How Often Should Phoenix Homeowners Flush Their Tankless Water Heater?

We get this question all the time — and the honest answer is that most Phoenix homeowners aren't flushing nearly enough. If you have a tankless water heater and you haven't thought about descaling in the past year, it's time to take a closer look.

Tankless water heaters are a fantastic upgrade for Valley homeowners — endless hot water, hot water reaching fixtures in 10-15 seconds, lower energy bills, and a compact footprint. But there's one thing they're vulnerable to, and Phoenix has it in spades: mineral buildup from hard water. Staying on top of regular flushing is one of the simplest things you can do to protect your investment and keep hot water flowing reliably.

25+ grains per gallon — Phoenix water hardness

2× faster scale buildup vs. national average

6–12 months — our recommended flush interval

Why Phoenix Is So Hard on Tankless Heaters

Most of the Valley's water comes from the Colorado River and the Salt River — both of which carry very high levels of dissolved calcium and magnesium. Phoenix water typically measures between 200 and 400 milligrams per liter of total dissolved solids (TDS), putting it firmly in the "very hard" to "extremely hard" category.

To put that in perspective, the U.S. Geological Survey considers anything above 180 mg/L to be very hard. We regularly exceed that here by a significant margin. When that mineral-rich water gets heated inside your tankless unit's heat exchanger, calcium carbonate crystallizes and sticks to the metal surfaces. Over time, it builds into a thick, stubborn crust — and that's when problems start.

What the numbers say

Just a 1/16-inch layer of limescale on a heat exchanger can reduce efficiency by up to 12%. Thicker buildup forces your unit to work harder and run longer to deliver the same result — raising your gas bill and wearing out components well before their time.

Most manufacturers recommend flushing once a year for areas with average water hardness. Phoenix is anything but average. In our experience servicing water heaters across the Valley, we recommend flushing every 6 months for most households — and some heavily used units benefit from descaling every 3 to 4 months.

Our Recommendation: Every 6 Months

For a typical Phoenix family using a single natural gas tankless unit, twice a year is the sweet spot. We like to suggest spring (March–April) and fall (September–October) — the shoulder seasons when it's easier to schedule service before the summer rush hits.

If your home has a properly maintained whole-house water softener, you may be able to get away with once a year. But only if it's actually working — an undermaintained softener gives a false sense of security. We've seen plenty of units in "softened" homes with serious scale buildup because the salt was running low.

Signs you may need to flush more often

• Large household with high daily hot water demand

• No water softener or conditioning system installed

• Your unit also supplies a spa, hot tub, or radiant floor heating

• Chalky white residue building up around faucets and showerheads

• Noticeable drop in hot water pressure or flow rate

• Error codes showing up on your unit's display

  
  

Warning Signs Your Heater Needs Attention Now

Don't wait for the calendar if your unit is already showing symptoms. These are the things we look for when a customer calls us saying something feels "off" with their hot water:

🌡️ Longer heat-up times

Takes noticeably longer than it used to get hot water at the tap.

💧 Reduced flow rate

Hot water pressure feels weaker than normal throughout the house.

⚠️ Error codes

Many units display codes like "Scale Alert" or "Flow Error" when buildup is significant.

🔊 Strange noises

Popping, rumbling, or crackling sounds coming from the unit during operation.

💸 Rising gas bills

A noticeable uptick in energy use with no change in habits or usage.

🚿 Temperature swings

Water fluctuating between hot and cold mid-shower.

What Does Flushing Actually Involve?

The process uses a small submersible pump to circulate white vinegar (or a commercial descaling solution) through the unit's heat exchanger for 45 to 90 minutes. The mild acid dissolves mineral deposits safely without damaging the unit's components. Here's the general process:

1. Shut off the gas and electricity to the unit and let it cool for at least 30 minutes.

2. Close the cold water inlet and hot water outlet valves. Most tankless units have dedicated service valves for this.

3. Connect a submersible pump and hoses to the service ports using standard washing machine hoses.

4. Circulate pure white vinegar (at least 4 gallons) through the heat exchanger for 45–90 minutes.

5. Flush with clean water for 10 minutes to rinse all vinegar residue from the system.

6. Reconnect everything, restore water flow, and relight the unit. Check carefully for any leaks.

   
   

Heads up: This is a task some confident DIYers handle themselves. But if you're unsure about working around gas shut-offs or your unit's service valves, we're always happy to take care of it for you — quickly and affordably.

Should You DIY or Call a Pro?

Flushing kits are available at most home improvement stores for $30–$60, and plenty of homeowners do this themselves without issue. If you're comfortable with basic plumbing tasks and your unit has accessible service ports, it's a reasonable DIY job.

That said, when our team handles a tankless flush, we're also inspecting the inlet filter screens, checking gas pressure, testing the ignition system, and looking for any signs of wear or early failure — things that are easy to miss if you're not doing this regularly. A professional service visit typically runs $100–$200 in the Phoenix area and gives you real peace of mind.

A practical approach many of our customers use: do one DIY flush per year, and schedule a professional visit for the other. Best of both worlds.

Will a Water Softener Help?

Yes — and we recommend installing one alongside a tankless heater, especially in the Valley. A properly functioning salt-based softener significantly reduces mineral load and can stretch your flush intervals while protecting the heat exchanger over the long term.

One important note: some manufacturers, including Rinnai and Navien, advise against running water that's too soft (below 3 grains per gallon), as very soft water can be mildly corrosive to certain internal components. Always check your unit's manual, or ask us — we're happy to walk you through the best setup for your specific model.

Salt-free conditioners (sometimes called conditioners or descalers) are another option. They don't remove hardness minerals but alter their structure so they're less likely to stick to surfaces. Not as powerful as traditional softeners for heavy scale prevention, but a good low-maintenance option for some households.

The Bottom Line

If you're a Phoenix homeowner with a tankless water heater and you're not flushing at least once a year, your unit is silently struggling. And if you're not doing it every six months, you're likely losing efficiency and lifespan without realizing it.

The good news: staying on top of this is easy, affordable, and pays for itself many times over. A tankless heater that lasts 20 years with proper care beats one that burns out in 8–10 years from neglect every single time.

Set two calendar reminders — one in spring, one in fall — and treat it like an oil change for your home. And if you'd rather have it handled by someone who does this every day, we're just a phone call away.

Ready to Schedule a Flush?

Our team serves the entire Phoenix metro area. We'll descale your unit, inspect/clean all inline filters/condensate trap and make sure your hot water is running exactly as it should.

Schedule a Service Call with Ascent Plumbing at (480) 818-4885 Today!

Same-week appointments available · Licensed & insured