How to Flush Your Tankless Water Heater: Gas and Electric

Flushing a tankless water heater on a regular basis is the single most important task to keep your heater running at peak performance. Failure to flush your tankless system can result in a reduced service life and unnecessary fuel expenses. Although, we highly recommend having a professional check your unit annually, the flushing task is not too difficult and can save you money if you do it yourself.

Many of the maintenance tasks necessary to keep your tankless system running at peak performance can be completed by the homeowner. Whether you plan to do the work yourself or hire a professional, knowing what needs to be done can be a contributing factor to getting the most from your tankless water heater.

Preparing to Flush Your Tankless Water Heater

Flushing your tankless water heater is an important maintenance task to keep your system operating at peak performance, but before starting, you'll need to get your supplies together. In addition, you'll want to keep safety top-of-mind. Here's a few safety precautions:

• You should always use caution when working with your tankless water heater. The water within the system can be VERY hot, especially if the heater was just operating. 
• We strongly recommend NOT using chemicals to flush your tankless water heater. Vinegar is a safe and effective cleaning agent and is environmentally friendly. 
• Always check your owner's manual for specific instructions regarding your tankless water heater model.

Necessary Supplies

Both gas and electric tankless water heaters need to be flushed regularly, however it's important to note that the procedures are entirely different. Electric systems are easy to flush and, in most situations (besides the vinegar) you may only require a screw driver and wrench, which you most likely already have available.

Gas systems are more complex. Here's a list of what most gas tankless water heaters require to flush limescale:

Some manufacturers provide Isolation Valves with Service Ports for their for their gas tankless water heaters. Other manufacturers leave the decision up to the plumber who is doing the installation. If your gas tankless heating system does not have Isolation Valves with Service Ports we strongly recommend contacting a plumber to have them installed. They are a worthwhile investment that you won't regret.

One important note, these are only for gas tankless water heaters. Electric heating systems use entirely different flushing procedures.

Deliming Solution

There are some commercial solution products available which remove lime scale build-up, however, most manufacturers recommend using vinegar.  It's safe, environmentally friendly, inexpensive, and very effective.

Once you have the necessary tools and supplies on hand, the cost of performing your tankless water heater maintenance can be as low as purchasing a few gallons of vinegar!

How to Flush a Gas Tankless Water Heater

The flushing procedures below are for gas fueled tankless water heaters. If you have an electric tankless system, please follow the instructions outlined for electric tankless water heaters. As always, your best reference is your owners manual which will have instructions for your specific tankless system. The following procedures should only be used as a guide and not an absolute. 

The amount of time recommended to circulate the vinegar varies between manufacturers, but as a general rule of thumb, longer is better. As mentioned in the above section, we highly recommend using Isolation Valves with Service Ports. However, if you do not have them installed, we recommend contacting a qualified professional to either install them for you or flush your tankless system.

Flushing Procedures for Gas Tankless Water Heaters

Rinse the Vinegar from the Tankless after Flushing

1. Remove the unattached end of Hose #2 from the bucket and place it in a drain or outside.
2. Remove Hose #1 and CLOSE the Cold Water Service Port and OPEN the Cold Water Shut-off Valve. Do not open the Hot Water Shut-off Valve.
3. Once the Cold Water Service Valve is opened, fresh water will begin to flush out any remaining vinegar within the unit thru Hose #2.
4. Allow the water to run for 10 minutes, then CLOSE the Cold Water Shut-off Valve. This will stop the water from flowing thru the unit.
5. Once the water stops draining, remove Hose #2 and  CLOSE the Hot Water Service Port.

Clean the Inlet Filter

1. Remove the Cold Water Inlet Filter.  (All tankless water heaters have a Cold Water Inlet Filter. A few manufacturers also install a Hot Water In-Line Filter which should also be cleaned at this time.) 
2. Flush the Cold Water Inlet Filter with tap water to remove any sedimjavascript:void(0)ent that has collected within the housing and screen.
3. Securely replace the Cold Water Inlet Filter and OPEN the Cold Water Shut-off Valve. 
4. Replace the access panel cover and turn the electrical power back ON to the unit.

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How to Flush an Electric Tankless Water Heater

The flushing procedures below are for electric tankless water heaters. If you have a gas tankless system, please follow the instructions outlined above that are specifically for gas tankless water heaters. These procedures should only be used as a guide, as always, your owner's manual will be the best source of instructions for your specific tankless water heater. Please read why it's important to check your owner's manual here.

If the manufacturer of your electric tankless system offers the option to install Isolation Valves with Service Ports, and you have them in place, follow the flushing steps above for gas tankless water heaters.

Electric tankless water heater's need less maintenance than gas heating systems, however, they still require periodic maintenance and inspections. Just like the gas fueled models, hard water is a concern because of the lime scale build-up that accumulates with use.

Most manufacturers recommend annual flushing, but it should be noted that ultimately, the flushing frequency needs to be determined by the hardness of your water. The harder the water, the more frequently you should flush.

Flushing Procedures for Electric Tankless Water Heaters

1. Turn OFF the electrical power to the tankless unit at the breaker.
2. CLOSE the Cold Water Shut-off Valve. This will prevent water from entering the tankless system.
3. Drain the water from the tankless unit by opening the faucets inside your home.
4. CLOSE the Hot Water Shut-off Valve.
5. Remove the cover and double check that no electricity is reaching the unit by using a non-contact voltage tester.

Remove the Electrical Heating Elements

1. Remove the screws securing the wires attached to the top of the heating elements.
2. Use a wrench to loosen the hexagon brass top of the heating element. Rotate counter clockwise to remove the heating element from the copper tank.
3. Inspect the heating elements. If any are cracked, they should be replaced.
4. Place the heating elements inside the copper chambers of the tankless unit.

Fill Copper Chambers with Vinegar

1. Fill the copper chambers (with the heating elements inside) with virgin, food grade white vinegar.
2. Allow the vinegar to soak inside the chambers for 90 to 120 minutes
3. Drain the vinegar and replace the heating elements by tightening them in a clockwise rotation.
4. Using a screwdriver, secure the wires to the top of the heating elements.

Flush the Vinegar from the Tankless

1. OPEN the Cold Water Shut-off Valve and allow the water to fill the tankless system.
2. While the tank is filling, check for leaks.
3. Open the Hot Water Shut-off Valve.
4. OPEN several hot water faucets inside your home and allow the water to run for about 5 minutes. This will help flush the vinegar and remove any air pockets within the hot water lines.

Clean the Inlet Filter

1. Remove the Cold Water Inlet Filter on the cold water line.
2. Flush out any sediment and debris from the filter housing.
3. Securely replace the filter.

Importance of Checking your Owner's Manual

Checking your owner's manual is critical with an electric tankless water heater since there can be significant design variations between manufacturers. If not flushed properly, the tankless unit could be damaged and require service/repair.

Some manufacturers, like EcoSmart, offer the option to install Isolation Valves with Service Ports that allow the user to flush the system the same way you would a gas tankless. Installing the ports will require a larger investment upfront, but overall, the task will be simpler since you'll no longer need to remove the electric heat elements.

Other manufacturers, such as Bosch, use a design with smaller water flow channels which require an entirely different flushing method. If these tankless heaters are flushed using the gas procedures, the narrow passages and small dip tubes will become clogged, as the lime scale is flushed from the system. If this happens, professional repair will most likely be necessary.

Bosch offers an O-ring kit that can be purchased. Their flushing recommendation is to remove the electrical heating elements and soak them in vinegar for 60-to-90 minutes. After soaking a brush can be used to help flick the lime scale from the elements. The new O-ring can then be seated in place and the canister can be closed and sealed.

How Often Should a Tankless Water Heater Be Flushed?

Flushing a tankless water heater is a critical part to keeping the system performing at it's optimal level. Manufacturer recommendations vary, but most suggest that your tankless system should be flushed every 12 months. 

Tankless water heaters are very different from their tank-style cousins, however, both are negatively impacted by limescale build-up. Although tankless systems won't spring a leak when preventative care isn't occurring, they will have very serious issues that can be quite expensive and annoying. 

Impact of Hard Water

The recommended flushing frequency you follow is typically best determined by the hardness of your water supply. Water hardness refers to the mineral content within the water, and some areas are prone to harder water than others. Learn how to protect your tankless from hard water.

Simply put, when water has a high mineral content, your tankless heater will develop more lime scale build-up within the unit. Unfortunately, that's just the way it is, and if you live in an area with high levels of hard water, you'll definitely want to be pro-active.

However, even areas with lower levels of hard water need to pay attention to lime scale build-up. Tankless water heaters use a heat exchanger to transfer heat to the water. Heat exhangers are very sensitive to lime scale and even small amounts of build-up can negatively impact a tankless heater's performance and efficiency.  

Damaging Effects of Lime Scale Build-up

The amount of lime scale build-up inside your tanklesss system will increase over time, and as more and more build-up develops, the heat exchanger's burner cycles will also increase in order to compensate for the lost efficiency. 

Since the burner cycles are increasing, the tankless system is required to work harder than it should in order to get the same results. Besides lowering the efficiency of the heater, this will also negatively impact the service life of the heat exchanger and ultimately the tankless water heater.

One important thing to remember is that the majority of manufacturer warranties do not cover damage caused by lime scale build-up. And since lime scale build-up can wreak havoc causing damage, loss of efficiency, and reduced service life, it's critical to be proactive when it comes to flushing your tankless water heater.

Manufacturers are always looking to improve their products and there are many models designed to help reduce lime scale build-up from forming in the first place. Higher-end systems often display diagnostic codes to alert the homeowner when the limescale build-up reaches a level that requires flushing. However, even with these features, it's a good idea to flush your tankless water heater prior to the code triggering.

Keeping Your Tankless Water Heater Healthy

Although, this article dealt specifically with flushing a tankless water heater, we highly recommend that reading our other articles that cover caring for your system.

With the proper care, a tankless water heating system can have a service life of up to 25 years and save you money every month in utility expenses. These articles will help you get the most from your tankless heater:

• How to Keep Your Tankless Water Heater Running at Peak Performance
• How to Protect your Tankless Water Heater from Hard Water
• Sediment Filters: Why Your Tankless Water Heater Needs One

If you're a first time tankless water heater owner, you may want to consider hiring a professional to flush your system the first time. You'll be able to observe, ask questions, and take notes. It'll be money well spent and you'll have the peace of mind that you'll know exactly how to perform the tasks in the future.