks. This is the expensive one. A scaled-up heat exchanger that overheats repeatedly will crack. Replacement cost: $1,050-1,500 for the part alone.
Brand-Specific Notes
Navien makes some of the best tankless units available. Their NPE-A2 series with the dual stainless steel heat exchangers is what we install most often. Navien's recirculation pump and buffer tank setup is more complex than competitors, which means more to maintain but a better end result. Their error codes (E003, E012, E016) almost always trace back to scale when the unit is 2+ years old.
Navien's warranty requires annual maintenance to remain valid. Skipping it doesn't just risk a breakdown; it risks paying full price for a repair that should have been covered.
Rinnai units are reliable and well-built. Their RUR and RSC series use a copper heat exchanger that's efficient but more susceptible to scale than stainless. Rinnai recommends descaling annually, and in hard water areas, every 6 months. Their error code 11 (no ignition) and code LC (scale detection) are the most common service calls we see, and both are preventable with maintenance.
Noritz uses copper alloy heat exchangers and has a built-in scale detection feature on newer models. When the unit detects flow restriction from scale, it displays error code 29 or LC. By that point, the scale is already significant. Don't wait for the error code.
What a Proper Tankless Descaling Involves
This isn't a DIY vinegar flush from YouTube. A professional descaling service includes:
Circulation descaling: We connect a pump and circulate commercial-grade descaling solution through the heat exchanger for 45-60 minutes. This dissolves the scale without damaging the heat exchanger material.
Flow rate measurement: Before and after descaling, we measure the unit's flow rate. This tells us how much restriction the scale was causing and confirms the flush worked.
Inlet filter cleaning: Every tankless unit has an inlet water filter that catches debris. These clog over time and restrict flow. Takes 30 seconds to clean and makes a noticeable difference.
Burner inspection: We check the burner for debris, spider webs (yes, spiders love gas appliance burners), and proper flame pattern.
Venting inspection: Condensing tankless units like Navien produce acidic condensate. The PVC vent connections need to be checked for integrity, and the condensate drain needs to be clear.
Error code history: Most modern tankless units log error codes. We pull the history to identify intermittent issues that haven't caused a full shutdown yet.
The Warranty Factor
This is the part most homeowners don't know: virtually every tankless manufacturer requires documented annual maintenance to honor the warranty. Navien, Rinnai, Noritz, and Bosch all have this clause.
A Navien NPE-240A has a 15-year heat exchanger warranty, but only if you can prove annual maintenance was performed. That's a $1,550 part you're gambling on to save $250 per year.
Hard Water Solutions
If you're descaling every year and still getting rapid buildup, a water softener or scale inhibitor system upstream of the tankless unit is worth considering. It extends time between services and protects the heat exchanger long-term.
McCorry Comfort is a Navien-certified installer and services all major tankless water heater brands across the Philadelphia area. Schedule your tankless descaling.