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Nightfall
My Goal Is To Deny Yours
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join:2001-08-03
Grand Rapids, MI
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Knocking hot water heater

We have a problem with a hot water heater that is knocking. It always happens after we use hot water such as in a shower. Obviously, the more we use, the longer the knocking happens.

I have done a little google searching on this and have found that draining the hot water heater works pretty well. I turned the heat way down, turned off the water to the heater, and drained it down as far as it would go. I was watching for some kind of discoloration but didn't see anything out of the ordinary. I filled it back up, and turned the heat back on and I heard the knocking sound.

I then read that the water pressure has to be turned on when you do a flush of the hot water heater. I tried that as well for about 5 minutes, same result happens.

Has anyone here had to deal with an issue like this?
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the Grumble
Yep, I changed my name again -

join:2009-05-11
You'll most likely get better answers here
»Home Improvement


hitachi369
Embrace Your Rights
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join:2001-10-03
Grand Rapids, MI
kudos:4
reply to Nightfall
How far is the room from the hot water heater? If it is far enough away the pipes could be cold. When the hot water turns on the pipes the pipes expand, and then cool back down. Causing a contraction in the pipes, causing it to knock, just like a hot car that cools down and pings.


shdesigns
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reply to Nightfall
Most likely there is a lot of sediment in the bottom of the tank. That insulates the tank from the water and the water boils and forms steam bubbles. Once they leave the bottom of the tank, they hit cooler water and collapse (condense) suddenly.

Not a whole lot you can do about it other than flushing it. If you flush it every year it won't get that way.

The problem occurs here often as there is a lot of rust in the water. WHen it gets worse, it will cause increased pressure and the pressure relief valve will often start venting due to the pressure from the steam.
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Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder


Nightfall
My Goal Is To Deny Yours
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Grand Rapids, MI
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reply to hitachi369
said by hitachi369:

How far is the room from the hot water heater? If it is far enough away the pipes could be cold. When the hot water turns on the pipes the pipes expand, and then cool back down. Causing a contraction in the pipes, causing it to knock, just like a hot car that cools down and pings.
I doubt it could be the cold, never had this problem until about 8 years after we owned the house.
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Nightfall
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Grand Rapids, MI
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reply to shdesigns
said by shdesigns:

Most likely there is a lot of sediment in the bottom of the tank. That insulates the tank from the water and the water boils and forms steam bubbles. Once they leave the bottom of the tank, they hit cooler water and collapse (condense) suddenly.

Not a whole lot you can do about it other than flushing it. If you flush it every year it won't get that way.

The problem occurs here often as there is a lot of rust in the water. WHen it gets worse, it will cause increased pressure and the pressure relief valve will often start venting due to the pressure from the steam.
We don't have a lot of rust in the water which is good.

I think there is sediment at the bottom of the tank. We have tried flushing with the water intake valve open and closed. Obviously when the valve is open the hot water heater never runs out of water so I can sit there and run it for 10-15 minutes. If the valve is closed, I can empty the tank.

What is the best way to flush it? Valve open or closed?
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scooper

join:2000-07-11
Youngsville, NC
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reply to Nightfall
If I needed to flush -
I'd close the Hot water outlet and open drain valve at the bottom of the Hotwater heater. (after turning the heater off / to pilot only).

That said - if you have THAT much crap down there - it would probably be easier to replace the water heater. if a 40 -50 gallon flush didn't clean it out - you got more serious problems.


tp0d
yabbazooie
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join:2001-02-13
Carnegie, PA
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2 edits
Only way to get it out is with a power flush.

When I flush a heater, I take out the drain valve, and screw in a 3/4" brass nipple attached to a 12' piece of 1" ID rubber hose. The hose end goes in the floor drain. Then, after the hot line is disconnected, high pressure water is applied from the top, with a concentrator on a hose (read-- 1/4" ID nozzle on hose). This agitation breaks up the solid crap on the bottom of the tank, and prevents heat stress on the steel. It then is flushed thru the 1" hose.

shdesigns described the problem well, but a relief valve will not leak due to this problem. That is a high pressure issue caused by thermal expansion.

So, what you can do... if you have a floor drain pretty close to the heater, drain the entire thing. Then remove the drain valve, and either run water from the cold feed, or blast water in through the drain tapping. You should see some white or light green chunks come out. You can also inspect the bottom of the tank thru this tapping, it should be a dark blue.

good luck

-j
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shdesigns
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said by tp0d:

O

shdesigns described the problem well, but a relief valve will not leak due to this problem. That is a high pressure issue caused by thermal expansion.
-j
I have seen it several times. Brothers was leaking through the TPV whenever the gas burner was on. The one that came with my house did the same.

The bottom will get real hot as it does not have water directly connected to it. The little that does soak through boils and generates steam causing high pressure.

Had me brother flush his and the leaking stopped, so did the funny noises. Mine got better after a flush, but developed a leak soon after.
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Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder