How to Flush a Tankless Water Heater (Step-by-Step Procedure)

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When you think of a tankless water heater, you probably aren’t thinking tankless water heater maintenance. Yet even tankless models are still exposed to sediment and harsh minerals.

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Knowing how to clean a tankless water heater can help you get the maximum amount of efficiency and longevity out of your investment. Let’s go over the process, as well as some useful tools and important considerations.

The Importance of Flushing Tankless Models

Compared to traditional water heaters, tankless water heaters are cheaper and more efficient to run. They also require far less maintenance or space to operate.

That’s why it’s important to know first why and when it’s needed, then how to flush a tankless water heater, be it gas or electric.

Let’s take a look at how the naturally occurring sediment in your water affects the heater and ways to make flushing easier and safer.

Tankless Water Heaters and Scale Buildup

Anyone who’s owned a regular water will be familiar with the need to flush the tank occasionally as a means of eliminating sediment. Tankless heaters are no exception.

Hard water contains minerals which can lead to lime scale build-up. Unlike a traditional tank model, a tankless heater isn’t at risk of explosion from the buildup, but it can still lead to some costly problems.

The heat exchanger is especially sensitive to scale. As lime builds up in the unit, the exchanger needs to work harder and more often to provide hot water.

This can significantly shorten the lifespan of this component, as well as cause a marked increase in your utility bills. To make matters worse, lime buildup isn’t covered in most warranties.

How Often Should You Flush a Tankless Water Heater?

how often to flush tankless water heater

As a general rule, your tankless water heater should be flushed once per year to maintain efficiency.

However, the hardness  of your water plays a part in this, and you may find it better to flush two to three times per year if your home’s water is especially poor quality. A sediment trap or softener can help with hard water to a point.

Isolation Valves

Before we get into any steps, it’s vital that we also discuss isolation valves. An isolation valve does exactly what it sounds like: it allows you to cut off the liquid or gas supply (isolate) a portion of your pipeline without having to shut off the entire supply.

The term is essentially synonymous with a shut-off valve. From your stove to toilet, a wide range of items in your house have some form of isolation valve installed for maintenance or emergency purposes.

These valves come in a variety of shapes, but they all boil down to two basic types: rotary movement (such as a ball or butterfly valve) and linear (ex: glove or diaphragm valves).

Depending upon the manufacturer and whether you had a technician do the installation, your tankless heater may or may not have an isolation valve installed.

In the event your heater lacks this feature, we strongly urge you to get one installed and hire a professional to flush in the meantime, as it can be a major safety factor when you’re performing a flush or other maintenance.

Using Pumps and Flush Kits

tankless water heater flush kit

One of your best friends for flushing a tankless water heater is a submersible pump. Unlike a tank heater, you don’t have large volumes of water to sweep up and pull the sediment out, so having a quality pump such as the 91250 from Superior Pump will save hours, if not days, of manual flushing.

Flushing will require a big bucket to put the pump in, as there’s no simple drainage option as there is with the average tank model, hence the need for a submersible model pump.

Another great tool is a good flush kit. These kits generally include everything you need, such as a pump, bucket, descaler, and hoses. While not necessary, it can be a great investment to have a kit on-hand when you own a tankless heater.

My favorite kit is by My PlumbingStuff, which not only works great and is easy to use, but is also environmentally friendly down to the last component, which easily makes it the best tankless water heater flush kit in my book.

How to Flush a Gas Tankless Water Heater

flush gas tankless water heater

Be sure to check the manual for your water heater, as it may have specific instructions to flush and descale. The three top brands for gas tankless models are Rinnai, Rheem, and Noritz. For your convenience, here are specific guides for each brand:

No matter which brand of gas tankless water heater you have, there are a few phases in flushing that must be done in order, but are all fairly simple to follow.

Phase 1: Preparation

  1. Disconnect the electrical power going to the heater.
  2. Remove the access panel and use a non-contact voltage tester to make sure there’s no longer power leading to the terminal.
  3. Locate and close the gas isolation valve.
  4. Locate the hot and cold shut-off valves and close them to prevent any water getting into the unit while you work.
  5. Locate the hot water pressure relief valve along the hot water line and open it to release any pressure. Be careful, as the hot water will be hot.
  6. There will be both a cold water service port and hot water service port. Connect hoses to both of these ports.
  7. Place your submersible pump into a five-gallon bucket.
  8. Connect the cold water hose to your pump.
  9. Place the free end of the hot water hose into the bucket.
  10. Pour four gallons of undiluted virgin food-grade white vinegar into the bucket. (Note: Noritz suggests one gallon of CLR to three gallons of water as a heavy-duty alternative)

Phase 2: Flushing

  1. Open the hot and cold service ports and turn the pump on.
  2. Allow the vinegar to circulate through the heater for a minimum of one hour (longer if the pump runs at less than four gallons per minute). This breaks down the scale and flushes out the sediment.
  3. Turn the pump off and dump the vinegar.
  4. Close the cold water service port and remove the attached hose
  5. Open the cold water shut-off valve to flush out any remaining vinegar through the second hose into your bucket for about ten minutes.
  6. Close the cold water shut-off valve.
  7. Allow the water to finish draining and remove the second hose.
  8. Close the hot water service port.
  9. Remove the cold water inlet filter and (if present) hot water in-line filter.
  10. Flush the filters at a sink to remove any trapped sediment.

Phase 3: Cleanup

  1. Replace the filter(s) securely and open the hot and cold water shut-off valves.
  2. Replace the access panel.
  3. Open the gas shut-off valve.
  4. Reconnect the electrical supply.

How to Flush an Electric Tankless Water Heater

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Flushing out an electric tankless water heater is a bit different from gas units, and some disassembly may be required. Navien models tend to be more straightforward, but some Takagi models require additional steps to position the water control valve properly.

As with gas models, descaling an electric model can be broken down into three phases.

Phase 1: Preparation

  1. Shut off the electrical power at the breaker.
  2. Close the cold water shut-off valve.
  3. Open your faucets to allow any water to drain from the heater.
  4. Close the hot water shut-off valve.
  5. Remove the access panel and use a non-contact voltage tester to make sure there’s no longer power leading to the terminal.
  6. Remove the screws which secure the wires to the heating elements.
  7. Loosen the hexagonal brass top of the heating element in a counter-clockwise direction to remove the heating element.
  8. Inspect the heating elements for any signs of damage. A cracked element should be immediately replaced.
  9. Store the heating elements in the cooper chambers of the heater so that they can also be cleaned.

Phase 2: Flushing

  1. Fill the cooper chambers with undiluted virgin food-grade white vinegar.
  2. Allow the vinegar to sit in the chamber for 90 minutes to two hours. This will break up any scale on the elements.
  3. Drain the vinegar.
  4. Replace the heating elements and re-secure the wires.
  5. Open the cold water shut-off valve and let the tank fill.
  6. Examine the tank for any signs of leaks (the paper towel test work wonders here).
  7. Open the hot water shut-off valve.
  8. Turn on several hot water faucets and let the water run for about five minutes to flush out the vinegar and clear any air pockets from the line.

Phase 3: Cleanup

  1. Turn off all hot water taps.
  2. Close the cold water shut-off valve.
  3. Remove the cold water inlet filter and flush it under a cold tap to remove any debris.
  4. Replace the filter securely.
  5. Close the access panel and restore power at the breaker.

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