If you have hard water, your hot water heater will slowly fill with mineral deposits that gather on the inner walls and prevent the heater from functioning properly. As calcium and lime build up in your hot water heater, the pipes clog up and decrease the heater's efficiency, which will increase your monthly bill. The acid content in apple cider vinegar is strong enough to break up mineral deposits and get your hot water heater back in full functioning condition.
Turn your hot water heater off. Locate its breaker on your electrical box and flip it to the "Off" position.
Locate the water supply pipe, which should be above the heater, and close it off by turning the lever to the "Off" position.
Drain the tank. Turn on the hot water at a nearby faucet. Locate the drain valve at the bottom of the tank; it will look like a spigot for a garden hose. Secure a garden hose to the drain valve and run the hose to a sink or outdoors. Straighten out any kinks in the hose.
Open the drain valve slowly and let the tank drain. Check the other end of the hose to make sure the water is coming out steadily. Allow the tank to drain completely and turn off the hot water faucet. Close the drain valve.
Locate the anode rod at the top of the hot water heater. This is secured by a large, recessed bolt that you may need to remove with either an Allen wrench or a socket wrench. Check your manual for specific information about the anode rod. Loosen the bolt with the appropriate tool and remove the anode rod.
Place a funnel at the opening left by the anode rod. Pour three to four gallons of apple cider vinegar into your hot water heater. Replace the anode rod. Open the water supply valve and let the hot water heater tank fill for about five minutes. Let the vinegar sit in the tank for 24 hours.
Open the drain valve and let the vinegar and water drain out of the hot water heater. If there were a lot of mineral deposits in your tank, the hose may become clogged and stop draining. Squeeze the hose until you find the deposits and massage the hose to break them up. Let the vinegar drain completely.
Remove the hose from the hot water heater and close the drain valve. Open the water supply valve and turn on the nearest hot water faucet. Wait until enough water is in the tank for it to come from the faucet. Turn the faucet off and turn the hot water heater back on.