How To Flush A Tankless Water Heater

How To Flush A Tankless Water Heater


Water heaters can be a great option for those looking to save some money as water is heated on demand, rather than a set volume of water being heated in a vast tank. A tankless water heater means you only heat the water you use which is going to save a good amount of energy.

Manufacturers often recommend that a tankless water heater be flushed on an annual basis. This will keep the water clean and prevent excessive mineral build-up which is important to keep the water clean, especially if you live in a hard water area. Ever looked at your heater and wondered exactly how you do that? 

Don’t worry. We have put together this handy article to talk you through flushing out your tankless water heater with ease.

How To Flush

  1. Turn off the heater. This needs to be done with the electrical circuit at the breaker panel for electric heaters, or by turning the gas cutoff to stop gas from entering the heater unit.
  2. Take off the cover plate of the water heater unit to expose the wiring – this usually needs to be done with a screwdriver. You now need to check that there is no power in the heater. Hold a non-contact electrical tester against the wiring that enters the terminals. If this tester shows that there is still power in the system, carry on turning off the breakers or go to the main breaker of the house and turn it off. Replace the cover.
  3. Turn off the water supply that gets water into the heater and turn off the hot water valve that gets water out of the heater.
  4. Attach one end of a garden hose to an outlet for a sump pump or something similar, and connect the other end to the cold water valve. Attach another hose to the hot water valve. Be sure to tighten the hose couplers as much as possible to keep the water contained and not bursting out.
  5. Put the other end of the second hose into a 5-gallon bucket with the pump, and fill with 5 gallons of fresh white vinegar.
  6. Open the hot and cold water valves on the heater and turn on the pump in the bucket. This will mean that the white vinegar is circulating the heater.
  7. Let this system run for about 45 minutes to an hour, then switch off the pump and wait for all the vinegar to drain from the heater into the bucket. You can now close the cold water valve.
  8. Get rid of the vinegar that should now all be in the bucket, then put the hose with the unattached end into the bucket. You now need to turn on the cold water valve and let the water fill the bucket for about five minutes. This is to help get rid of any residual vinegar.
  9. Turn off the cold water after these five minutes and detach the two garden hoses from the valves.
  10. Turn off the hot water valve and remove the filter screen and rinse to remove any accumulations, before reinstalling. If you are unsure how to remove your screen refer back to the manufacturer’s instructions.
  11. Turn the hot water valve back on, then turn the cold water valve back on. Turn on a few hot water taps in the house to get rid of any air that is in the hot water pipes and turn off when the water is running smoothly.
  12. Check that all connections have been reattached properly so there are no leaks and then you can turn on the gas supply valve – if your heater has one – and then flip back the circuit breaker.
  13. Enjoy your clean, fresh hot water.

Frequently Asked Questions

When Do I Need To Flush My Tankless Water Heater?

As a baseline, you need to flush your tankless water heater once a year. This may be more often if you live in a hard water area and experience a lot of mineral build-up. Some tankless water heaters will indicate when the unit needs to be flushed, usually with a light that turns on, but there are some other signs your unit needs flushing too.

When Do I Need To Flush My Tankless Water Heater

How Do I Know When To Flush My Tankless Water Heater?

Some models will have an indicator system to let you know when you need to flush your system, but there are some other things to be aware of.

Weird noises: noises from the unit can be a suggestion of sediment build-up, meaning a flush is long overdue.

Colored water: if the water coming out of your taps is a weird color, it is likely that your tank has a mineral build-up but may also mean that the anode rod needs to be replaced. Either way, annual flushing will help keep you on top of both these issues.

Odd smells: funny odors coming from the water may mean that bacteria are growing in the unit and it needs to be flushed to get rid of any microbial cultures.

No hot water: this is a more obvious indicator that your heater needs to be flushed as no hot water is likely due to excessive sediment blocking the burner.

Can I Flush My Own Tankless Water Heater?

Yes, you absolutely can flush your own water heater with just a few tools and about an hour free. Just remember to work safely, make sure the power is turned off, and refer back to the manufacturer’s instructions if you are unsure. 

Why Use Vinegar To Flush A Tankless Water Heater?

White vinegar is used to flush a tankless water heater because it does a great job of sanitizing the system, as well as effortlessly removing any mineral build-up within the system or on the anode rod. It is also cost-effective, easy to get hold of, and completely safe for human consumption.

How Do I Get Rid Of Calcium Deposits In My Tankless Water Heater?

You need to flush the system a little more regularly if you are noticing calcium build-up in the unit, especially if you live in a hard water area. All you need to do is increase the flushing to once or twice a year, following the instructions set out in this article.

Fortunately, you do not need any additional equipment as vinegar – otherwise known as acetic acid – will easily combat any alkaline calcium deposits.

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