Frozen pipes under ground in upstate NY cold winter

The U shaped pipe under my garage floor has frozen the last 2 winters in the lengthy extreme cold spells. I have been away for 10 weeks each of the last 2 winters. The first winter my house watcher flushed toilets and ran water once a week. This resulted in a mess inside the house from backing up after a few weeks. The second winter I decided to skip having him run any water in the house to avoid a repeat situation. What I did not realize was the furnace gave off condensation (about 3 gal per 24 hours) and goes through the waste water pipes under the garage floor through the U shaped pipe and out towards the sewer. This amount of water also froze over time. The 2 feet depth on each side of the U freeze solid. I am not 100% sure but it may start in the section of pipe under ground just outside the wall of the garage. It took 4 hours of umbels time and 1 hour of time from a guy with a steam jenny (sp?) to thaw it out.
I have no idea how to keep underground pipes from freezing. This is a townhouse built on a slab. I know of 2 other homes on the street with no one home for an extended period of time during this cold period that did not have any issues. Any suggestions on a solution?
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In alt.home.repair, on Mon, 07 Sep 2015 03:44:01 +0000, GMC

Why? Do you mean that when he came over he wanted a drink of water and needed to use the toilet?
Because I was going to suggest while your away you turn off the water, drain the pipes, and pour winnebago antifreeze in your traps, sinks, toilets, and maybe even dishwasher? In case the heat goes out.
Is this only a problem when you're away, 6 days at a time, or also when your home, using the toilet every day, twice a day. Can it freeze if you sleep for 8 hours and go out for 8 more?

Go talk to the owners.
Redirect the condensation from your furnace, at least when you are not home. Even if you have to run it out through a tube through a hole in the wall. And
Any suggestions on a solution?
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On 9/6/2015 11:56 PM, micky wrote:

In ten weeks the water would dry up and sewer gasses would get in the house. This can be prevented by pouring cooking oil to prevent evaporation.

Make sure it does not freeze going through the wall though.
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Ed Pawlowski wrote:

Sounds like something is not right. What is frost line there? Hard to believe P trap freezes if every thing is done right.
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On Monday, September 7, 2015 at 1:26:47 AM UTC-4, Tony Hwang wrote:

RV/non-toxic antifreeze would be one solution. Or maybe a heat pad or similar over the area.
What is the purpose of this "U" which is apparently a P trap? Is it a garage floor drain? But from the description it sounds like maybe it's in the main sewer path? That would be unusual. If it's a floor drain, once it has antifreeze it will stay full unless someone uses it.
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trader_4 wrote:

As long as water does not run into it. Otherwise antifreeze will run away.
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On Sun, 06 Sep 2015 22:44:01 -0500, GMC

Farmers in times past would've put square straw bales over the area of the suspected freeze up. Would the insulation blankets used by concrete workers be any help? My second thought was to constantly run RV antifreeze into the pipes. Irrigation wells have oil drippers to constantly oil them. A couple pictures: http://preview.tinyurl.com/okzaw63 The brass knob adjusts the flow rate. It's typically a few drops/minute. The black piece is an automatic shutoff. It can be set to run constantly. Another picture: http://preview.tinyurl.com/nqnumh4
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On 9/6/2015 8:44 PM, GMC wrote:

How do you *know* it is a "U shaped pipe"? Do you have visual access to it? Have you probed it with a snake, etc.? Or, has someone just told you that it is U shaped and that it is under your garage floor?

Don't you LIKE the cold weather? :>

Why did it back up? Did the house-watcher not notice that the pipe was blocked/frozen? Or, did this happen in his absence?

Again, how do you *know* this? I assume your garage floor is a concrete slab?

You could "shade"/insulate the pipe before burying it! I.e., put it in a material that is less of a thermal conductor.
You could also wrap them with a heating element and deliberately keep them warm.
You could ensure an adequate flow of "room temperature" water is flowing through them to keep ice from forming.

So, what's different between your property and theirs? Sun exposure? Soil composition? Different construction techniques??

What is the diameter of the pipe in question? Is it a typical 4" drain? Or, something that *feeds* the drain pipe?
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