How to Flush a Hot Water Heater and Prevent Problems
(updated 2020)

How to flush a hot water heater and how flushing helps to keep the heater’s performance high, remove the sediments and gunk, bad odor and bacteria that causes it.

Periodic tank flushing is necessary and recommended by every manufacturer (Rheem, AO Smith, Bradford White and other) and for every type of water heaters; tankless, tank type, electric, gas and solar.

A guide on how to flush a hot water heater guide will also help you with the chlorination process to clean the heater from the rotten egg odor, discolored water condition and eliminate bacteria.

In this article:

  1. Why and when to flush a hot water heater
  2. Instructions on how to flush electric and gas water heater
  3. How to flush with chlorine
  4. How to flush with hydrogen peroxide

Why and when to flush a hot water heater

Image by Jerzy Górecki from Pixabay

Perform flushing because hot water is rusty or brown in color. The brown color is usually the sign of the material buildup inside the tank due to rust from plumbing, water softener resin, sand and clay sediments, and other earth materials. If the home plumbing is plugged into the water mains, usually constructed of steel or cast iron pipes, there is a lot of rust coming into the tank.

Perform flushing because of the rotten egg smell. The rotten egg odor is actually a hydrogen sulfide odor caused by bacteria. The only way to control the smelly water is by eliminating the bacteria.

Water heater flush is necessary to remove the sediment and lime build-up. Lime and sediment build-up are the major causes of the slow and inefficient water heating, rumbling and pounding noise and that is why you should use the softener, sediment cleaner or delimer solution, in combination with tank flushing.

Flushing also increases the life of your water heater.

When is the best time to flush

If you are asking yourself how often to flush a hot water heater, simply follow the manufacturer recommendation which is usually once every 6 months to one year.

Flushing schedule depends on the local water condition also. If the water is hard and it contains more minerals, you may need to flush more often. The minimum requirement is to drain 1 to 2 gallons from the drain valve monthly.

Required tools for the DIY project

  1. Adjustable pipe wrench
  2. Screwdriver
  3. Hammer
  4. Teflon tape
  5. Garden hose
  6. Shop-Vac
  7. Ball valve
  8. Fittings

Flushing electric and gas water heater - Step by step guide

  • If using the gas water heater turn the gas control valve to the OFF position. You can also close the valve on the gas pipe that supplies the heater.
  • If using the electric heater, turn the electricity OFF on the circuit breaker (breaker box).
  • Turn OFF the cold water supply to the heater, on the main shut-off valve (usually installed near the cold water inlet). It can be recognized as the valve with the lever (ball valve) or with the wheel (gate valve).
  • Open a hot water faucet to prevent the vacuum from forming in the heater while draining.
  • Drain the heater by connecting one side of the garden hose to the drain valve, and place the other end to the floor drain or outside. You can speed up the process if you open the TPR valve, located in the upper part of the unit and placing the bucket under to catch the water.

    The water should be running from the relief valve, and if not (if it is clogged), replace it with the new one with the same specs. If the purpose of draining is to remove the sediments, drain it until the water runs clear with no debris. If the situation is worse than you expected, you might have to drain it completely.
  • Turn back ON the cold water into the heater.
  • Allow water to run through the heater and out of the drain valve. Flush the heater for five or ten minutes.
  • Close the drain valve and keep the tap open while refilling the tank heater.
  • Once the tank is full of water, open the pressure relief valve to bleed the air.

Next, use the instructions found on the unit's label to light the pilot, or resume the power. It is essential to have the tank full of water, especially on electric models, because if the heating element is exposed to air, it burns out easily.

Alternatively, you can watch the video guide ...

Flushing with chlorine

Flushing the water heater through the drain valve provided at the bottom of the heater, with the appropriate dissolver like chlorine is one way to control and eliminate bacteria from the heating system. Below is the proper procedure for chlorinating a heater, recommended by the Bradford White manufacturer:

  • Turn off the water, power and/or gas supply to the heater.
  • Use the drain valve located at the bottom of your heater and drain several gallons.
  • Remove the anode rod.
  • Pour a 1/2 to 1 gallon of bleach into the water heater through the hot water outlet opening.
  • Install the new anode rod or bring the old one back if it is still functional.
  • Re-connect the hot water supply line to the outlet on the heater.
  • Turn on water supply and draw water to every hot water fixture in your home until the chlorine smell is detected. Keep in mind that all plumbing lines must receive the treatment.
  • Once the chlorine odor is noticed, turn off the faucets and allow bleach to sit in the heater and plumbing lines for a minimum of 3 hours, a full day is desired.
  • Turn on and draw water at each tap in your home, to flush all chlorine from the piping, till the odor is no longer present.
  • Turn on the power and/or gas supply to the heater.

Water softeners, long periods of no water movement or if using a well supply, are all the potential causes of the bacteria presence. This is why shock-chlorination of the heating system is recommended. Combine the above process and flush hot water heater often if you have a heavily infected system.

Flushing also gives you an opportunity to check out some of the elements; an anode rod and drain valve. If the anode rod has been consumed, and it doesn't perform as before, replace it.

How to flush a water heater with the hydrogen peroxide

  • If the water heater is electric, turn the electricity on the breaker box to OFF, and do the same thing if the unit is gas-powered, or simply turn the gas control valve pilot position.
  • Open the nearby hot water tap and TPR valve.
  • Drain enough water to provide space for hydrogen peroxide, or below the TPR valve level.
  • If the water heater has a capacity of 40 gallons, add 1-2 pints of 3% hydrogen peroxide. If it is a different size, change the amount accordingly. Pour the peroxide through the opening of TPR valve, anode rod or the water outlet if possible.
  • Open the cold water inlet valve to fill the tank and close it.
  • Leave the hydrogen peroxide for a few hours in the tank’s water.
  • Open the cold water valve again and hot water tap so the hydrogen peroxide can flush the supply lines.
  • Drain all the water and peroxide from the tank (including debris).
  • Close the drain valve.
  • Fill the tank with water thoroughly.
  • Repeat flushing through the pipes and drain water again, if necessary (if for example smelly water is still present or water is not clear).
  • Fill the tank with water (drain valve is closed).
  • Purge the air from the tank with the open hot water faucets. Also, use the pressure relief valve to bleed the air.
  • Resume the power and heating operation.

Note: You can also use vinegar, even apple cider vinegar for flashing, in the amount of one gallon.

Most of the water heaters have the plastic drain valve factory installed, so while flushing the unit debris can clog the valve and cause leaking due to improper seating. Replace it with the ball valve.

Related articles

How to maintain a tank-type water heater
Maintaining an electric water heater
How to drain a water heater
How to flush a water heater
How to extend the life of a water heater
How to winterize a tankless
Cleaning tips
How to vent a gas water heater
How to improve water heater safety at home
How to prevent water damage and avoid costly repairs
How to protect a water heater from corrosion

› How to flush a water heater

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