How to Flush a Tankless Water Heater: Ultimate Guide

A tankless water heater is a major upgrade for any home, offering endless hot water on demand, improved energy efficiency, and a compact design that saves space. Unlike traditional water heaters that continuously heat a stored tank of water, tankless units heat water only when needed, eliminating standby energy loss and reducing overall utility bills.

However, like any appliance, proper maintenance is crucial to keep your tankless water heater operating at peak performance. One of the most important maintenance tasks is flushing the system regularly to remove mineral buildup, sediment, and scale that can accumulate inside the unit over time. If left unaddressed, this buildup can cause a variety of problems, including lower efficiency, higher energy bills, water pressure issues, and even system failure.

What Happens If You Don’t Flush Your Tankless Water Heater?

A tankless water heater is designed to provide consistent hot water for years, but like any appliance, it requires proper care. Failing to flush your unit regularly can cause several problems, including:

Loss of Energy Efficiency

According to the U.S. Department of Energy, when properly maintained, tankless water heaters are 24%–34% more energy efficient than traditional storage water heaters. However, failing to flush your system allows scale buildup to insulate the heat exchanger, reducing heat transfer efficiency and forcing the heater to consume more gas or electricity. As a result, energy efficiency drops, increasing your monthly utility bills.

Reduced Hot Water Flow and Pressure

Mineral deposits accumulate inside the inlet and outlet valves, restricting water flow and leading to lower water pressure in sinks, showers, and appliances. If the buildup becomes severe, water may trickle out instead of flowing at the expected rate. Homeowners in hard water areas may experience this issue within six months if the heater is not flushed regularly.

Overheating and System Failure

Tankless water heaters use safety sensors to detect excessive heat levels. When mineral buildup blocks water flow, the system overheats, triggering error codes and automatic shutdowns. Ignoring these warnings can permanently damage the heat exchanger, requiring costly repairs or an early unit replacement.

Shortened Lifespan of the Unit

A properly maintained tankless water heater can last 20+ years, while a neglected unit may fail in as little as 10 years. The U.S. Department of Energy warns that excessive scale buildup can cut the lifespan of a tankless system in half, leading to premature replacement costs ranging from $1,500 to $3,500.

Increased Risk of Costly Repairs

If scale buildup damages internal components, repair costs can quickly add up. Professional flushing services range from $150 to $300, while major component replacements, such as clogged heat exchangers or malfunctioning sensors, can cost between $500 and $1,000. In severe cases, replacing the entire unit may be the only solution.

Fluctuating Water Temperature

One of the most noticeable effects of excessive scale buildup is inconsistent water temperature. Homeowners may experience sudden bursts of cold water during showers or delayed heating, as the unit struggles to maintain a consistent hot water supply. This issue becomes more frequent as the heat exchanger loses efficiency due to mineral insulation.

How Often Should You Flush Your Tankless Water Heater?

The recommended flushing frequency depends on several factors, including water hardness, household water usage, and the manufacturer’s guidelines. However, a general rule of thumb is:

If you live in an area with hard or very hard water, you may need to flush your tankless water heater twice a year or more. Installing a water softener or sediment filter can help reduce the frequency of descaling and extend the life of your unit.

Signs That Your Tankless Water Heater Needs Flushing

Even if you don’t track your water hardness levels, your tankless water heater will often give clear warning signs when it needs a flush:

• Water temperature fluctuates or doesn’t stay hot
• Decreased water flow or pressure from faucets and showers
• Unusual noises (popping or rumbling sounds) coming from the unit
• Error codes displayed on the heater’s control panel
• A noticeable increase in energy bills despite regular usage

If you notice any of these symptoms, your tankless water heater is likely struggling with scale buildup, and flushing should be done as soon as possible to prevent further damage.

DIY or Professional Flushing: What’s the Best Option?

Flushing a tankless water heater is a doable DIY project for homeowners who are comfortable with basic plumbing maintenance. It requires a few essential tools, such as a submersible pump, hoses, a descaling solution (or white vinegar), and a bucket. However, if you’re unfamiliar with the process or don’t have the right equipment, hiring a licensed plumber is a good option to ensure the job is done safely and effectively.

Many professional plumbing services offer comprehensive tankless water heater maintenance packages, including flushing, inspecting components, and checking for leaks or damage. This not only saves time but also helps identify any potential issues before they become costly repairs.

Tools and Materials Needed for Flushing a Tankless Water Heater

Proper preparation ensures an efficient and hassle-free flushing process. Below is a concise list of essential tools, recommended accessories, and flushing solutions.

• Submersible Pump – Circulates descaling solution through the heater.
• 5-gallon Bucket – Holds the cleaning solution during flushing.
• Hoses (Two 3/4-inch hoses) – Connect the heater’s service ports to the pump and bucket.
• Descaling Solution or White Vinegar (4 Gallons) – Breaks down mineral buildup.
• Adjustable Wrench & Pliers – Loosens and secures service port connections.
• Non-Contact Voltage Tester (For Electric Models) – Ensures power is off before maintenance.
• Phillips Screwdriver – Needed for access panel removal on some electric models.
• Towels or Rags – Prevents spills and water damage.

Optional Accessories

• Sediment Filter – Reduces debris entering the heater, minimizing scale buildup.
• Water Softener – Prevents calcium and magnesium accumulation, reducing flushing frequency.
• Isolation Valve Kit – Allows easier flushing without disconnecting the plumbing.

White Vinegar vs. Commercial Descaling Solution

• Vinegar is sufficient for regular maintenance (every 6-12 months).
• Use a descaling solution for severe buildup.

Pre-Flushing Setup

1. Turn off the power and gas supply.
2. Close cold and hot water supply valves.
3. Relieve pressure by opening a hot water tap.
4. Attach hoses to service ports.
5. Place pump in a bucket and fill with cleaning solution.
6. Ensure all connections are secure before starting.

Step-by-Step Guide to Flushing a Tankless Water Heater

Flushing your tankless water heater is essential for removing mineral buildup and restoring efficiency. This guide covers the step-by-step process for both gas and electric models, ensuring a thorough and effective flush.

Flushing a Gas Tankless Water Heater

1: Preparation

1. Turn Off Power and Gas Supply
   • Switch off the gas valve to stop the fuel supply.
   • Unplug the heater from its electrical outlet.
2. Close Water Valves
   • Shut off the cold water inlet valve to prevent new water from entering.
   • Shut off the hot water outlet valve to stop water flow to faucets.
3. Relieve Pressure
   • Open a hot water tap in your home to release pressure.
4. Attach Hoses to Service Ports
   • Connect one hose from the submersible pump to the cold water service port.
   • Connect another hose from the hot water service port to a 5-gallon bucket.
5. Prepare the Descaling Solution
   • Pour 4 gallons of white vinegar or descaling solution into the bucket.
   • Place the submersible pump inside the bucket.

2: Flushing the Heater

1. Turn on the Submersible Pump
   • Allow the cleaning solution to circulate through the heater for 45 to 60 minutes.
   • Observe the flow of liquid; discolored water indicates scale removal.
2. Turn Off the Pump and Discard the Solution
   • After flushing, switch off the pump.
   • Dispose of the used descaling solution safely.

3: Rinsing and Reassembly

1. Flush the System with Clean Water
   • Close the cold water service port and remove its hose.
   • Open the cold water inlet valve to flush fresh water through the system for 5-10 minutes.
2. Clean the Inlet Water Filter
   • Remove the cold water inlet filter and rinse any trapped debris.
3. Reconnect and Restore Operation
   • Close all service ports and remove hoses.
   • Turn on the hot water outlet valve.
   • Restore gas supply and power.
4. Test the Heater
   • Run hot water from a faucet to confirm proper operation.

Flushing an Electric Tankless Water Heater

1: Preparation

1. Turn Off Power at the Circuit Breaker
   • Verify with a non-contact voltage tester that no power is flowing.
2. Close Water Valves
   • Shut off both the cold and hot water supply valves.
3. Relieve Pressure
   • Open a hot water tap to release pressure.
4. Remove the Heating Element (If Necessary)
   • Some models require the removal of the heating element for thorough cleaning.

2: Flushing the Heater

1. Fill the Heat Exchanger with White Vinegar
   • Pour 4 gallons of vinegar directly into the unit’s copper heat exchanger.
   • Let the solution sit for 2 hours to break down scale deposits.
2. Drain and Rinse the System
   • Open the drain valve to remove the solution.
   • Flush with clean water for 5-10 minutes.

3: Reassembly and Testing

1. Reinstall the Heating Element (if removed).
2. Reconnect Power and Restore Water Supply.
3. Test by Running Hot Water to Ensure Proper Function.

Final Checks After Flushing

• Ensure the water flows properly with no pressure loss.
• Check for any error codes on the heater display.
• Inspect for leaks at service ports and water connections.
• If issues persist, consult a professional plumber.

Flushing your tankless water heater removes scale buildup, restores efficiency, and prevents future damage. Regular maintenance keeps your unit performing at its best, extending its lifespan and reducing energy costs.

Conclusion

Regularly flushing your tankless water heater is essential for maintaining its performance and preventing scale buildup. Over time, minerals can accumulate in the inlet and outlet valves, restricting water flow and reducing efficiency. By properly closing the hot water valve and using washing machine hoses to connect a submersible pump, you can circulate a descaling solution through your tankless heater and remove sediment buildup. Regular maintenance, including checking hot valves and ensuring your system is free of obstructions, will help extend the life of your heater and maintain strong water pressure. Additionally, keeping other plumbing systems, such as your sump pump, in good condition ensures your home’s water systems function efficiently. And that’s how to flush a tankless water heater.

For professional tankless water heater flushing, installation, or general plumbing services, Prestige Water Group is your trusted provider in Northern New Jersey. Our team is equipped to handle everything from hot water valve inspections to sump pump maintenance. Call us today at (973) 227-4740 or email info@pwgroupnj.com to schedule your service. Keep your tankless heater running efficiently with expert care from Prestige Water Group.

FAQs

Can I flush my tankless water heater myself?

Yes, you can flush a tankless water heater yourself if you are comfortable with basic plumbing tasks. The process involves turning off the power and water supply, connecting hoses to the service ports, using a submersible pump to circulate a descaling solution or white vinegar through the system, and flushing it with clean water. If your heater has significant scale buildup or you are unsure about handling the procedure, hiring a professional is recommended to avoid potential damage.

How often should a tankless water heater be flushed?

A tankless water heater should be flushed at least once a year to prevent scale buildup, but homes with hard water (high in minerals) may need to flush every six months. Regular maintenance ensures optimal performance, prevents clogging, and extends the unit’s lifespan. Installing a water softener can help reduce the frequency of required flushes.

What chemical do you use to flush a tankless water heater?

White vinegar (four gallons) is a common and natural descaling solution for routine maintenance, as it effectively dissolves mineral buildup without harming the system. For severe scale buildup, commercial descaling solutions like Flow-Aide or CLR are more effective, but they must be thoroughly rinsed out after use to prevent potential damage to internal components.