Tankless Water Heaters Need Descaling in North Texas
A tankless heater has no tank to flush, which leads people to think it needs no maintenance. In hard water it needs more, not less, and the warning signs are easy to miss.
August 29, 2026 · 5 min read
A tankless water heater heats water on demand by running it through a compact heat exchanger with a large burner or heating element behind it. The passages in that exchanger are narrow, and the water inside them gets hot fast. That combination is exactly what mineral scale likes. Our North Texas water is hard enough that a tankless unit which is never descaled will lose capacity within a few years and can fail outright well before it should.
What scale does inside a tankless unit
As water heats, dissolved calcium and magnesium come out of solution and stick to the hot surfaces. In a tank heater that sediment settles to the bottom, where it is a nuisance. In a tankless exchanger it coats the passages the water has to flow through. Two things happen: flow drops as the passages narrow, and the metal behind the scale runs hotter because the scale insulates it from the water. The unit compensates by firing harder, efficiency falls, and eventually the exchanger overheats or cracks.
Signs a unit is overdue
- Hot water flow at the taps has dropped, so a shower that used to be strong is now weak while cold flow is fine.
- Temperature swings mid-shower, or the unit shuts off briefly when two fixtures run together.
- An error code on the display relating to overheating, flow or the exchanger. Codes vary by brand; the manual lists them.
- A rumbling or crackling noise from the unit when it fires.
- Longer wait for hot water, or water that is noticeably less hot than the set temperature.
- The gas bill has crept up without a change in use.
What the descale service involves
Most tankless units have isolation valves with service ports on the hot and cold connections, fitted at installation. We close the isolation valves, connect a small pump and hoses to the ports, and circulate a descaling solution through the exchanger for around an hour. The solution dissolves the scale, which flushes out with it. Then we rinse with clean water, check the inlet filter screen, clean it, and put the unit back in service. If the unit has an air intake filter, that gets cleaned at the same time.
If the unit was installed without isolation valves, the flush cannot be done without cutting the pipes. Fitting a valve kit is a one-time job that makes every future service straightforward, and it is worth doing at the first maintenance visit.
Why it matters for the warranty
Most manufacturers list periodic descaling in their maintenance requirements and may decline a heat exchanger claim if the unit is found packed with scale. Keeping a record of the service dates is worth the small effort. We note it on every visit.
Can I do it myself?
With isolation valves, a submersible pump, a bucket and a descaler, a handy homeowner can. The risks are getting the sequence wrong on the valves, using the wrong solution and damaging the exchanger, and missing the inlet screen and air filter. If you are comfortable, follow the manufacturer's procedure exactly. If you would rather not, it is a short visit for us and we check the burner, venting and condensate drain while we are there.
Family-owned in Rockwall, licensed and insured. TX Master Plumber TXM11693. (469) 338-5533.
Frequently asked
Do electric tankless units need descaling too?
Yes. The heating elements scale in the same way, and a scaled element runs hot and burns out. The procedure is similar.
Will a water softener eliminate the need to descale?
It reduces it substantially. Softened water leaves very little scale, and many owners with a softener extend the interval to every couple of years. An annual check of the inlet screen is still worthwhile.
Is a tankless heater a bad idea in hard water?
No, provided it is maintained. It just does not tolerate neglect the way a tank does. With annual descaling or a softener, tankless units do well here.
What is the descaling solution?
Food-grade white vinegar is the traditional choice and is what many manufacturers specify. Purpose-made descalers work faster. Never use anything not approved for the unit.