Do Tankless Water Heaters Need Flushing?

You switched to tankless to avoid the problems of traditional water heaters. No tank means no sediment buildup, right?

Not quite. Tankless water heaters don't accumulate sediment the same way, but they do develop scale buildup — and that buildup can cause serious problems if ignored.

Why Tankless Heaters Need Flushing

Tankless units heat water on demand by passing it through a heat exchanger. In areas with hard water, minerals (calcium, magnesium) deposit on the heat exchanger surfaces over time.

This is called scale buildup — and it's different from sediment in a tank, but just as problematic.

What Scale Does to Your Tankless Heater

• Reduces efficiency. Scale insulates the heat exchanger, forcing the unit to work harder.
• Limits hot water output. Restricted flow means less hot water at the tap.
• Triggers error codes. Many tankless units have sensors that detect flow or temperature issues caused by scale.
• Shortens lifespan. A scaled-up heat exchanger wears out faster and may need expensive replacement.
• Voids warranty. Most manufacturers require regular flushing to maintain warranty coverage.

How Often Should You Flush a Tankless Water Heater?

It depends on your water hardness:

• Hard water areas: Every 6-12 months
• Moderate water: Every 12-18 months
• Soft water or with water softener: Every 18-24 months

Check your owner's manual — most manufacturers specify a flushing interval. If you're unsure about your water hardness, your local water utility can tell you, or you can use a home test kit.

Signs Your Tankless Heater Needs Flushing

Don't wait for problems. But if you see these signs, flushing is overdue:

• Fluctuating water temperature — hot, then cold, then hot again
• Reduced hot water flow — weaker pressure than usual
• Error codes on the display — especially flow or ignition errors
• Unit cycling on and off — struggling to maintain temperature
• Longer wait for hot water — takes longer to heat up

Can You Flush a Tankless Water Heater Yourself?

Yes, but it's more involved than flushing a tank water heater.

What's Required

• Flushing kit: Includes a small pump, hoses, and a bucket (or you can buy components separately)
• White vinegar: Food-grade white vinegar is the standard descaling solution (4-5 gallons)
• Isolation valves: Your unit needs service valves installed to flush it — most newer units have them
• Time: About 45-60 minutes for the full process

The Basic Process

1. Turn off the gas or electricity to the unit
2. Close the hot and cold isolation valves
3. Connect hoses from the pump to the service ports
4. Submerge the pump in a bucket of white vinegar
5. Circulate vinegar through the unit for 45-60 minutes
6. Flush with clean water to remove vinegar
7. Reconnect everything and restore power

Where DIY Can Go Wrong

• No isolation valves: If your unit wasn't installed with service valves, you can't flush it yourself without modifications.
• Gas units: Relighting pilots and checking gas connections requires care. Mistakes can be dangerous.
• Warranty concerns: Some manufacturers require professional service to maintain warranty.
• Heavy scale: If it's been years since flushing, vinegar may not be enough — professional descaling solutions are stronger.

When to Call a Professional

Consider professional flushing if:

• Your unit doesn't have isolation valves installed
• It's a gas unit and you're not comfortable with gas appliances
• The unit has never been flushed (or not in 2+ years)
• You're seeing error codes or performance issues
• Your warranty requires professional service

A professional flush typically costs $100-$200 and includes inspection of the unit. Compared to a $1,500+ heat exchanger replacement, it's cheap insurance.

Tankless vs Tank: Maintenance Comparison

Factor	Tank Water Heater	Tankless Water Heater
Problem	Sediment buildup	Scale buildup
Flush frequency	Every 1-2 years	Every 6-24 months (depends on water)
DIY difficulty	Easier (drain valve)	Harder (pump + vinegar circulation)
Equipment needed	Hose	Pump, hoses, vinegar, bucket
Professional cost	$80-$150	$100-$200
Consequence of neglect	Reduced efficiency, tank damage	Error codes, heat exchanger failure

The Bottom Line

Tankless water heaters need maintenance too — just different maintenance. Scale buildup is the enemy, and regular flushing is the solution.

If you're comfortable with the process and have the right setup, DIY flushing works fine. If not, a professional flush once a year is a small price for a unit that can last 20+ years.

Related Guides

• Water Heater Sediment — Overview
• Can You Flush a Tank Water Heater Yourself?
• When to Call a Plumber