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Tankless is the right call in some Woodbridge houses and an expensive mistake in others, and the thing that decides it is scale. Every major manufacturer writes scale damage out of the warranty, which makes descaling a running cost rather than an upsell.
A tankless unit fires a burner only while water is moving through it, instead of holding forty or fifty gallons hot around the clock. Nothing stands there losing heat. Nothing rusts from the inside out. The Department of Energy, modelling appliance lifetimes from survey and census data, estimates an average of around 20 years for instantaneous water heaters against around 15 for storage tanks. That is a real advantage and we fit these gladly.
The 20 years has a condition attached, and it is the part the brochure leaves out.
Scale is the chalky mineral deposit hard water leaves on anything hot. A tank copes with it: volume for sediment to settle into, a sacrificial anode rod protecting the steel, a drain valve to get the sediment back out. A tankless unit has none of the three. Pacific Northwest National Laboratory, writing for DOE, notes it cannot use an anode rod at all, so makers build the exchanger from copper and stainless instead.
A.O. Smith explains the failure in its own guide better than we could. Scale coating the inside of the exchanger pipe "acts as a thermal insulator", heat stops crossing into the water, and the metal "retains this excess heat, eventually overheating and becoming damaged". Then the line that should decide how you treat one: "Once scale forms, scale removal maintenance can be performed, but the heater will never return to peak efficiency." Left alone, it says the piping eventually leaks. Bosch explains its own overheat code the same way.
The lab work agrees. In accelerated testing PNNL reviewed, gas tankless efficiency slid from 80 percent to 72 percent inside the equivalent of 1.6 years, and then the units did not limp on. They failed on a clogged pressure sensor or other control device and needed deliming, which returned them to 77 percent rather than the original 80. Read the conditions before the numbers: that rig ran at roughly 444 mg/L hardness and 600 gallons of hot water a day, nothing like a house in Dale City. The pattern transfers, the figures do not. A neglected tank gets worse. A neglected tankless stops.
Hard enough to matter, nowhere near the limestone extremes the sales literature is written for. Prince William Water publishes hardness by system rather than by town. Its East and West systems and Carter's Grove are listed at 3.5 to 7.0 grains per gallon, 60 to 120 mg/L, Moderately Hard. Bull Run Mountain, Evergreen and Hoadly Manor run higher at 7.0 to 10.5 gpg, Hard. Fairfax Water describes its own water as "moderately hard to hard (5 - 10 grains per gallon, or 84 - 170 mg/l)". Manassas, on its own plant, reported 47 ppm in 2025.
Nobody is delivering bad water, either. The same page says there is no primary or secondary standard for hardness, because it is an equipment nuisance rather than a health problem.
Four of these five draw their hardness line at 200 mg/L, which every published figure here sits under. But two start asking for treatment and annual flushing at around 3 to 4 grains per gallon, and nearly every local system is at or above that. By the manufacturers' own tables, a tankless unit here is in the maintenance zone. Not near it. In it.
| Brand | Published hardness ceiling | How it asks you to flush | Scale in the warranty |
|---|---|---|---|
| Noritz | 200 mg/L (12 gpg), its stated maximum suggested limit | A table by hardness band. No treatment and no flushing below 3 gpg. From 3 gpg up, a treatment device, with annual flushing required if no device is fitted | Excludes water over 12 gpg and poor water quality, and specifically the case where the unit displayed its C1# scale service reminder and the heat exchanger was not flushed |
| Navien | Up to 200 mg/L (12 grains per gallon) | Bands as well. Nothing below 4 gpg; a softener or its own device plus annual flushing from 4 gpg; the device required above 12 gpg | States that damage caused by poor water quality is not covered under warranty |
| Rinnai | 200 mg/L (about 11.7 gpg) | No calendar interval published at all. Annual professional inspection, plus flushing when the unit throws its Service Soon code. In known hard water areas it says flushing may need to be more frequent | States three separate times that water quality damage, including heat exchanger damage from lime build-up, is not covered |
| Rheem | Its RTGH warranty chart says up to 200 mg/L, while its own documents elsewhere say 11.68 grains and 250 mg/L. We are not going to resolve that for them | Counted in combustion hours, selectable at 500, 750 or 1,000. The reminder is inactive as default | Excludes failures caused by lime, mineral build-up or scale, heat exchanger leaks included. Separately, all warranties void after 12,000 hours of operation |
| A.O. Smith | None published in its tankless use and care guide | Annual is the implied baseline. It markets a cartridge that removes the need for annual descaling | Excludes damage from lack of maintenance including scale. Units with its integrated anti-scale technology get 15 year heat exchanger coverage against hard water scale, and that coverage is explicitly not extended to its bypass manifold models even if the cartridge is added |
Two things fall out of it. There is no single tankless flushing interval, so the honest answer is whatever your manual says for your water. And the reminder ships switched off on Rheem and Navien, so a homeowner waiting to be told is waiting for nothing.
Every tankless manufacturer we checked writes scale damage out of the warranty, in plain language, in a document handed over at purchase. Rheem excludes failures caused by lime, mineral build-up or scale, heat exchanger leaks included. Rinnai states the same exclusion in three places. Noritz goes furthest and voids coverage where the unit displayed its scale service reminder and was not flushed. A.O. Smith sells the exception as a feature, covering hard water scale for 15 years on units with its anti-scale technology while keeping the exclusion on its bypass models.
So the part most likely to fail in Northern Virginia water is the part the warranty will not cover unless the unit was looked after. That is not a reason to avoid tankless. It is a reason to read your own certificate, because they differ.
Tank for tank is one trade and one afternoon. A conversion is a different job, and the cost sits in the parts nobody photographs.
One licensing note, whoever you hire. Under DPOR's classifications neither a plumbing nor an HVAC classification covers gas piping; gas fitting is its own. That is why a tankless job can mean several trade permits and sometimes two contractors. We hold Virginia Class A license #2705131990 and run both trades in house.
If you have read that gas tankless heaters all have to go condensing by 2029, that is out of date. DOE did publish such a rule in December 2024. Congress disapproved it under the Congressional Review Act, the resolution was signed 9 May 2025, and DOE withdrew it on 20 May 2025 stating it shall have no force or effect. Non condensing units are still legal and nothing federal is pending.
The 2029 date that does exist belongs to storage heaters, and it is a date of manufacture, not installation. From 6 May 2029 most common sizes of new electric storage tank will need heat pump technology, with units of 35 gallons or less carved out. Our water heater page covers that properly.
Budget without a tax credit, too. The 25C credit was terminated early by the July 2025 law and is not allowed for property placed in service after 31 December 2025. Anyone still advertising a federal credit on a water heater is quoting a dead statute.
We service these as well as fit them, and most of what shuts one down is not terminal. Ignition faults, flow sensor trouble, restricted venting, overheat codes from scale. A unit locking out on overheat is usually telling you about the exchanger. A leaking exchanger is the other conversation, the one those exclusions were written for.
One call covers plumbing and HVAC. Talk to somebody who has been doing this for decades.
