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Old 05-09-2014, 02:37 PM   #1
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Running PEX through COPPER ?


I have a slab leak in my copper pipe that supplys hot water to my kitchen sink and dishwasher. Had a plumber come out and cap off the hot water line with a sharkbite connector, so now I have no hot water to the kitchen sink/dishwasher. My question is, would I be able to run 3/8 pex tubing through the exsisting 1/2 copper pipe ? Its roughly about 20' and believe it is a pretty much straight run. Then I was thinking of getting two sharkbite fittings 1/2-3/8 to re-connect the fixture pipe to the pex on the sink side, and same for the line that is currently capped off.

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Old 05-09-2014, 02:52 PM   #2
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Running PEX through COPPER ?


Might work if you do not mind the flow and pressure being reduced 50%.

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Old 05-09-2014, 03:02 PM   #3
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Running PEX through COPPER ?


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Originally Posted by wannadiy View Post
I have a slab leak in my copper pipe that supplys hot water to my kitchen sink and dishwasher. Had a plumber come out and cap off the hot water line with a sharkbite connector, so now I have no hot water to the kitchen sink/dishwasher. My question is, would I be able to run 3/8 pex tubing through the exsisting 1/2 copper pipe ? Its roughly about 20' and believe it is a pretty much straight run. Then I was thinking of getting two sharkbite fittings 1/2-3/8 to re-connect the fixture pipe to the pex on the sink side, and same for the line that is currently capped off.
Technically it would fit but if you have any bends I don't think you would get it in. 3/8 Pex is less than 1/2 the area of 1/2" copper pipe.
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Old 05-09-2014, 04:27 PM   #4
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Running PEX through COPPER ?


The inside diameter of 3/8 inch PEX is 0.35 inches, the outside diameter is 0.5 inches. The inside diameter of 1/2 inch copper Type L (commonly used in slabs) is 0.545 inches. This gives you less than 0.05 inches clearance, which is a pretty tight fit. If there is the slightest kink in the copper, you will never get it through.

If you can push it through, you will not have a 50% reduction in flow or pressure. The actual flow and pressure reduction is tricky to compute. You will have half the area, but it is a common misunderstanding that half the area means half the flow, it does not. All it means is there is more pressure loss per foot of pipe than through a larger pipe. The total flow through a system is based on the total pressure loss from one end of the system to the other. A few extra psi of pressure loss due to a short (in your case 20 foot) length of narrower pipe will result in a small reduction in flow. The flow reduction percentage increases as the total flow through the pipe increases, since pressure loss is proportional to the square of the velocity, but typically a short section of narrower pipe is not very noticeable.

Conclusion: If you can get the PEX through, you should be fine for flow.

Note: It is pretty common practice in the utility industry to insert a smaller diameter plastic pipe inside a corroded steel or iron pipe. There is an entire industry built around this practice, since it avoids the need to dig up pipe just to reline it. In most cases, the flow reduction is small enough that it is not a problem. Where it is a problem, alternative techniques are specified by the engineer.
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Old 05-09-2014, 04:44 PM   #5
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The inside diameter of 3/8 inch PEX is 0.35 inches, the outside diameter is 0.5 inches. The inside diameter of 1/2 inch copper Type L (commonly used in slabs) is 0.545 inches. This gives you less than 0.05 inches clearance, which is a pretty tight fit. If there is the slightest kink in the copper, you will never get it through.

If you can push it through, you will not have a 50% reduction in flow or pressure. The actual flow and pressure reduction is tricky to compute. You will have half the area, but it is a common misunderstanding that half the area means half the flow, it does not. All it means is there is more pressure loss per foot of pipe than through a larger pipe. The total flow through a system is based on the total pressure loss from one end of the system to the other. A few extra psi of pressure loss due to a short (in your case 20 foot) length of narrower pipe will result in a small reduction in flow. The flow reduction percentage increases as the total flow through the pipe increases, since pressure loss is proportional to the square of the velocity, but typically a short section of narrower pipe is not very noticeable.

Conclusion: If you can get the PEX through, you should be fine for flow.

Note: It is pretty common practice in the utility industry to insert a smaller diameter plastic pipe inside a corroded steel or iron pipe. There is an entire industry built around this practice, since it avoids the need to dig up pipe just to reline it. In most cases, the flow reduction is small enough that it is not a problem. Where it is a problem, alternative techniques are specified by the engineer.
I read somewhere that a person had a similar if not exact situation where it was an kitchen island sink, like mine and it went through without a hitch. I do have a fish tape for electrical cabling, so instead of trying to push it through maybe try pulling it instead, along with lubricating the outside of the pex tubing.
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Old 05-09-2014, 04:48 PM   #6
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Running PEX through COPPER ?


You will never make the bend from horizontal to vertical to come up out of the slab.

In the south we run the PEX in the attic to replace bad copper under the slab.
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Old 05-09-2014, 05:23 PM   #7
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You will never make the bend from horizontal to vertical to come up out of the slab.

In the south we run the PEX in the attic to replace bad copper under the slab.
Well its not a 90 degree bend, guess all I can do is try, the option of running it from the attic is not an option as I said the sink is on an island, so I would have no way of getting the pex to the sink, unless I built a channel from the island to the ceiling which would look very un-sightly.
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Old 05-09-2014, 05:29 PM   #8
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Running PEX through COPPER ?


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I read somewhere that a person had a similar if not exact situation where it was an kitchen island sink, like mine and it went through without a hitch. I do have a fish tape for electrical cabling, so instead of trying to push it through maybe try pulling it instead, along with lubricating the outside of the pex tubing.

Wonder how long it will last. Since everytime you use hot water. The pex will expand and rub against the copper. And then when it cools it will contract and again rub the cooper. Eventually rubbing a hole in the pex.
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Old 05-09-2014, 05:33 PM   #9
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Running PEX through COPPER ?


At some point all that old copper is going to fail if it was run through the slab.
How it needs to be redone depends on where you live.
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Old 05-09-2014, 05:40 PM   #10
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At some point all that old copper is going to fail if it was run through the slab.
How it needs to be redone depends on where you live.
I live in Southern California. I have spoke to several plumbers and they mention that they never break into the slab anymore, instead run pex through the exsisting copper, if the copper corrodes, it no big deal as the pex is in place. From what I gather pex is relatively new in the U.S. but it is common in Europe.
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Old 05-09-2014, 05:48 PM   #11
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Running PEX through COPPER ?


If well over 10 years is new so be it.
As we have all stated your going to be losing a lot of pressure and flow.
Might be ok for a kitchen sink but what about the washing machine and shower?
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Old 05-09-2014, 05:53 PM   #12
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If well over 10 years is new so be it.
As we have all stated your going to be losing a lot of pressure and flow.
Might be ok for a kitchen sink but what about the washing machine and shower?
I dont think so....This hot water line ONLY supports the kitchen sink and dish washer. No shower or clothes washer, plus its rare we use the dishwasher
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Old 05-09-2014, 06:42 PM   #13
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Running PEX through COPPER ?


If you can't push or pull it through, you might try connecting the pex to one end of the copper and then pulling the copper out from under the slab.
As the copper is removed it is replaced with pex.

I have done this several times for water service replacement on homes. However, I used the hydraulics of a backhoe to do the pulling. But if you have pea gravel or sand under the slab it might go easily.

As mentioned, I dont think you'll make the turn going inside the pipe. Copper tends to oval in shape when bent by hand.
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Old 05-09-2014, 07:37 PM   #14
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Running PEX through COPPER ?


That is a cool concept and I can't think of any reason it would work well in theory. If it according to code, I can't think why not - but I have no idea.

In practice, PEX won't follow any bends that copper makes. If the copper makes some gentle curves (certainly not any 90 or 45 bends), then again in theory PEX can certainly be made to follow that curve. But in practice, I seriously doubt you could ever make it bend while inside the copper pipe by pushing it trough. Simply too much friction with no room inside the copper pipe.

I recently had a leak somewhere in my main supply line from the meter to my house. Shoving 3/4" PEX through the old pipe would have been WAY easier than digging a trench to run the new line! However there's no way I could fit 3/4" PEX in the existing pipe, and I would never reduce the main line down to 1/2".
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Old 05-10-2014, 05:52 AM   #15
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Running PEX through COPPER ?


Time to break out the concrete saw and cut a new trench over to the island for the new pipe. While your at it replace both hot and cold. Repair or replace the kitchen floor.

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