Flushing your water heater properly will remove rust sediment that settles at the bottom of your tank. This sediment is created from the heating process in which the iron separates from the water in the form of sludge. This sludge falls to the bottom of the tank and can show up in the water as a yellow discoloration. Keep in mind the flushing has to be done at least every six months to a year or as required.

If you have a power vented tank, that would be a tank with a motor on the top, unplug it from the wall receptacle. If the water heater has a metal chimney, turn down the temperature to vacation on the gas control. This will prevent the burner from coming on but does not turn off the pilot.

Turning down the temperature control to vacation or its lowest setting, will ensure no fuel being wasted during the flushing process. On a regular water heater, the control is located on the front of the water heater towards the bottom. Most power vented water heaters have their temperature control hidden and access is not required for flushing. Do not turn off anything else including the water supply.

We are going to use the town or city water pressure to do the work for you. Attach a garden hose to the shut off valve at the bottom of the tank. It can be identified to look like a tap. Place the other end or the hose in a white laundry tub. If a white laundry tub is not available, place something white in the tub or where you are draining your flushed water too so when the water runs over it you can compare the colors.

Turn on the drain valve approximately one quarter turn where the hose is attached at the tank and watch the water coming from the hose into the sink or draining area.  When the water runs clear, turn the valve open a little more to increase the flow and once again watch the water coming from the hose till it runs clear. Continue this process until the water is clear.

Unfortunately this is a slow process but necessary. The process can take from 10 minutes to 45 minutes to complete. This will be based on the amount of sediment in the tank. If you turn the water on full it will just stir up the rust sediment in the water heater and make a complete mess. It will take approximately 72 hours to for the tank to settle down and then repeat the process properly.

Close up the valve snugly but do not force it. Most of the valves are made from nylon and can crack very easily leaving a possibility of a flood in the basement. If this unfortunate situation occurs, shut off the water supply to the tank. Open a hot water tap in the house somewhere to relieve the pressure and drastically reduce the leak. 
If you own your tank it is advisable to replace the drain valve with a brass shut off valve to prevent any future problems. If your tank is rented or leased and the valve leaks, contact your supplier for repairs.

Please watch this short video for a more illustrated explanation.