hose connected, burst pipe?

Science question: I live in the  south east U.S. We don't usually have sub freezing temperatures, however when we do, the local news always says to disconnect external water hoses to prevent waterpipes from bursting.
I'm trying to understand if there is any true science to back up this claim. Why would the water hose itself have any affect on pipes bursting?  Wouldn't it only occur if water freezes between the faucet and the pipes and expands and cracks the pipes? The hose connected doesn't seem relevant.
Any science to back this up?
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Scott FellDeveloper & EE ModeratorCommented:
I am in the north midwest and it is just common to disconnect your hose pre winter.  

Statefarm recommends this practice:
Before winter hits, disconnect garden hoses and, if possible, use an indoor valve to shut off and drain water from pipes leading to outside faucets. This reduces the chance of freezing in the short span of pipe just inside the house.
Dave BaldwinFixer of ProblemsCommented:
Are you sure they're not talking about the hose bursting?  That would be because the hose may not drain if it is still connected.

Here you go, this is a more complete explanation: http://homerepair.about.com/od/plumbingrepair/ss/thaw_frzn_pipe_6.htm  This is page 6 and you might want to go back to page 1 and read thru it.

Oh padas, I had to finish a phone call before I could post!
Paul SauvéRetiredCommented:
Yes - the scientific explanation is that water expands when it freezes.

What I do is disconnect the hose from the faucet, close the valve indoors and leave the faucet open! But then again, you may not have the indoor valve on that water line if you live in the South!

Not only can the hose burst, but the water pipes as well.
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guidwayAuthor Commented:
Thanks for the links.  I read them but don't see explanations for why removing the hose matters. Just basically saying to do it or it could cause broken pipes. Why though? Would it be back pressure from the hose?

Dave, I don't know.  Maybe it is just to keep the hose itself from bursting and they are just lumping it in with the pipes. I'm not sure what they mean. Just wanted to see if there could be a reason the hose could matter.
guidwayAuthor Commented:
Paul, How would water freezing in the hose and expanding cause the pipes to burst?  Or am I misunderstanding what you mean?

Sorry guys not trying to be difficult just trying to understand. Thanks for the help!
Scott FellDeveloper & EE ModeratorCommented:
I am no scientist, but if your hose is completely empty and your spigot never drips, you are probably ok.   Chances are you have water in your hose and there could be a slow drip you don't realize.  Now water could be backed up in that area, freeze, expand and do damage.
Scott FellDeveloper & EE ModeratorCommented:
Dave, I gave it a standing 10 count and didn't see anything before I posted.
Dave BaldwinFixer of ProblemsCommented:
No problem @padas, I was on the phone.  @guidway, the whole point is to let all the water drain out that would otherwise freeze.  No water = no problem.
"I'm trying to understand if there is any true science to back up this claim.

The problem is not science but English.  Newpapers do not always write exactly as they intend.
The links given tell you the science.
Paul SauvéRetiredCommented:
If you leave the hose connected and the faucet closed, it is the water inside the water pipe that will freeze (and also in the hose).

Since water expands when it freezes, it will expand inside the water pipe causing it POSSIBLY to burst. I know this because one year I forgot to disconnect my hose and the pipe froze AND burst! I live in Quebec and it freezes for at least four months every year!

Bu disconnecting the hose, the ice will expand out of the end of the hose, so there is no buildup of pressure. Bu closing the valve INSIDE the house and leaving the faucet OPEN, any ice buildup in pipe between the valve and the outside of the house will come out the open faucet.

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Outside hoses would be most exposed to freezing temperatures, and ice blockages within the hose could prevent relief of pressure by flow along the pipe and out open faucets.
Scott FellDeveloper & EE ModeratorCommented:
I think Paul has give you the most complete answer so far http:#aa39744312
guidwayAuthor Commented:
Thanks again to everyone! Paul's answer really helped me to understand it and makes sense.

I've always disconnected it anyway (just in case), but never understood really why so started questioning it. This helped a lot!
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