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Old 05-04-2010, 06:07 AM   #1
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How to properly drain and flush a hot water heater?


Background- We recently had a water main break near our home and was without water for 3-4 hours. After the water was turned back on it was full of sediment and debris. My toilet intake filter got clogged and I had to remove all the aerators on my sink faucets to clear the grit and sand that accumilated. It was at that point I heard some water dripping from the vent on my water heater. I had heard that the most common reason for getting water out of the vent was becuase of high pressure build up due to restricted water flow in the tank. Most likely due to sediment. the remedy is to drain the tank and flush it properly.

I've never drained a water heater so I'm looking for guidence on how to properly do this. The water heater is a 2005 40 gallon whirlpool that runs off natural gas. A friend of mine recommened the following method but I wanted to run it past some of you plumbing experts first.

1- Turn heater to the OFF position.
2- Shut off water supply to the water heater
3- Connect garden hose to the bottom drain on tank.
4- Open bottom drain and let the hot water flow into a drain, or in my case the wooded gully behind my house.
5- Once the tank is drained turn the water supply to the hot water heater back on for a few minutes in order to flush any settled material from bottom of tank.
6- Close bottom drain and turn water supply back on in order to refill tank.
7- once tank is refilled relight the pilot light on tank and turn tank back on.

Sound right? Do I need to do anything to the vent release switch?

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Old 05-04-2010, 06:18 AM   #2
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How to properly drain and flush a hot water heater?


Yes turn down the heater to min or to off. No need to kill the pilot. I usually leave the water on, then run a hose to a 5 gal. bucket, from the bottom drain. Flush untill the water clears up. Leaving the water on keeps the bottom stirred up, so more of the junk gets flushed. You can also run the hose to a sink or floor drain, but harder to monitor.

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Old 05-04-2010, 06:22 AM   #3
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How to properly drain and flush a hot water heater?


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Yes turn down the heater to min or to off. No need to kill the pilot. I usually leave the water on, then run a hose to a 5 gal. bucket, from the bottom drain. Flush untill the water clears up. Leaving the water on keeps the bottom stirred up, so more of the junk gets flushed. You can also run the hose to a sink or floor drain, but harder to monitor.
Interesting. So no need to drain the entire tank? I forgot the mention, I've noticed the temperature of my water has gone up relative to what the temp was prior to the water outage.
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Old 05-04-2010, 07:43 AM   #4
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How to properly drain and flush a hot water heater?


There may have been sediment accumulated over the years before the water main break called your attention to the sediment. This may be the reason for a change in temperature.

Alternatively you might not have set the temperature dial in the same position when you turned the heater back on.

There will be variation in water temperature depending on how long it has been since the heater last kicked on.

You only need to drain one more gallon after you see the water run clear. But for some kinds of fine sediment this could mean draining the whole tankful. For a large amount of sediment as you saw, do a repeat draining the next day.

Do not turn the heater back on until the water has been on long enouth that you have water gushing out of hot water faucets upstairs.

If you see "rusty water" from the cold faucet or if you know of some "event" with the water system such as a hydrant flushing (to get sediment out of water mains) or a water main break, try not to use hot water until you have used or flushed a lot of cold water including doing lawn watering and gotten the sediment out that way. This minimizes the amount of sediment going into the water heater and possibly resulting in stained clothes when you do a wash.
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Last edited by AllanJ; 05-04-2010 at 07:49 AM.
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Old 05-05-2010, 05:39 AM   #5
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How to properly drain and flush a hot water heater?


I ran a short hose into a 5 gallong bucket last night and drained about 3 gallons of water from the bottom valve of the hot water heater. I really didn't notice any debris at all. Emptied the bucket and drained another 3 gallons. Still nothing. about 5 minutes later I had about 8 ounces of water drip out of my pressure relief valve. So now I'm confused. Should I do a complete drain of the tank or should I double check the water pressure in the house. I guess it's possible the the incoming debris somehow has effected my pressure regulator but if that is the case I'd expect the pressure to decrease instead of increase.
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Old 05-05-2010, 08:02 AM   #6
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How to properly drain and flush a hot water heater?


You need an expansion tank for your water heater, or if you already have one then you need to recalibrate it.

Every time the water heater kicks on, the water in the tank will expand slightly. Water does not compress. The water will either back up into the house inlet towards the pump or water main, or will make use of an expansion tank, or will force out the water heater relief valve.

A rough calibration is had by turning off the water and draining the expansion tank (no need to drain the water heater), closing the air valve on the expansion tank again, and then turning the water back on. Because you are not sure whether the water heater may empty partially during this process, turn off the water heater before starting calibration, and have a hot water faucet running before turning the heater back on.
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Last edited by AllanJ; 05-05-2010 at 08:04 AM.
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Old 05-05-2010, 08:42 AM   #7
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How to properly drain and flush a hot water heater?


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Originally Posted by AllanJ View Post
You need an expansion tank for your water heater, or if you already have one then you need to recalibrate it.

Every time the water heater kicks on, the water in the tank will expand slightly. Water does not compress. The water will either back up into the house inlet towards the pump or water main, or will make use of an expansion tank, or will force out the water heater relief valve.

A rough calibration is had by turning off the water and draining the expansion tank (no need to drain the water heater), closing the air valve on the expansion tank again, and then turning the water back on. Because you are not sure whether the water heater may empty partially during this process, turn off the water heater before starting calibration, and have a hot water faucet running before turning the heater back on.
The thing is that the water heater was installed in 2005 and has never had an expansion tank. And never has a drop of water came out of that vent until my most recent water outage. So I'm convinced that something during this event has changed the way the water heater is working. It worked fine for 5 years without an expansion tank. Now all of a sudden I need one?
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Old 05-05-2010, 12:41 PM   #8
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How to properly drain and flush a hot water heater?


Sediment from the water main incident could well have affected the behavior of your system resulting in the water heater using its relief valve but I am sure the "problem" is not in the water heater.

Perhaps there was a faucet (hot or cold) which, alghogh it was not leaking all the time, did leak under greater pressure so it rather than the water heater relief valve relieved the pressure of the expanding hot water. Then sediment made that faucet tighter. Or the regulator (you said you ahd one) for better or worse let expanding water push back through and sediment now made it tighter.

Since you said that the temperature of the water has gone up, the greater the rise in temperature the more it expands. The expansion of the water in the water heater is slow enough that any restricted flow (not a complete blockage) won't make a difference inside the heater.
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Last edited by AllanJ; 05-05-2010 at 12:48 PM.
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Old 05-05-2010, 09:23 PM   #9
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How to properly drain and flush a hot water heater?


How I flush a water heater: Shut off the water feeding the tank. With a garden hose on the drain valve, open the drain on the tank and an upstairs faucet. Once the tank has drained down, completely remove the plastic junky drain valve. Now replace the plastic drain valve with a 3/4 X 1 1/2 stainless steel nipple and a good quality ball valve. Install a MIP to garden hose adapter in the ball valve. Close the valve and upstairs faucet and refill the tank. Now with the water pressure on and a garden hose connected to the ball valve, you can open the ball valve and REALLY flush the tank with some serious water flow. After flushing, close the valve, remove the hose and cap the adapter with a garden hose cap and garden hose washer. If you don't change out that plastic valve that comes in the tank, chances are you will never flush any sediment through that tiny little valve. Those valves just don't have a big enough opening to pass any sediment. I flush my tank a couple times a year this way and the water is always clean and clear. I am on city water with a softener so tank junk is really not a problem.

Last edited by rustyedge; 05-05-2010 at 09:26 PM.
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Old 04-17-2012, 09:55 PM   #10
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How to properly drain and flush a hot water heater?


Here are some great --------- among them is how to drain a water heater, turning the gas off to the house or the water heater, and a lot of other good info!


Last edited by Gary in WA; 04-17-2012 at 11:01 PM. Reason: Removed web link that belongs under your signature line.
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