To descale a tankless water heater, mix equal parts of white vinegar and water, then circulate the solution through the system using a pump for 45 minutes.
Descaling your tankless water heater removes mineral buildup that reduces efficiency and shortens lifespan. This guide provides detailed instructions for safe, effective descaling using vinegar or commercial solutions.
Why Tankless Water Heaters Need Descaling
Hard water causes scale buildup in tankless units just like traditional water heaters. The difference is tankless models have narrower heat exchanger passages that clog faster. Left untreated, scale can:
- Reduce heating efficiency by up to 30%
- Increase energy bills significantly
- Cause premature component failure
- Restrict water flow through the unit
For tankless water heater replacement parts, check our recommended components after descaling.
How Often to Descale
Descaling frequency depends on water hardness:
| Water Hardness | Descaling Frequency |
|---|---|
| Soft (0-3 gpg) | Every 2-3 years |
| Moderate (3-7 gpg) | Annually |
| Hard (7+ gpg) | Every 6 months |
Signs You Need to Descale
- Reduced hot water flow
- Fluctuating water temperatures
- Unusual noises from heater
- Error codes indicating flow problems
Descaling Supplies Needed
Gather these materials before starting:
- Submersible pump (1/6 HP minimum)
- 2 washing machine hoses
- 5-gallon bucket
- Descaling solution (4 gallons vinegar or commercial descaler)
- Adjustable wrench
- Towels for spills
For recommended descaling pumps, see our expert reviews.
Step-by-Step Descaling Process
1. Turn Off Power and Water
- Shut off gas supply (for gas units)
- Turn off electrical power at breaker
- Close cold and hot water isolation valves
2. Prepare the System
- Open hot water faucet to relieve pressure
- Remove service port caps from isolation valves
- Connect hoses to service ports (hot to bucket, cold to pump)
3. Circulate Descaling Solution
- Submerge pump in bucket with solution
- Open service valves
- Run pump for 45-60 minutes (longer for heavy buildup)
According to Oatey’s technical guide, never run the pump dry.
4. Flush the System
- Drain solution and refill bucket with clean water
- Flush for 5-10 minutes until pH neutral
- Check pH with test strips if available
5. Reassemble Components
- Close service valves and remove hoses
- Replace service port caps
- Open isolation valves fully
6. Restore Power
- Turn gas supply back on (if applicable)
- Restore electrical power
7. Test Operation
- Run hot water at faucets for 2-3 minutes
- Check for leaks at service ports
- Verify normal operation
Descaling Solution Options
Two effective descaling solutions:
White Vinegar
- Cost-effective and readily available
- Food-grade (5% acidity) works best
- May require longer circulation time
Commercial Descalers
- Formulated specifically for tankless units
- Faster acting than vinegar
- More expensive but more effective
As noted by Lenox Plumbing, commercial descalers work 30-50% faster than vinegar.
Maintenance Tips
- Mark descaling date on unit with permanent marker
- Install whole-house water softener if hardness exceeds 7 gpg
- Consider annual professional inspection
- Keep exterior clean and check for leaks monthly
For homes with very hard water, anode rods can provide additional protection between descaling sessions.
