To bury below or above the frost line?

[john M]

In the begining of my research it seemed everyone said to bury my underground pipes from my OWB only about 12" down - in the frost line - to keep them away from the water table. I was shocked at first as we always burried "normal" water lines 4' down to prevent frezeing. But keepign the insulated pipe away from the water table made a lot of sense. BUT lately, I am seeing several posts with people talking about buring their pipes 3 or 4 feet underground. Is there a concensus on this topic?

 

[Bob Rohr]

the deeper bury will take advantage of the warmer ground.

Heat moves from one place to another based on delta T. Delta T being the "temperature difference". The higher the delta T between the pipe and surrounding ground, the higher the rate of transfer. Hot always goes to cold, by the way.

No amount of insulation can completely stop heat transfer, but only slow the transfer. So the trade off becomes how much insulation to slow the heat flow.

Also the warmer the surrounding ground the less the thermal transfer.

Seems the shallow 12" bury is more of a convience "number" not a best performance number? I dropped below typical frost level around here by 6" or so. Why not?

hr

 

[john M]

certainly the pipes will loose less heat the deeper I go, but is not the trade off the possibility that my pipes could be sourrounded by water much of the time? And of course if I ever developed even a pin hole of a leak, the resulting heat loss to the water would be ten times higher.

 

[JPapiPE]

The water company will bury the main water line to your house at 6' below ground. I think that is an indication of what works best to protect from freezing pipes.. So I would want the pipes to be below the frost line, plus a little deeper in case of an extremely cold winter. In Portland, Maine the frost line is 4' below grade, but 30 miles north in cornish the frost line is 5'.
I hope this helps, Joe

 

[Bob Rohr]

certainly the pipes will loose less heat the deeper I go, but is not the trade off the possibility that my pipes could be sourrounded by water much of the time? And of course if I ever developed even a pin hole of a leak, the resulting heat loss to the water would be ten times higher.

Yeah, you really need to stay out of any ground water, running or still. Water will really conduct heat away. And it is hard to waterproof insulated underground piping.

Stay out of the water or install a drain field with gravel below it, open to daylight if possible.

hr

 

[JPapiPE]

If ground water is 5-6" feet below grade , you can bet your boots that the water will be above freezing....What do you need a geiger counter?

 

[Jerry_NJ]

Keep it out of the water, I don't have the numbers at hand but my money says you'll lose more BTUs per foot in 50 degree water than you will in 10 degree soil if you have insulation on the pipes. Water is a great conductor of heat, that's why the "water cooled" engines run at a lower temperature than air cooled engines. That's why geothermal heat pumps use a liquid either in the closed loop or directly from the ground, and better yet if the loop is down in the water, here we want to exchange BTUs, heat or cool.

 

[flyingcow]

ive read that using solid foam will "raise" the frostline . solid foam & consideration for frost penetration alteration vs uninsulated ground pertaining to frostwalls.....good luck

I've got well piping less than 12 inches below the lawn in a couple of spots(this is due to ledge). Its inside 4" pvc with solid foam sheets on top of it. 2 layers of 2".The foam was 2ft wide. This has worked well. I thought that frost will generally go straight down, and will not run sideways. We usually get 4 to 6 feet of frost, depends how much of an open winter we have in Jan. Don't know how this will preform with heat loss in furnace piping.

 

[JPapiPE]

Oh, sorry i missed the part of your shallow buried pipe being encased in PVC with additional insulation. A very wise and keen solution to a immovable problem, i.e, hitting bedrock so close to the surface. At least if it fails you haven't got too much to excavate. Wind is usually more the culprit than sheer cold...I think you'll be ok as you are out of the wind and if you do have a problem it will show itself immediatly and then you can dribble a faucet during the coldest nights. And thats what the old timers did before all this new scientific data barragged us. And it worked
Joe

 

[MrEd]

If ground water is 5-6" feet below grade , you can betyour boots that the water will be above freezing....What do you need a geiger counter?

Yea, it might be 40-45 degrees...but what do you think it would do to your heat loss to have your pipes saturated by 45-degree water wicking away the heat? Better to have a plan to keep those pipes dry no matter the temp of the water.

 

[granpajohn]

I am not familiar with OWB tubing, but in distributed energy.
Don't confuse average frost penetration with extreme frost depth. A building code uses the latter. I don't think frost heave every 5 years would hurt an insulated PEX pipe. Yes, the warmer the ground, the better, but perhaps not at the expense of deep excavation.

This may be good for home use: http://www.rovanco.com/flexible_pipe.html I like the supply and return line in one insulated, flexible, casing.
They give heat loss numbers at their site.

Just my take....

 

[JPapiPE]

i think these outside boilers are so efficient that 40 degree water couldn't be any worse than pipes passing throught 40 degree air temperature space. I think there will be so much extra heat created that you will have heat to spare... Just a sophomoric guess. as always the proof is in the pudding.
Joseph