Why tankless heaters need flushing
Water passes through a narrow heat exchanger in a tankless heater, and minerals in the water can leave a crust on its walls. That scale acts like insulation, forcing the unit to work harder, producing less hot water and triggering error codes. A flush removes the scale before it hardens.
A flush is not difficult, but it does involve gas or electric power, water and a descaling solution, so it deserves care. If you are not comfortable, a plumber can do it quickly. Our tankless water heater page covers service and installs.
Always read the manufacturer's manual first. Brands differ in valve layout, flush time and the solution they recommend, and the manual is the final word on your unit. What follows is the general process most units share.
A general tankless flush, step by step
What you need, and what to be careful about
- A small submersible pump, two hoses with fittings and a bucket.
- Descaling solution. White vinegar works for many units, though some makers recommend a specific product.
- The manual, for service valve layout and flush time.
- Safety. Turn off gas and power, wear gloves and eye protection and work in a ventilated space.
- Care with gas. If you smell gas at any point, leave and call your gas utility from outside.
- A way to catch drips, since the heat exchanger holds some water.
How often to flush
The right schedule depends on your water. In soft water, a flush every couple of years may be enough, while in moderately hard or hard water it is common to flush once a year. Some makers specify a schedule in the warranty, and a record of flushes can matter if you ever make a claim.
Watch for signs between flushes. Lukewarm water, rumbling or popping, error codes and reduced flow at hot taps suggest scale. If you see those and have not flushed for years, do it soon. A neglected heat exchanger can scale badly enough to need replacement.
Why Apple Valley water makes this a yearly job
Liberty Utilities, which serves about 81 percent of Apple Valley, reported 2022 hardness from 57 to 130 ppm and an average near 80 ppm. That is moderately hard on the common scale, so a yearly flush is a reasonable starting point. Other purveyors in the Town, which lists 11 in total, report their own values, and you can test your tap to see what reaches your unit.
Cold weather adds a second duty. Apple Valley winter lows reach the low 20s, and a tankless unit in an unheated garage or on an outside wall can freeze. Flush in the fall, check the freeze protection and insulate exposed pipes before the first cold snap.
The maker's service guidelines in your manual decide the flush time, the solution and the valve layout for your model. Follow them, since they also matter for the warranty.
Source: Liberty Utilities 2022 water quality data and Town of Apple Valley climate profile.
Signs and what they suggest
| Sign | Likely cause | What to do |
|---|---|---|
| Lukewarm water | Scale on the heat exchanger | Flush the unit |
| Rumbling or popping | Scale boiling in the exchanger | Flush soon |
| Error code | Scale, venting, gas or flow | Check the manual, then call |
| Low flow at hot taps | Clogged inlet filter | Clean the filter |
| Leak at the unit | Failed seal or crack | Shut off and call |
When to leave it to a plumber
Call a plumber if the service valves are missing, since a unit without them needs valves installed before it can be flushed. Call one if the unit leaks, if it shows a code the manual cannot explain or if you smell gas. Call one if you are not comfortable working with the gas and power supply.
A plumber can flush quickly, check the venting and the gas connection, clean the filter and test the unit. If the unit is badly scaled, we will say so and explain the options, which may include a stronger descaling or a repair. For a unit that is failing, our water heater replacement page explains the next step.
Preventing scale in the first place
A softener or a scale-inhibiting filter reduces scale at the source. Our water softener and filtration page explains how to test your tap and choose a system. Even with a softener, a yearly flush helps and keeps a record for the warranty.
Set the temperature no higher than you need, since higher temperatures speed scale formation. Homes in Victorville and Hesperia have different water, so check your own water report.
Tankless flush questions from Apple Valley homeowners
Once a year is a common schedule in moderately hard water. Follow the maker's manual.
Many makers allow white vinegar. Check your manual, since some specify a product.
Circulating the solution often takes 45 to 60 minutes, plus setup and rinsing.
Yes. Turn off the gas and the power before connecting hoses.
Service valves need to be added first. A plumber can install them.
Still have a question about your situation? Call (760) 986-1999 any hour and ask.