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How to Flush a Tankless Water Heater
Quick answer: Flushing a tankless heater means circulating a descaling solution through the heat exchanger for about 45–60 minutes using the service valves, then rinsing with fresh water for about 5 minutes. Most homes flush once a year; hard water may need every 6 months. Follow your manufacturer's manual and hire a pro if unsure.
- Turn off electrical power (breaker) and, for gas units, the gas supply before any service.
- Scald risk: water and parts may be hot — let the unit cool and protect skin and eyes.
- Follow your manufacturer's manual for your exact model.
- Your warranty may require service by a licensed professional — check before doing it yourself.
Context: why flushing matters
Hard water leaves mineral scale (mostly calcium carbonate) inside the heat exchanger. Scale insulates the metal, so the unit works harder, may throw error codes, and can wear out early. Regular flushing removes that scale. Many manufacturers list descaling as required maintenance, and some warranties require proof of service.
This page gives a high-level outline only, not model-specific instructions. Your manufacturer's manual is the authority for your unit, and a licensed professional is the right choice if you are unsure, uncomfortable, or if your warranty requires professional service.
Terms
- Service (isolation) valves: valves on the hot and cold lines with small ports that allow a pump to circulate solution.
- Descaler: a solution (often food-grade acid or white vinegar) that dissolves scale. See the descaler guide.
- Inlet filter: a small screen on the cold inlet that traps debris.
- Error codes: many units display a scale or maintenance code when flushing is overdue.
How often to flush (example guide)
| Water hardness | Example interval |
|---|---|
| Soft (under ~60 ppm) | Every 1–2 years |
| Moderate (60–120 ppm) | Yearly |
| Hard (120–180 ppm) | Every 6–12 months |
| Very hard (over 180 ppm) | Every 6 months, consider a softener |
High-level overview (not model-specific)
- Power and gas off per the safety box above; let the unit cool.
2. Close the service valves; connect hoses and a small pump per the manual.
3. Circulate descaler for the time the manufacturer specifies.
4. Drain the solution, rinse with clean water.
5. Clean the inlet filter, restore valves, restore power.
If anything is unclear, stop and call a licensed professional.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: moderate hard water, yearly flush.
- About 4 gal of solution in a bucket
- ~60 min circulation, ~5 min rinse
- DIY supplies (pump, hoses, descaler): example $60–$120 one-time, then ~$10–$25 per flush
- Professional service: example $100–$250 per visit
Over 10 years, DIY supplies might total ~$250 vs ~$1,500 for yearly pro visits. Either path costs far less than an early heat-exchanger replacement.
Signs it is time
- Hot water fluctuates or takes longer to reach temperature.
- A maintenance or scale error code appears.
- Reduced flow at the tap.
- Unusual noises from the unit.
Mistakes to avoid
- Skipping power or gas shut-off.
- Using a chemical not approved by the manufacturer.
- Forgetting the inlet filter.
- Ignoring warranty service requirements.
FAQ
Can I use vinegar?
Many manuals allow white vinegar; some specify a dedicated descaler. Follow your manual.
Do I need service valves?
They make flushing practical. If missing, ask a licensed plumber to add them.
Does flushing void the warranty?
Not if done per the manual, but some warranties require professional service — check first.
Does a softener remove the need to flush?
It reduces scale but most manuals still recommend periodic service.
Safety and code reminders
Gas, venting, combustion air, condensate disposal and high-amperage electrical work all carry real safety risks, including carbon monoxide, fire, scalding and shock. They are also governed by local code and permits that differ between towns and provinces or states. Treat every step on this page as preparation for a professional visit, not as instructions to do the work yourself. Keep the manufacturer's manual for your exact model, set water temperature with scald risk in mind (120°F is a common household target), and stop and call a licensed professional whenever something is unclear.
Quick reference: typical fixture flow rates
These example flow rates help you translate everyday use into the GPM numbers used throughout this site. Check the rating stamped on your own fixtures, as older fixtures often run higher.
| Fixture | Example hot-water flow |
|---|---|
| Low-flow showerhead | 1.5–1.8 GPM |
| Standard showerhead | 2.0–2.5 GPM |
| Bathroom faucet | 0.5–1.2 GPM |
| Kitchen faucet | 1.0–1.8 GPM |
| Dishwasher | 0.8–1.5 GPM |
| Washing machine (warm) | 1.0–2.0 GPM |
| Bathtub fill | 2.0–4.0 GPM |
Add only the fixtures that genuinely run at the same time. Running the dishwasher or laundry outside shower hours is the simplest way to stay within a unit's capacity.
Questions to ask a licensed installer
Bring these questions to every quote so you can compare them fairly. Good installers welcome them, and the answers often reveal hidden costs before work starts.
- What peak GPM and temperature rise did you size this unit for, and what inlet water temperature did you assume for winter?
- Is my existing gas line, meter or electrical panel large enough, and if not, what exactly needs upgrading and what does that line item cost?
- What venting route will you use, how long is it, and does it meet the manufacturer's instructions and local code?
- Does the quote include a permit, isolation (service) valves for future flushing, a condensate drain if needed, and removal of the old heater?
- What are the labour warranty and the manufacturer's heat-exchanger and parts warranty terms, and do they require registration or yearly professional service?
- How long will the install take, and will there be a period without hot water?
How to read the numbers on this site
Every figure on TanklessMath is a planning estimate. Ranges reflect typical North American examples as of 2026-10-29 and will differ with your region, home, fuel prices and installer. Equipment ratings vary by model, so always check the specification sheet for the exact unit you are considering. When our example and a licensed installer disagree, trust the installer who has seen your home.
Next steps
Write down your peak fixtures, coldest incoming water temperature, fuel type and possible locations, then ask at least two licensed installers to confirm sizing, venting, gas or electrical capacity, permits and local code. Compare written quotes line by line so the unit, labour, materials and upgrades are separated. Questions about this site: hello@charlandandco.com.
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