How to Flush Your Tankless Water Heater: A 7-Step Guide

by Mark Soto
Residential Condensing Hybrid Tankless Water Heater

Flushing a Tankless Water Heater at a Glance

  • Step 1: Turn of isolation valves and gas
  • Step 2: Connect hoses
  • Step 3: Attach hoses to circulation pump
  • Step 4: Add cleaning agent
  • Step 5: Open valves
  • Step 6: Flush cleaning agent
  • Step 7: Reconnect

One of the most important maintenance tasks you can do for your water heater is to flush it. Sediment like calcium and other minerals can build up inside the tank over time, causing it to erode, which shortens its lifespan. To deal with this problem, you need to flush it to get rid of these pesky minerals and other unwanted debris.

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Whether you hire a professional or do it yourself, flushing your tankless water heater is something you need to add to your list of yearly maintenance tasks. By doing so, you can improve your water heater’s efficiency and delay an expensive water heater replacement down the road.

Why You Need to Flush Your Tankless Water Heater

Mineral sediment made up of calcium and magnesium can build up inside your water heater over time and disrupt efficiency as well as reduce its lifespan. Mineral buildup can also cause loud, unpleasant noises to come from the water heater. When you flush a water heater, the cleaning agent used helps remove this buildup in the process and restores the machine.

Water heaters aren’t cheap by any means, so by performing routine maintenance like flushing them, you’ll help increase your machine’s longevity.

How Often Do You Need to Flush Water Heaters?

You should strive to flush your water heater at least once a year at the minimum. This will keep things running smoothly and ensure it doesn’t need replacement before its time. If the area you live in has hard water, you’ll want to flush it even more often.

DIY or Hire a Professional?

With the right tools and knowledge, you can certainly flush a tankless water heater by yourself. In fact, there are water heater flushing kits you can buy online for the job, which come with everything you need. They include things like a circulation pump, hoses, a bucket and a cleaning agent.

However, this is not a project where you can put caution aside. If you’re not confident working with tools, it’s best to leave it up to a professional. Depending on where you live, you can expect to pay anywhere between $150 to $250 per flush. Assuming you only do it once a year, the yearly costs of flushing are minimal compared to the costs of replacing a water heater. So, in the end, it’s a good long-term investment.

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How to Flush a Tankless Water Heater: Step By Step

Step 1: Turn Off the Isolation Valves and Gas

The first step is to turn off the isolation water valves that connect to your water heater — a blue inlet valve for the cold water and a red outlet valve for the hot water. A valve is turned off when it’s perpendicular to the pipe. If the valve is in the same direction as the pipe, then the valve is turned on. Always double-check that the valves are closed. You also need to turn off the gas valve by following the same steps above. You don’t need to turn off the power to your water heater.

Step 2: Connect the Hoses

Connect the hoses to the inlet and outlet valves. There might be a service port cap on both valves you need to remove before connecting them. If you didn’t buy a flushing kit that already comes with water hoses, you could always use water machine hoses as well. Connect the hoses to the vales and use pliers to ensure that the hose and valve are watertight. You don’t want the water to start leaking when you’re in the middle of flushing.

Step 3: Attach Hoses to the Circulation Pump

Now you need to use the circulation pump that came with your water heater flushing kit. Attach the other end of the inlet hose to the circulation pump and place it in the bucket. You only need to attach the inlet hose to the discharge side of the pump. For the outlet hose, you can simply place it in a bucket since the system you have set up will ensure the cleaner goes through the structure and cleans off any magnesium and calcium.

Step 4: Add Cleaning Agent

Once your system is all set up, simply add the cleaning agent to the bucket. The bucket should have at least one gallon of water in it before beginning the process.

Quick tip: If you don’t have a cleaning agent, you can always use vinegar. However, you will need to use at least 2 to 3 gallons of vinegar and let it circulate longer.

Step 5: Open the Valves

At this point, you just need to open up the water valves to let water flow through them and start the pump. You need to let everything circulate for at least 45 minutes and up to an hour and a half. The instruction booklet that comes with the cleaning agent will tell you how long you should let things circulate, but plan on at least an hour. If you’re using vinegar, you need to let it run even longer — for at least an hour and a half to 2 hours.

Step 6: Flush Out the Cleaning Agent

After the time has passed, turn off the pump to stop the circulation. Then turn off both valves and disconnect the inlet valve hose. You now want to flush out any remaining cleaning agent or vinegar. To do this, keep the hose connected to the hot water outlet valve and turn on the cold water inlet valve. This will push water up the system and flush out everything through the hose on the hot water side. Let the water flush for 5 to 10 minutes, then turn off the service port and the cold water inlet valve again. Finally, simply remove the remaining hose.

Step 7: Reconnect Everything

The tankless water heater has now been flushed, so now you can put everything back the way it was. Plug the service port caps back in and then turn on both water valves and the gas valve.