Step by step guide: How to flush a tankless water heater for non-plumbers

Do you have a tankless water heater (also known as an on-demand water heater)? These tankless models can last 20 years if they have proper maintenance.

A tankless hot water heater needs to have regular maintenance, meaning they need to be flushed out every 12 to 18 months, depending on the hardness of the water in your area. If you have hard water you will likely have mineral deposits and it’s better to do routine maintenance every 12 months.

I tried for several months to get the professional plumber that installed my tankless system to come back and flush it for me. He wasn’t interested, so I asked google. Turns out this project is something anyone can do with just a few tools and about an hour.

I’m going to share the process step by step. Before you start you have to make sure that your tankless heater has a flush kit installed. This is a set of service ports or valves that are below the water heater. They have caps on them, most of the time, and they are specifically designed to allow you to flush your unit.

If you have soft water or a water softener built into your house you don’t have to flush your unit as often as there will not be as much mineral buildup.

Supplies You Will Need

A 5 gallon bucket
4 gallons of vinegar (the vinegar at Target was less expensive than Home Depot)
2 supply hoses (called a 2-pack fill hose, 3/4 inch in the washing machine area)
A submersible sump pump (these are much less expensive on Amazon than Home Depot)

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If you want everything in one package for convenience you can buy a “Flow Aid kit” that contains all the necessary supplies.

Parts Identified

Before I show you how, let’s get clear on what’s what.

Here’s a step-by-step guide to the process. The water heater I have is gas, but the flushing process will be the same for an electric tankless water heater, with one fewer steps.

The first step is to turn the power off to the water heater at the breaker, even gas units have electric power. You will need a working power source nearby, so this might require some testing if they are on the same breaker.
Turn off the gas line. This might seem obvious, but it wasn’t to me. The off position is where the handle is perpendicular to the pipe vs parallel.
Turn the cold and hot water valve to the off position, again perpendicular to the pipe.

Remove the flush valve caps (it’s possible that water inside the unit will run out). Connect the two water hoses to the water valves, one for hot and one for cold. These valves are designed to make flushing the unit easy.
Attach the hose that goes to the cold side to the pump. Put the submersible pump inside the 5-gallon bucket. Drop the hose that is attached to the hot side into the bucket (not attached to anything). Fill the bucket with 4 gallons of white vinegar, this essentially functions as a descaling solution. Plug the pump in. The pump circulates vinegar through the heat exchanger.
Open the flush valves, it can be a bolt that turns or a regular valve like this.

Turn the pump on and let it run for 45 minutes. If your pump has a ball on it, you have to make sure that ball is higher than the pump or it will shut off.
When the time is up, turn off the pump. This next step runs water through the system to flush out the vinegar. Close the cold water valve. Disconnect the hose that’s hooked up to cold side only. Turn the cold water supply on (not hot). Pull the red hose out of the bucket and you will see that clean water is now flushing through the water heater. Let that run for 1-2 minutes. Turn water back off.
Clean the small water filter that’s under the unit. You can either pull it out with your fingers or a small long nose pair of pliers (or tweezers). It’s a very small stainless filter. Rinse out any debris in the filter and pop it back in.

You are done. The only step left is to close the flush valves (isolation valves), then turn on the cold and hot water (parallel position), turn on the gas and plug the unit back in.

It’s a good idea to do this every 12 to 18 months. With routine tankless water heater maintenance your tankless unit can last 20 years, much longer than traditional water heaters.

I found a great video tutorial, but it was hard to keep pausing and rewatching it so I thought photos might be helpful too.

Before you go, if you liked this post read more about whether tankless water heaters are really worth it.