Disinfecting the Hot Water Supply, Step-by-Step

Excerpted from “Shock Chlorination: Disinfecting the Hot Water Portion of Household Plumbing,” by Mark Walker, et al. See full citation below. 

Gazette Introductory Note: These rather elaborate instructions from the Univ. of Nevada describe a way to disinfect your hot water heater without cutting or disconnecting pipes. An easier way is to have a water filter somewhere in front of the heater and use the filter housing to introduce household bleach into the water line.

1. If the water heater is electric, turn the power off. If the water heater is gas or propane, set the thermostat as low as it will go. This keeps the water temperature cool, which preserves the active ingredients in bleach.


2. All hot water heaters have a pressure relief valve that prevents explosions. In most cases the relief valve is connected to a pipe that leads outside to prevent damage from water discharges in the house. Be sure that the pressure relief piping is unobstructed. The end of the pipe should always be open and dry. If you can see water dripping steadily from the end of the pipe, the relief valve should be replaced.


3. Make sure that all hot water faucets are off. Bleach is introduced into the system by siphoning, which will not be possible if a hot water faucet is left on.


4. Locate the cold water valve leading into the hot water tank and turn it off. Normally this valve is on top of the tank. By closing this valve the hot water tank will be isolated from the rest of the water system in the house.


5. If the garden hose that was used to drain the sediment from the tank has been removed, reattach it and direct the hose to a location, preferably outdoors, where hot water discharges will not cause injury or damage.


6. Attach the short piece of garden hose to a hot water tap nearest to the water heater. In most cases this faucet will be located in a laundry room. If there is a hot water tap available that has male hose thread (such as the hot water tap for the washing machine), attach the hose there. If such a connection is not available use a piece of hose that can be slipped over the end of a faucet and attached with a hose clamp to make the connection air tight. Make sure that the hose is sturdy, because it will be used to siphon the bleach solution and flimsy hoses may collapse when the siphon begins. Also do not use a tap with a decorative brass finish. Bleach will discolor brass.


7. Use gloves and goggles when pouring and mixing bleach solutions. Dilute a suitable quantity of bleach (see Table 1, below) in the two-to-five gallon plastic bucket. The bleach solution may damage fabrics and other surfaces, so be sure to not splash or overflow the bucket. The actual amount of bleach to use is dependent upon the size of the hot water tank and the amount of time that can be dedicated to the process. In general, if you create a 250 ppm solution of sodium hypochlorite in the hot water tank and plumbing, it should rest for twelve hours before being purged. Table 1 provides guidelines for the amount of bleach needed to disinfect the hot water heater and associated household plumbing. Table 1 assumes that the volume of water in the hot water part of household plumbing is approximately five gallons.

Table 1: Amount of Bleach Needed to Disinfect Water Heaters of Specific Sizes, with Associated Household Plumbing:

40 Gallon Tank= 3.25 cups of bleach.

50 Gallon Tank = 4 cups of bleach.

80 Gallon Tank = 6 cups of bleach.

120 Gallon Tank = 8.75


8. Place the free end of the drain hose in the plastic bucket containing the bleach solution, making sure that it reaches all the way to the bottom of the bucket.

9. This is the point in the process when having two people involved is helpful. In order to start the siphon, the drain on the heater must be opened at the same time that the hot water faucet equipped with the hose is opened. Open the hot water heater drain first, then open the hot water faucet.


10. Watch the bucket containing the bleach solution carefully. If the siphon is working properly, the level of bleach solution should be dropping as it is drawn into the hot water tank and piping. If the liquid level in the bucket is rising, turn off the hot water faucet immediately and start again. When most of the bleach solution has been siphoned out, shut the hot water tank drain, then shut the hot water tap and remove the short hose. Be sure to rinse the tap to avoid corrosion.


11. After the bleach solution has been siphoned into the hot water tank, open the cold water valve to the water heater.


12. One by one, open each of the hot water faucets in the house until you can smell bleach in the water. After water with bleach is introduced throughout the hot water system, do not use hot water faucets.


13. After at least twelve hours, drain the hot water tank to a location where it will not cause damage. Never drain water with a high concentration of bleach to the septic tank. A septic tank is dependent upon bacteria to operate properly and bleach could kill these beneficial bacteria.


14. Go to the hot water tap furthest from the water heater and let it flow for a few minutes to purge any introduced air from the system and remove any bleach from the pipes. Repeat this at each of the hot water taps until the chlorine smell goes away. Be sure to check all hot water taps to verify that all bleach has been purged from the hot water system


15. Reset the water heater thermostat.


The instructions above are excerpted from a University of Nevada Cooperative Extension publication.  Go here for the full article.


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