drain water heater mystery

I drained my water heater yesterday (1 year after installation). I turned off the cold water supply into the water heater, turned off the gas, open a hot water faucet on the second floor, and then open the drain valve on the water heater.
About one gallon of water comes out, then no more. I have to open the pressure relief valve to let the rest of the water out. Why is this? Shouldn't the open faucet on the second floor break the vacuum and allow all water to come out?
BTW, the water being drained out was clear with some black specks floating in it. I was expecting murky water.
Anyone knows how to adjust the air pressure in the thermal expansion unit on the water heater? Or at least check if it is correct? The manufacturer's instruction only says to hire a plumber :(
http://www.amtrol.com/thermxtrol.htm
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I'd be tempted to leave the cold water on. Then it woul dhave cold water pressure at the top of the WH.
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Christopher A. Young
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Stormin Mormon wrote:

The cold water goes to the bottom via the dip tube, not the top. You do want to turn on the cold water in a few spurts while draining the heater since the rush of water from the dip tube can help flush out and sediment accumulating.
The expansion tanks, at least all I've seen, have a regular tire type valve under the little plastic cap. A regular tire gauge can be used to check the pressure and a regular tire pump or compressor can be used to increase the charge. Pressing on the valve core will of course release the charge.
The charge should be a little above normal water system pressure (a few PSI) to insure the bladder / diaphram is distended in the correct direction and allow it maximum working displacement. If it's below the water pressure for some reason the bladder / diaphram will be collapsed or distended in the wrong direction and be unable to absorb thermal expansion.
Pete C.
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Stormin Mormon wrote:

I like opening the T&P valve better.
Your way, the cold water would be entering (through the dip tube) near the drain valve, and it'd take quite a while before the water at the top of the tank "really" drained out.
Course, 99.9% the crap is at the bottom of the tank anyway, so my point is sort of moot, huh?
Hey, maybe the best thing would be a combination of both. Let the tank drain by gravity with the T&P open, then open the inlet valve full on for a short while so that water coming out of the dip tube splashes around the bottom of the (almost) empty tank and maybe sweeps off some more crud?
I've heard tell the "way to do it" for electric water heaters is to remove the lower element and stick in a garden hose with a straight nozzle. Then turn on the water to the hose and twist and bend it around so that the strong water stream from the nozzle blows stuff off the inside of the tank bottom. I wonder if anyone has ever actually gone through the bother of doing that?
Jeff (Straining at a gnat again...)
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Jeffry Wisnia
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Think 3,000 PSI pressure washer, and really get it right. Or unscrew the top element, and pour in a galon of vinegar, to dissolve all the lime scale. Phosphoric acid works well, too.
OTOH, I like to just open the stupid drain and let it run awhile. Close the drain and call it good for another year. I get about 94.867% that way.
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Some T&P's won't close evenly and start leaking, necessitating replacement.
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turn your sink hot/cold water on to let air in as it drains.
http://www.minibite.com/america/malone.htm
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peter wrote:

Normally I'd expecy it should, but maybe it wouldn't work as expected if you had an unusual plumbing run and the hot water line from the heater first went down to near the heater's floor level somewhere before it headed upstairs.
Just a WAG; someone tell please me if I'm way off base on the physics of that one.
Jeff
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Jeff Wisnia wrote:

I'm still scratching my head trying to think of why opening the hot water faucet didn't work. I don't think your explanation would explain it. Might slow it down a bit but it should still drain.
Harry K
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If the hot water pipe does indeed go downwards below the level of the drain valve before it goes back up, then the weight of the water trap in that downward pipe would resist the weight of the water trying to escape from the water heater.
Next time I'll try blowing into the hot water faucet to force the trapped water out of the pipes. Then the draining should start.
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peter wrote:

Hmmm....mentally picturing the set up. I am of two minds on it. One says it will drain, the other says you are right.
Being retired I will try to fit an experiment into my busy schedule ;). Should be able to build a quicky test with a white sealable bucket and a piece of hose.
My 'it will drain' side says the system would drain as the total resistance to air entering would be the height of the water colum from the bottom of the "U" to the top of the water level in the tank. Not very much PSI there. .46 psi per foot an average tank will be about 5 ft so it would only take about 2 1/2 pounds of 'suck' to draw air in. That decreases as the water level drops....
but then, due to the "U", there will always be more water column in the pipe than in the tank....
Why did I ever open this thread!
Harry K
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There is most likely a heat trap in the line. It is a plastic ball bearing check valvee usually located right at the outlet of the tank. Only allows water to leave the tank, now come back in.
-Brian
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Brian V wrote:

Good call!
Now why didn't I think of that?
Prolly because I only thought of them as "gravity valves" to keep thermosyphoning from occuring and never considered their effect on water trying to flow the OTHER way.
Jeff
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Keep in mind also that the flush valves on water heaters are notorious for clogging with pieces of crust from the heater. And it is sometimes hard to "work" the valves to clear the junk. OTOH, your heater is only 1 year old, so I would not expect that. Also, it is unlikely that the flow would stop completely -- unless something more substantial got into the valve to block it. The only good way to test all the theories of what is the cause of your problem, would be to open the Temp/Pres relief valve on the heater and see if that gives you flow. Also, I hope you have tried opening more hot water valves than just the one on the second floor. --Phil
peter wrote:

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Phil Munro Dept of Electrical & Computer Engin
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It will not drain (siphon) if there is water in a pipe that is lower than the drain valve. You cannot siphon to a level higher than the level of the source. Air needs to enter at a point above the valve. That is why it drained when you opened the relief valve.
Frank
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