How to Flush a Tankless Water Heater on Your Own in Just 8 Steps

To start, you’ll want to shut off the red and blue isolation valves—red for the hot water and blue for the cold water. Place a bowl or bucket beneath the valves in case any water leaks out. Try to open the valves with your hands, but you can use the channel lock pliers if they give you any trouble. The valves are shut off when they are positioned perpendicular to the pipe and on when the handles run parallel to the pipe.

Shutting off the cold water valve prevents more water from coming into the water heater as you’re cleaning it, while the hot water valve will prevent the distilled white vinegar or other cleaner from flowing from the water heater to your home’s fixtures and appliances.

With these water lines shut off, you’ll also need to shut off the gas line to prevent the water and cleaner from heating up while you’re working. To turn off the gas supply to the water heater, turn the gas valve about 1/4 of a full turn, until it is perpendicular to the supply pipe.

If the valves have caps, make sure to remove them to attach the water hoses to the isolation valves. Your water heater flushing kit typically includes hoses, or you can use washing machine hoses. Use your pliers to tighten the connections, but be careful not to overtighten. Be firm, but don’t overdo it.

Once you’ve confirmed the hoses are tightly secured to the isolation valves, you’ll connect the other end of the cold water hose to the circulation pump. This device should be included in a tankless water heater flushing kit, or you can purchase it separately. Place the circulation pump into your 5-gallon bucket.

Wondering about the hot water hose? The other end of that will go into the 5-gallon bucket as well.

Pour at least two gallons of a 1:1 mixture of water and vinegar or store-bought water heater cleaner into the bucket. The circulation pump should sit completely below the level of the liquid, as should the end of the hot water hose. If not, you risk burning out your pump.

Open the valves and start the circulation pump to begin flushing the system. For vinegar, let the water and vinegar circulate for one to two hours. For cleaning agent, flush the system based on the time recommended on the label, usually 45 to 90 minutes.

Turn off the circulation pump and shut off the water valves as you did at the beginning of the flushing process. Disconnect the hose from the cold water valve, then turn this cold water valve back on. Keep it on for about 10 minutes to rinse away any remaining vinegar or cleaner. Once this process is complete, you can turn the cold water valve back off and disconnect the hot water hose.

Now, you likely have a bucket of dirty vinegar and sediment on your hands. You can safely dispose of vinegar down a drain or even outside without contaminating community water sources, but if you use a store-bought cleaner, follow the manufacturer’s instructions for disposal. Make sure to rinse out the bucket thoroughly after disposing of the vinegar or commercial cleaner.

Place the service port caps back over the isolation valves if you had to remove them earlier. Turn on the isolation valves and then the gas supply valve, all of which should now be sitting parallel to the pipes. As a final step, turn on a faucet and let it run for one to five minutes to flush out any possible remaining vinegar or cleaner. Do a quick check for leaks, and then call this project done!