How to Flush a Tankless Water Heater
Part of our Ventilation & Hot Water guides.
Annual descaling keeps a tankless heater efficient; in hard water, flush more often. Turn off the water (and power at the breaker for anything electrical) before starting, and follow the product manual where it differs from these general steps.
| Time | 1-1.5 hours |
|---|---|
| Difficulty | Easy |
| Tools | Flush kit (pump, pail, hoses), white vinegar or descaler |
Step by step
- Power off. Switch off power (or gas) to the unit.
- Close isolation valves. Close hot and cold service valves.
- Connect hoses. Attach pump hose to the cold service port and return hose to the hot port, into the pail.
- Add descaler. Fill the pail with vinegar or descaler.
- Circulate. Run the pump for the time the manual recommends, often 45-60 minutes.
- Rinse. Flush with clean water.
- Restore. Remove hoses, open valves, purge air, restore power.
Tips & common mistakes
- Clean the inlet screen filter while you're there.
- Mark your calendar for next year.
How often to flush
Manufacturers commonly recommend once a year. In hard water areas (lots of white scale on faucets), every 6 months keeps efficiency up. Signs it's overdue: fluctuating temperature, reduced flow, error codes or new noises.
Vinegar or descaler?
White vinegar works for most units and is inexpensive; about 4 gallons is typical for circulating. Commercial descaling solutions work faster on heavy scale. Use whatever your heater manufacturer allows.
Don't forget the inlet filter
Most tankless units have a small mesh screen at the cold inlet. Unscrew it while the unit is isolated, rinse it and refit. A clogged screen alone can cause low hot water flow.
Gas vs electric units
The process is the same for both. On gas units, turn the gas valve off as well as electrical power. On electric units, make sure the breakers stay off until the heater is full of water again.
Kit or DIY setup
A flush kit like VEVOR's bundles a pump, 5-gallon pail and two hoses (plus descaling powder in one version). You can also assemble a pump, bucket and washing machine hoses yourself.
- VEVOR Tankless Water Heater Flushing Kit (Pump, Pail, Hoses) — $77.90View
Frequently asked questions
How often should I flush?
Typically once a year; every 6 months in very hard water.
Can I flush without isolation valves?
Without service valves, flushing is much harder; consider having them installed.
Do I need to flush electric tankless heaters?
Yes, scale affects both electric and gas units.
What if my heater shows an error after flushing?
Make sure air is purged and power restored correctly; consult the manual's error codes.
Can I use citric acid instead of vinegar?
Many descalers are citric-acid based; follow the heater manufacturer's guidance.