The Classic Plumber — Knoxville's Plumbing Expert
Burst pipe, sewer backup, or no hot water? Call (865) 659-8294Human service Mon–Fri 7:30–5 · After hours, call anyway—we’ll confirm availability right away · AI guidance 24/7

Basic Tankless Maintenance Information

Annual flushing is essential for tankless units. Here's what's involved and why most homeowners hire a plumber for it.

Professional recommendedCaution — requires proper equipment

Tools You'll Need

Submersible pumpTwo hosesIsolation valve kit (if not already installed)Descaler (white vinegar or manufacturer-approved solution)Bucket

Step-by-Step

1

Turn off the unit

Turn off the power (electric) or gas valve. Follow the manufacturer's shutdown procedure.

2

Connect the pump and hoses

Connect a hose from the cold water isolation valve to a bucket with the pump. Connect another hose from the hot water isolation valve back to the bucket. This creates a loop through the heat exchanger.

3

Open the isolation valves

Close the main water valves and open the service valves on the isolation valve kit. This redirects water through the pump loop instead of the house.

4

Run the descaler

Fill the bucket with descaler (vinegar or manufacturer solution). Turn on the pump and circulate the descaler through the heat exchanger for 45–60 minutes.

5

Flush with clean water

Disconnect the pump, reconnect the hoses to a drain. Open the cold water supply and flush clean water through the heat exchanger for 5 minutes to remove the descaler.

6

Restore and test

Close the service valves, open the main valves, remove the hoses, restore power/gas, and test the unit.

WHEN TO STOP

  • If your unit doesn't have an isolation valve kit, stop — one needs to be installed first.
  • If you're not comfortable working with gas or electrical connections, call a plumber.
  • If the unit displays an error code, note it and call a professional.

WHEN TO CALL A PROFESSIONAL

  • Most homeowners hire a plumber for annual tankless flushing — it requires the right equipment and procedure.
  • Your unit doesn't have an isolation valve kit — one needs to be installed.
  • The unit displays an error code or isn't heating properly after flushing.
  • You're due for annual maintenance and want it done right.
Learn how this works

Still not sure what's wrong?

Ask our AI Plumber about this topic — your conversation will start with context from this guide so you don't have to start over.

Back to the Plumbing Guide

Helpful answers wherever you live. Local service when you need it.

In Knoxville or the surrounding area?

The Classic Plumber provides onsite plumbing service in Knoxville, Karns, Powell, Farragut, and nearby communities. Tell us what you noticed and we will confirm coverage and availability before scheduling.

Request local plumbing serviceCheck our service area

Outside our service area?

Our guides and AI Plumber are available to help you understand the problem wherever you live. For an onsite diagnosis or repair, contact a qualified plumber in your area. Local requirements and plumbing systems can differ.

Ask the AI Plumber

What to tell your plumber

Note when the problem started, which fixtures are affected, whether it is constant or intermittent, and any recent plumbing work. Photos of accessible equipment and model labels can help. Describe any checks you have already tried without opening equipment or entering an unsafe area.

Call Request Service Help