Draining the Water Heater: Water Does Not Come Out

[skeezix]

In March 2020 I installed a new A.O. Smith gas 40-gallon water heater. Now I want to flush it and this is what I did:

1. Turned the heater control to "Pilot".
2. Turned the cold water valve to the heater off.
3. Attached a hose to the drain valve.
4. Put the other end of the hose into the adjacent wash tub.
5. Opened the hot water tap on the wash tub. (No water came out).
6. Opened the drain valve (a ball-type that I install when I installed the heater).

After an initial gush lasting 2-3 seconds, no more water came out of the drain. So I loosened the cold water union until I heard air enter, and then the water flowed out of the drain.

What's going on here? Why did I have to loosen the cold water union? I thought that opening a hot water tap would let air into the tank as the water flowed out.

In all of the "tutorials" I viewed about draining the water heater, it was never mentioned that one should loosen the water input union. All that was said was to open a hot water tap.

 

[Twowaxhack]

You could’ve opened the relief valve to give it air.

It was air locked. Opening the laundry faucet still left a loop of pipe with water in it.

 

[skeezix]

Now why didn't I think of that...

 

[Joacchim57]

When I do this, I just shut the heater off and open the drain/flush valve. Leaving the cold water supply "on" prevents the problem you had. Any reason why this is the wrong approach?

 

[RS]

The only time I flush our water heaters is when I am winterizing in the fall, I use compressed air to blow it out, the air going down the dip tube agitates any silt and it drains out with a great force, I think this cleans out the heater quite well. We then leave the heaters empty all winter as we are gone, but a person could just start it up again.