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Old 10-13-2010, 01:03 AM   #1
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Cold water is warm - hot water leak under slab?


Our house is built on a concrete slab. The water line from the city enters the slab at the laundry tub. The hot and cold water pipes are copper and run under the slab. A picture of the layout (not to scale) follows:



When I turn the cold water on in the master or bedroom bath basins, the water runs cold for about 5 seconds then gets warm until 40 seconds then gets cold again. This happens even at 12 am midnight which indicates it is not a sun heating the cold water pipe problem. I believe there's a leak in the hot water pipe under the slab which is heating the ground which in turn is heating the cold water pipe running adjacent to the hot water pipe. I really don't have any idea how the supply pipes are running under the slab so this is just a guess.

Our house is 39 years old and to the best of my recollection, the problem started about 9 months ago. It was barely noticeable at first but has progressed to the symptoms above.

Any professional plumbers out there?

1. Do you think the hot water pipe is leaking under the slab or can there be another reason for my symptoms?

2. If there is a hot water leak under the slab, with the 40 seconds of warm water, how far away from the bath basins would you guess the leak is? The hot and cold water pipes coming up from the slab at the bath basins are both 1/2" copper pipes. I can actually see that using a flashlight and a mirror since I have an access hole in the double wall about 2 feet from where the pipes come up through the concrete.

3. If there is a hot water leak under the slab, should I just run new hot water pipes from the water heater to the Kitchen, laundry and bathrooms? The pipes would probably have to be mounted under the eaves outside of the house to the different points since we have a 2.5"/foot roof pitch and there is very little room above the ceiling.

I'm wondering if option-3 above would be the sensible way to go since Googling water leak under the slab, some folks have had a leak under the slab repaired only to have more leaks happen one-by-one.

Thanks for any help,
HomeRepairGuy

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Old 10-13-2010, 02:44 AM   #2
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Cold water is warm - hot water leak under slab?


You should have a shutoff valve at the water heater (if not, might want to install one on the cold side) I'd shut off the water at the water heater and keep it off for a few hours. At that point, you should have your answer about a leak under the slab.
If you don't have a shutoff, you could just turn off the hot water.
My old setup had the hot water line from heater to kitchen going under the bathroom floor, I could feel the warmed floor after running the hot water... You could be getting heat transfer from the concrete.
From the looks of your setup, you might be best to run new insulated hot one the side where the kitchen/baths are, I was told not to go overhead, I dug up a run for all my services. You might have to cut some concrete and dig a bit to get there.

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Old 10-13-2010, 05:27 AM   #3
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Cold water is warm - hot water leak under slab?


Quote:
Originally Posted by KarlJay View Post
You should have a shutoff valve at the water heater (if not, might want to install one on the cold side) I'd shut off the water at the water heater and keep it off for a few hours. At that point, you should have your answer about a leak under the slab.
If you don't have a shutoff, you could just turn off the hot water.
My old setup had the hot water line from heater to kitchen going under the bathroom floor, I could feel the warmed floor after running the hot water... You could be getting heat transfer from the concrete.
From the looks of your setup, you might be best to run new insulated hot one the side where the kitchen/baths are, I was told not to go overhead, I dug up a run for all my services. You might have to cut some concrete and dig a bit to get there.
That's a good idea about not letting hot water get into the pipes to verify if a leak is the problem. There is a ball valve on the cold water line to the water heater. Tomorrow night I'll tell my wife to stop using hot water by 8pm and turn off the cold water to the heater. Hopefully by midnight, 4 hours later, the hot water in the pipes under the slab should be cold and I can run the test. I'll post the results.

Running a new hot water line from the heater under the ground will be a major problem. All of the area in the picture I posted is concrete slab. On the right side of the water heater and the front of the house is a covered concrete sidewalk which is not shown. I'll have to check with our city to see what the code is for running supply hot water pipes above the ground. BTW, I live in an area where it never snows or gets below 50 degrees so water pipes outside can't freeze.

Thanks a lot for your help,
HomeRepairGuy

Last edited by Homerepairguy; 10-13-2010 at 08:12 AM.
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Old 10-13-2010, 08:40 AM   #4
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Cold water is warm - hot water leak under slab?


Have you noticed an unusually high summertime gas bill or anytime electric bill? A leak in the hot water pipes will certainly cause this.
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Old 10-13-2010, 09:28 AM   #5
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Cold water is warm - hot water leak under slab?


If you have a leak you should see your water meter spinning. Any increase in your water bill? Also check your faucets. They can pass hot water to the cold side if they are failing
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Old 10-13-2010, 05:17 PM   #6
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Cold water is warm - hot water leak under slab?


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If you have a leak you should see your water meter spinning. Any increase in your water bill? Also check your faucets. They can pass hot water to the cold side if they are failing
I didn't think the water meter would show a small leak but I checked it and was surprised to see the small analog dial spinning. It is spinning at a rate of 1/2 revolution in 1min, 18sec.

I shut off the ball valve that feeds cold water into our water heater and the dial stopped spinning. So that verifies there is a leak in the hot water line somewhere.

Although this is probably moot since a hot water leak has been verified, I'll supply the info in case it might help in some way. All of the water faucets for the kitchen sink, laundry tub and both bathroom basins in our home have separate hot and cold water knobs which feed the common water outlet. Both bathroom tub/shower stalls also have separate hot and cold water knobs. So I don't think hot water can be passed to the cold side via the faucets.

Thanks much for your help.

And AllanJ,
Thanks for taking the time to respond too.

Any suggestions for my options now?
Thanks,
HomeRepairGuy
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Old 10-13-2010, 07:35 PM   #7
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Cold water is warm - hot water leak under slab?


If you are not using cold water, hot water will not pass to the cold side even if you left both shower faucets on (but shut off the water at the shower head, common cause of gettting hot water in the cold or cold water in the hot when you are using another bathroom).

I would seriously consider rerouting the hot water via a new overhead pipe or a through-the-wall pipe. Just be sure to disconnect the leaky pipe at all ends when abandoning it. If you are lucky there are separate runs from the water heater to the bathrooms and from the water heater to the laundry so you only have to run one new length of pipe and decommission only two ends. By the way my uncle had this problem a few years ago and had a pipe run overhead (exposed) to fix it.
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Last edited by AllanJ; 10-13-2010 at 07:48 PM.
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Old 10-13-2010, 08:05 PM   #8
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Cold water is warm - hot water leak under slab?


You might want to check codes in your area, someone told me you can't run water overhead. He was just a co-worker, might have been full of it... but worth checking into just in case you sell the house later.
One upside is that now you can insulate the pipes! I ran the over the pipe foam rubber insulation and then stuck the pipe inside a 2" PVC pipe.
One other side note:
If you do the math, you'll find that a 3/4" pipe is more that twice the volume of a 1/2" pipe. Therefore when you turn on the hot with a 1/2" pipe, it get's hot more than twice as fast (less cold water to push out of the way). If you look at the final opening size of most things (shower head/ sink...) you'll see many of them are less than 1/2"... No need to have 10 gallons of cold water sitting in the way of your hot water! Maybe even 3/8" will do the trick. Worth checking into and it's nice to have quick hot water and save $$$ to boot!
Look under your sink, I bet the supply tube is pretty small.
Lastly, don't forget the "no knock" thing... short overrun (goes up past the destination) of pipe that holds air so that when you shut off the source, it doesn't 'knock' the pipes.
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Old 10-13-2010, 09:08 PM   #9
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Cold water is warm - hot water leak under slab?


Quote:
Originally Posted by AllanJ View Post
If you are not using cold water, hot water will not pass to the cold side even if you left both shower faucets on (but shut off the water at the shower head, common cause of gettting hot water in the cold or cold water in the hot when you are using another bathroom).

I would seriously consider rerouting the hot water via a new overhead pipe or a through-the-wall pipe. Just be sure to disconnect the leaky pipe at all ends when abandoning it. If you are lucky there are separate runs from the water heater to the bathrooms and from the water heater to the laundry so you only have to run one new length of pipe and decommission only two ends. By the way my uncle had this problem a few years ago and had a pipe run overhead (exposed) to fix it.
Thanks for explaining how hot can be mixed into the cold line with the water shut off at the shower head. That makes a lot of sense, though it's not the cause of my problem.

Unfortunately it will really be hard to isolate the hot water line to the bathrooms since the hot water pipe from the heater goes into the slab and they emerge from the slab at each location. Googling I found that the hot water line is more prone to leaking than the cold water line since hot water is more corrosive. Also that rust from old water heaters can leave deposits in the hot water pipe resulting in pin hole leaks in the copper due to electrolysis. I think that is what happened to our copper hot water pipes since the heater we had before going to a new solar heater a few years ago was really old and starting to leak when replaced. So folks, replace your old water heaters! Much cheaper than installing new hot water lines.

I'm definitely going to install new copper hot water pipes and try like hell to run them in the attic if local codes allow it. Second choice will be to run them under the eaves if code allows it.

Thanks,
HomeRepairGuy

Last edited by Homerepairguy; 10-13-2010 at 11:00 PM.
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Old 10-13-2010, 09:17 PM   #10
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Cold water is warm - hot water leak under slab?


Quote:
Originally Posted by KarlJay View Post
You might want to check codes in your area, someone told me you can't run water overhead. He was just a co-worker, might have been full of it... but worth checking into just in case you sell the house later.
One upside is that now you can insulate the pipes! I ran the over the pipe foam rubber insulation and then stuck the pipe inside a 2" PVC pipe.
One other side note:
If you do the math, you'll find that a 3/4" pipe is more that twice the volume of a 1/2" pipe. Therefore when you turn on the hot with a 1/2" pipe, it get's hot more than twice as fast (less cold water to push out of the way). If you look at the final opening size of most things (shower head/ sink...) you'll see many of them are less than 1/2"... No need to have 10 gallons of cold water sitting in the way of your hot water! Maybe even 3/8" will do the trick. Worth checking into and it's nice to have quick hot water and save $$$ to boot!
Look under your sink, I bet the supply tube is pretty small.
Lastly, don't forget the "no knock" thing... short overrun (goes up past the destination) of pipe that holds air so that when you shut off the source, it doesn't 'knock' the pipes.
Yup, I left call back messages at two local city departments to see if hot water pipes in the attic or under the eaves are allowable but no one called back all day. Pretty poor service but what I expected from our city departments.

Thanks for the heads up on the volume of water difference between 3/4" and 1/2" pipes. I might have installed 3/4" pipes if you hadn't mentioned that. Sure makes a lot of sense to minimize the amount of water wasted waiting for the hot water to arrive. I won't be going smaller than 1/2" copper pipes though. My wife only takes baths and I don't want her waiting too long for the tub to fill...

I'll just be splicing in the new pipes where the old pipes feed each location so any water hammer arrestor piping will still be intact. Thanks for the heads up on this aspect too.

Great forum!
HomeRepairGuy

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