How To Flush A Tankless Water Heater? (9 Step Guide)

What You Need To Know About Flushing Tankless Water Heaters

Descaling a tankless water heater is performed by circulating distilled white vinegar or a descaling cleaner through the tankless water heater for 30-min to 1-hour. A small circulation pump and a bucket is required to circulate the cleaning solution as well as flush valves that have been previously installed.

The Flush Valves

If your tankless water heater doesn't have flush valves, then the water hoses can't be connected to the tankless unit for descaling. After shutting off all water and gas valves, you will need to connect two water hoses (washing machine hoses will work) to the flush valves, and then open the flush valves.

Circulation Pump & Bucket

The small circulation pump (in the bucket) will pump the cleaning solution into the tankless water heater, and it will come out of the hot water side through the flush valve—and back into the bucket—creating a closed cleaning loop.

It is recommended to flush your tankless water heater at least every year, but you can go 3-4 years if you have soft water (low mineral content). If you don't flush your tankless water heater regularly, there is a risk of corrosion from scaling, which is mainly calcium and magnesium deposits on the heat exchanger. For more details, keep reading...

Tools And Supplies Needed To Clean Tankless Water Heaters 

• small channel lock pliers
• small circulation pump
• washing machine water hoses/lines
• bucket
• vinegar or descaling cleaner

Read Also: What Are The Best Condensing Tankless Water Heaters?

9 Steps For A Tankless Water Heater Flush

Step 1 - Unplug Tankless Water Heater

For most households, you will want to unplug the tankless water heater when you are flushing it.

There is also a safety hazard if the tankless water heater is plugged in while doing a flush. If a hose isn't properly connected, or if someone is careless, you can easily get scalded.

Simply unplug the unit from the outlet or you can also shutoff the electrical breaker.

Step 2 - Shut Off Isolation Valves

The next step is to shutoff the water going to and from the tankless water heater.

There should be two small isolation valves underneath the tankless unit that you can turn perpendicular to the water line to close it. If the tankless heater is gas-fired, you will also want to shutoff the gas valve, which should be right next to the water shutoffs.

Read Also: How To Adjust Temperature On Rinnai Tankless Water Heaters?

Step 3 - Connect Water Hoses

Now it's time to connect the rubber (or steel braided) water hoses to the tankless water heater. There may be a screw cap on the flush valves which will need to be removed first.

At least one of the water hoses should have two threaded female ends. The hose with two female ends should go onto the inlet flush valve which is the cold side, or water going IN. The other hose should go on the hot side or outlet flush valve.

Make sure the connections are tight, and that they won't leak.

Before turning on the flush valves, put at least the outlet hose (hot side) into the bucket because the tankless water heater may drain completely of the water that is inside.

Step 4 - Open Flush Valves

After you have connected the water lines, and with at least the outlet hose into the bucket, you can open the flush valves.

You may need the channel pliers to open the valve. Turn the valve parallel with the water line to open it.

Water should be draining from the outlet valve and into the bucket, draining the water that was left in the heater.

Read Also: What Are The Best Tankless Water Heaters For Cold Climates?

Step 5 - Connect Inlet Flush Valve Hose To Pump

Once you have connected the water hoses to the flush valves, you will want to connect the inlet/cold flush valve hose to the discharge/outlet side of the circulation pump.

The 'inlet' port of the circulation pump will not have a hose connected, and it should have a filter over it. So again, the outlet port of the circulation pump is connected to the cold water hose or the inlet flush valve.

The hot water side or outlet flush valve hose will just be dropped into the bucket, creating a closed loop system.

So the water gets sucked into the circulation pump, gets discharged into the connected hose, which then goes into the cold water side of the tankless water heater. Then the water goes out of the hot water side of the tankless unit, and then gets discharged back into the bucket.

Step 6 - Pour Cleaning Agent Into Bucket

For this step, you can use either professional descaling cleaner and water or you can opt for the much cheaper white vinegar option. Since vinegar is a mild acid solution, it is an inexpensive and safe way to descale your tankless water heater.

The main downside between a professional cleaner and vinegar is that vinegar can take twice as long to clean the tankless water heater. Homeowners probably have the patience and time, but contractors likely do not.

Flushing a tankless water heater with vinegar will require it to circulate in for at least an hour, and you can go longer if you wish. For most descaling cleaners, you will only need to wait about 30-minutes.

For vinegar, you should use between 1-4 gallons of vinegar, and one gallon is probably the minimum. You want the vinegar to at least cover the inlet port of the circulation pump. With pro descaling cleaners, you will need to follow the instructions on the container, but you will likely need to mix it with at least a gallon of water.

Read Also: What Are The Best Outdoor Tankless Water Heaters?

Step 7 - Flush The Unit With Clean Water

After you have circulated the vinegar for at least an hour through the tankless heater, you will want to flush out the unit with clean water.

Unplug the circulation pump, and then turn off the cold water flush valve, and turn on the cold water isolation valve. This will circulate fresh water through the tankless water heater, and it will be dumped into the bucket.

Once the bucket has filled up with water, turn off the cold water isolation valve, and dump the water onto the ground or into a sink. I recommend doing this process at least a few times, but don't dump out the water the last time because it will be used to clean the pump.

Step 8 - Flush Pump With Clean Water

After you clean the tankless water heater with fresh water at least a few times, I recommend that you do the same thing with the circulation pump. If you leave acidic vinegar in the circulation pump, it will corrode and lower its life expectancy.

With the bucket filled with water, simply disconnect the cold water flush valve hose from the tankless unit, and point the end into the bucket.

Keep the other side of the hose connected to the pump, so that when you plug the pump back into the outlet, it will discharge the water back into the bucket. Just run the pump for a few minutes to remove all of the acidic vinegar.

And it is also a good idea to make sure both water hoses are completely clear of vinegar as well which can cause the metallic ends to rust.

Read Also: What Are The Best Point Of Use Tankless Water Heaters?

Step 9 - Open All Valves And Plug It Back In

After you have cleaned the pump and tankless water heater with fresh water, you can disconnect the other flush valve hose, and turn all of the valves back on.

And don't forget to open the gas/propane valve if you have one.

You can plug the tankless water heater back in, and turn it on.

Final Thoughts On How To Descale Tankless Water Heaters

Most homeowners shouldn't have any trouble descaling their tankless water heater themselves, but if you have any doubts, hiring a qualified plumber to flush it for you is the way to go.

The circulation pump will need to be from 1/6-HP to 1/4-HP and it will last many years if make sure to clean it after running vinegar through it.

The other main component is a set of washing machine hoses. And don't forget to clean these hoses of vinegar or cleaning solution as well.