peter wrote:
"Harry K" wrote in message
oups.com...

Jeff Wisnia wrote:
peter wrote:
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?

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

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"What do you expect from a pig but a grunt?"

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.

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.

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