How to Flush a Rinnai Tankless Water Heater – An Ultimate Guide

Appliances involving water use require periodic maintenance. This is because they are prone to rust over time. As for the water heater, the core working principle revolves around warming up the water, which puts it at the highest risk of damage. 

The water that passes through the heater is not perfectly clean – it has minerals and dirt. These residues settle down at the bottom of the heater, deteriorating its power. 

Rated as one of the best tankless water heaters, Rinnai works its best to produce high-quality and effective heating solutions. If you own one, you must know how to flush a Rinnai tankless water heater for regular maintenance. Keep reading for details.

What Is Flushing?

Flushing means draining the old water or oil thoroughly to clean the machine from all kinds of sediments. It also involves replacing it with fresh liquid for better performance.

All the machines around us make our daily tasks more accessible. These machines also demand care in return. They require periodic maintenance to operate correctly and last longer.

Imagine driving a car without clean engine oil. It will work for the first few weeks without any problems, and then the symptoms will manifest. The performance of that car will start to deteriorate and gradually damage the engine, shortening its life.

The same situation arises for all the types of machinery, be it a heater or a ship engine – all require maintenance. The first step is cleaning. 

The most commonly used method of cleaning machines, especially water heaters, is — flushing. In flushing, you can clean those machine components that can not be cleaned with a cloth or hand. 

You can do this by completely draining the dirty liquid and sending in pressurized air or water to clear the sediments, such as mud or minerals. After cleaning the residues, you can fill the tank again with clean water or oil, whichever the machine needs. 

How to Flush a Rinnai Tankless Water Heater

Rinnai produces high-end water heaters. You can get more than twenty years out of them, but they require routine maintenance, which is easy to do yourself. If you plan to do it, this tankless water heater maintenance guide can come in handy.

The first step of maintenance is flushing the unit. This begs the question — how to flush a Rinnai tankless water heater? 

Rinnai has provided a brief official step-by-step guide to flushing the tankless water heater, but it might be confusing for first-time users to understand where to start and what supplies they should arrange before starting the process.

Read on to find a detailed breakdown of this process in a well-elaborated way, with its dos and don’ts and the importance of flushing your unit.

Handling Rinnai Tankless Water Heater

You can install Rinnai’s water heater inside and outside your home. Whichever option you choose, you have to follow the same flushing methods. However, the heater mounted indoors last longer.

These water heaters do not have a tank to store water and have to warm up the flowing water as soon as it passes through. This means they require enough power to heat the water immediately.

These tankless units have a massive gas engine inside to make this work. When this gas fires, it can build up a small scale inside the unit. The intensity of the scale depends on the hardness of the water.

When your water has excessive minerals, such as calcium and magnesium, these deposits will build up in the heater and its pipes. To clean this scale, you need to flush your unit. 

It’s recommended that you do it annually. Or if you have a water softener, you can go for around three years without flushing. 

If you neglect the maintenance routines, Rennai’s system will detect the lime buildup and show error codes like 00 or L0, indicating that it’s time to flush. Try not to drag it until the code appears – wash it periodically for longevity.

Preparing to Flush

kelaro tankless water heater flush kit

To start flushing, you will need three essential things – a submersible water pump, a hose kit, a five-gallon bucket, and a flushing fluid. You can use food-grade white vinegar if you don’t have access to the flushing fluid.

The first and most crucial step is to switch off the main power and unplug it. Electricity and water combined can be lethal. Also, cut off the gas connection to the heater, or if you have an electric unit, only the electric disconnection will do.

After electricity and gas, you have to turn off the water valve for both the cold water inlet and the hot water outlet. 

Rinnai’s tankless heaters have pre-fitted flush valves. It’s important to note that you must call your plumber if your heater doesn’t have a flush valve. Because without these valves, you can not flush your water heater yourself.

Setting up the Flushing System

Now that all the external connections are off, you can start flushing. Start by attaching the hoses to these flush valves. Attach the hose to the hot water side first, where the flush water will come out.

Then move forward with attaching the hose to the cold water side, which is connected to the submersible pump at the other end. This pump has enough power to drive the water through the unit under pressure.

The water bucket should have an adequate amount of water. You need to add a quart of flushing fluid for every gallon of water. Once you have connected both hoses and submerged the pump in flushing fluid, you are set to start flushing.

Start Flushing

Put the hose from the hot water side into the bucket to make the loop and roll the water back. Plug the pump in, and the water will flow through your heater and back into the bucket. Keep this loop running for 45 minutes or an hour.

Stop the pump after the desired time. At this point, all the debris inside the water heater has been loosened up. The final steps are critical – the loosened junk must be flushed out of the unit.

You can do this by plugging in the standard water and allowing it to flow through the system for two to five minutes. It will drag all the impurities outside. Ensure you only allow cold water to pass through. All other components of the heater should remain idle.

Putting It Back

Once you’re done, disconnect the hoses and remove the filter. You’ll find it underneath the unit – it filters the water from the heater. Clean the filter by blowing it or running it under the faucet, then put it back after it’s cleaned.

Now it’s time to turn the unit back on and reverse all the steps. Open the valves for both hot and cold water. Turn on the gas and plug in the heater. 

You are all set – the water heater is now flushed.

Importance of Flushing a Water Heater

Imagine having to shower in freezing weather without warm water – it sounds very uncomfortable. From cooking to laundry, hot water is essential to our daily lives.

As we discussed earlier, the water is not always clean. It brings various minerals and residues, such as calcium, magnesium, and iron, inside the water heater. These sediments will settle at the bottom of your water heater, slowing down its performance. 

It may also cause blockages in your water pipes, which could lead to a leak. Usually, the heaters have burners at the bottom. If the sediments settle at the bottom of your heater, it will have to make more effort to warm up the water due to residue blocking the heat.

Due to this, your heater consumes more gas than usual to warm up the water, increasing your gas bills. In addition, if you don’t flush your heater during periodic maintenance, it will eventually have pipe blockages or rust. The repairs will be costly.

Hence, you must flush your heater to prolong its life and performance, saving you from hefty repair costs or heater replacement. If your water heater is not heating up properly even after flushing it, you can check out other possible reasons.

If you feel your heater has rust and leak problems, you can follow a few steps to inspect these problems. In addition to that, you can also learn about other common issues with water heaters.


To prolong your water heater’s life and make it work effectively, you should keep up with periodic maintenance. You must flush your unit once a year to keep it going for more than twenty years.

If you own one, you must know how to flush a Rinnai tankless water heater. It takes only one hour of your time and a few tools to flush it and keep it going till another maintenance checkpoint.

With years of experience under his belt, Adam decided to put his water heater knowledge to work. By providing high-quality content and expert guides, Adam hopes to help anyone looking for expert water heater advice.