Scale is the quiet enemy of a tankless heater. Every time hot water passes through the heat exchanger, dissolved calcium and magnesium can settle on the walls of the narrow passages. A thin film does little. A thick crust restricts flow, forces the burner to work harder, and eventually trips a temperature or flow fault.
A flush dissolves that film before it hardens. The job is simple if the unit has service valves, and awkward if it does not.
What do you need for a flush?
Gather the kit first, because the procedure takes about an hour once it starts.
Flush kit
- A small submersible or utility pump rated for the flow, with two hoses
- Two hoses with washing machine style fittings that fit the service valves
- A five gallon bucket
- Two to three gallons of undiluted white vinegar, or a descaler your unit's manual approves
- A garden hose or second bucket for the rinse
- Gloves and eye protection for splashes
Not sure your unit is flushable or the isolation valves are installed? Call (855) 575-2890 and we will walk through it.
Call (855) 575-2890How do you flush it, step by step?
Read the manual for your brand first, because some manufacturers specify a particular descaler or time. The sequence below fits most gas units with isolation valves on the hot and cold lines.
- Switch the unit off at its power switch or unplug it, and close the gas valve to the heater.
- Close the cold water and hot water isolation valves on the lines under the heater.
- Connect one hose from the pump to the cold side service port and a return hose from the hot side port into the bucket.
- Pour vinegar into the bucket and open both service port valves.
- Run the pump so vinegar circulates through the heat exchanger for 45 to 60 minutes.
- Drain the vinegar, then close the ports, open the cold isolation valve, and flush the unit with clean water for five minutes.
- Clean the inlet filter screen, close the ports, open both isolation valves, restore gas and power, and run a hot tap until the water clears.
Vinegar has a strong smell and can leave a faint taste for a few minutes, so open a window and run a tap after the rinse.
How often should you flush?
Manufacturers differ, so use this as a way to think about the schedule, not a fixed rule.
| Situation | Reasonable approach | Reason |
|---|---|---|
| Moderately hard water, no softener | Flush on the schedule in your manual and check the screen at each visit | Scale builds slowly but steadily |
| Very hard water, no softener | Flush more often and watch for slow warm-up | Passages narrow faster |
| Softener or filter upstream | Follow the manual, inspect the screen yearly | Less mineral reaches the exchanger |
| Heater never flushed in several years | Have a plumber inspect before a DIY flush | Heavy scale may need a stronger approach |
| Display shows a flow or high-limit code | Flush, then call if the code returns | The code may mean scale or a real fault |
How Los Gatos hardness changes the schedule
San Jose Water serves Los Gatos, and its 2025 report lists three water sources with different hardness. Mountain surface water averages 151 ppm, or 9 grains per gallon. Groundwater averages 288 ppm, or 16.8 grains per gallon, with results from 175 to 451 ppm. Treated surface water bought from Valley Water averages 99 ppm, or 5.8 grains per gallon.
The practical point is that two neighbors can have different scale rates. A tankless heater on the groundwater end of that range will collect scale faster than one on Valley Water supply. Check which source feeds your home by asking San Jose Water, and look at the inlet screen and the heater's performance rather than guessing.
San Jose Water also advises cleaning faucet aerators regularly. White grit in an aerator is a cheap early warning of scale, so check one when you flush the heater. If the aerators clog quickly, that is a reason to flush sooner and to consider treating the water.
Is a softener better than regular flushing?
A flush treats scale after it forms. A softener or scale-control system treats the water before it reaches the heater. For a house on the harder end of the local range, a whole-house system can reduce how often you need to flush at all.
Softeners need salt refills, a drain connection, and some space. A cartridge filter style conditioner needs less room but works differently. Our water softener and filtration team can look at your plumbing, your heater, and your household use and tell you which option fits.
If your unit has no service valves, ask about adding them during the same visit. Any work on the heater itself can be handled by the plumber we send for tankless water heater service, including checking the heat exchanger for damage.
What can go wrong during a flush?
Scalding, leaks, and gas
Hot water may remain in the unit and lines, so let the heater sit idle for a while before opening ports. Close the gas valve before you start. If you smell gas at any point, leave the house, avoid switches and flames, call 911 from outside, and then call the gas utility.
If a service valve drips after you close it, do not keep tightening. The valve may need replacing, and that is a plumber's job.
Does this apply in nearby neighborhoods?
The procedure is the same across the area. Homeowners in Saratoga and Cambrian Park should still ask which water source serves their address, because that answer sets the schedule more than the town name does.
If you hear a rumble or kettle-like popping while the unit runs, scale is likely already heavy. That sound is steam bubbles forming in the narrowed passages, and it is a sign to flush soon.
Flushing a tankless heater, answered
White vinegar is widely used for descaling, but check your manual first. Some manufacturers name an approved descaler and warn against others.
Not easily. Without isolation valves and service ports you cannot circulate a solution through the exchanger, so a plumber should add them first.
It can. San Jose Water reports groundwater at 288 ppm against 99 ppm for Valley Water supply, so scale risk depends on which source serves your address.
Circulating the solution takes 45 to 60 minutes, plus setup, rinsing, and screen cleaning. Plan on roughly two hours for a first-time job.
Then scale may not be the cause. A failing sensor, weak gas supply, or vent problem can show similar symptoms, so have a licensed plumber test the unit.