"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.