If your area has hard water, minerals such as calcium and magnesium slowly build up inside a tankless water heater’s heat exchanger. This scale insulates the metal, makes the unit work harder, narrows passages and can eventually trigger error codes or reduced hot water flow. The remedy is a periodic flush with a mild acid solution. This guide explains how to flush a tankless water heater, what you need, how often to do it and the safety points to respect.
Always follow your unit’s installation and maintenance manual first; it overrides general advice here. If you are not comfortable working with gas appliances or plumbing, hire a licensed professional. Maintenance may also be tied to your warranty terms, so check them.
Why Scale Builds Up
Heating hard water makes dissolved minerals come out of solution and form a crusty deposit on hot surfaces. Inside a tankless unit, the heat exchanger is the hottest surface and has narrow passages, so even a thin layer matters. Signs of scale include:
- Reduced hot water flow or a longer wait for heat
- Fluctuating temperatures
- Noises such as rumbling or popping
- Error codes related to temperature, flow or ignition (check your manual)
- Visible mineral deposits at fittings or filters
How Often Should You Flush?
Manufacturers often suggest an annual flush, and more often in very hard-water areas. Some recommend a water softener or a scale-inhibiting filter. If you do not know your water hardness, test it with strips or ask your water supplier. A good habit is to pick a date, such as the start of the cold season, and flush every year.
What You Will Need
- A small submersible pump (sump or utility pump) rated for the job
- Two hoses with fittings that match your service valves (typically washing-machine-style hose connectors)
- A 5-gallon bucket or similar container
- Descaling solution: either undiluted white vinegar (commonly about 2 to 3 gallons for a typical flush) or a descaler recommended by the manufacturer. Check your manual for the approved product and amount.
- Isolation (service) valves installed on the hot and cold lines. If your unit does not have them, ask a plumber to add them; they make future maintenance far easier.
- Adjustable wrench or pliers
- Towels for drips
- Gloves and eye protection
Safety Checklist Before You Begin
- Turn off the power to the unit. For gas models, switch off the gas supply at the service valve.
- Allow the unit to cool if it has been running.
- Do not mix vinegar with bleach or other chemicals.
- Ventilate the area.
- Keep children and pets away.
- Wear gloves and eye protection when handling acid solutions.
Step-by-Step Flush
1. Shut off power and gas
Unplug the unit or switch off the breaker. Close the gas supply valve for gas models.
2. Close the water valves
Close the cold-water inlet and hot-water outlet service valves.
3. Connect the hoses
Most isolation valves have hose-style ports with caps. Release any pressure by opening the pressure relief port carefully. Remove the caps and connect one hose from the cold-side valve port to the pump outlet, and another from the hot-side valve port to the bucket. Make sure the bucket sits on a stable surface.
4. Fill the bucket with descaler
Pour about 2 to 3 gallons of white vinegar or the recommended descaler into the bucket. Place the pump inside, with the hose from the cold-side port connected to the pump outlet.
5. Open the service valves
Turn the service valves so water can flow through the hoses into the unit (check the valve markings). Make sure that the main isolation valves are positioned so that the solution flows only through the heater.
6. Start the pump
Run the pump so that the solution circulates through the heat exchanger and returns to the bucket. Circulate for around 45 minutes to an hour or as the manual specifies. Watch the hoses for leaks and the bucket level.
7. Stop and drain the solution
Turn off the pump. Close the valves, disconnect the hoses and drain the solution into a drain or sink. Do not pour it on plants.
8. Flush with clean water
Reconnect the hoses, then flush the unit with clean water for several minutes. A cold-water supply can be used to flush the system and rinse out any remaining solution.
9. Clean the inlet filter
Many units have a small mesh filter at the cold-water inlet. Remove it, rinse out debris and reinstall it. Replace it if it is damaged.
10. Restore everything
Remove the hoses, replace the caps, and open the isolation valves fully. Turn on the gas and power. Run a hot tap to purge air from the lines. Check for leaks.
11. Test
Run hot water at several fixtures and confirm normal flow and temperature. Watch for error codes.
Troubleshooting a Flush
| Issue | Possible cause | Fix |
|---|---|---|
| Little flow through the pump | Hose kinked or pump too weak | Check hoses, try a stronger pump |
| Leaks at the caps | Missing or worn gaskets | Replace the washers or caps |
| Water is still discoloured | Residual sediment | Flush longer with clean water |
| Error codes remain | Scale not fully removed or another fault | Repeat the flush or call a professional |
| No isolation valves | Not installed | Hire a plumber to fit them |
Preventing Scale Build-Up
- Water softener or conditioner. If your water is very hard, this is the most effective approach, though it adds cost and maintenance.
- Scale-inhibiting filters. Some reduce scale formation, but their effectiveness varies.
- Moderate temperature settings. Higher temperatures can speed scaling. Setting 120°F is common and also helps prevent scalding.
- Regular schedules. Mark your calendar for annual maintenance.
When to Call a Professional
- You smell gas or suspect a leak
- Your unit shows persistent error codes
- You cannot access isolation valves
- Your warranty requires professional servicing
- You are uncomfortable with gas appliances
If your unit gives lukewarm water rather than scale-related symptoms, our guide to troubleshooting lukewarm water may help. For a general overview of how these heaters work, read how tankless water heaters work.
Tankless Water Heater FAQ: Lifespan, Noise and Venting
Frequently Asked Questions
Can I use vinegar to descale a tankless water heater?
Many people do, but check your manual. Some manufacturers specify an approved descaler and quantity, and using the wrong product could affect the warranty.
How long does a flush take?
Plan on one to two hours including setup, circulation and rinsing. The exact time depends on your unit and the scale level.
Do I need isolation valves?
They are not strictly required for every unit, but they make flushing much easier and are widely recommended. A plumber can add them.
What if my water is soft?
Scale builds up more slowly, but regular inspection is still wise. Check your manual’s schedule.
Conclusion
Flushing a tankless heater is a manageable DIY task for many homeowners, provided you follow safety steps and your manufacturer’s instructions. With isolation valves, a small pump and the right descaler, an annual flush helps protect efficiency and lifespan. If anything seems off, call a licensed pro.
