heat loss underground


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Old 02-28-12, 02:55 PM
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heat loss underground

100 feet of underground pex moving at about 8 GPM 180 degree feed water losing 15 degrees

how much (or what is the formular) to figure the btu loss per hour?
 
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Old 02-28-12, 03:15 PM
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Hi billie,
The HVAC pros probably have an easier method, but the basic conversion is one pound of water raised or lowered 1 F equals 1 btu. So, gallons per hour times 8.33 pounds per gallon times 15 degrees. Thus 8 x 60 x 8.33 x 15 = 60,000 btu's per hour. Now the pros will give us the rest of the story.

Bud
 
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Old 02-28-12, 04:02 PM
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Sounds like the ground above the buried pipe would be a good place to put a cold frame to get an early start on some vegetable seedlings! Probably makes a dandy snow melt system to keep the path from the house to the wood boiler snow free!

You got it all right Bud, no reason for to get any more cipherers in here!

BUT... (there's always a but!)

Since we know that 1 GPM at 20 = 10,000 BTUH

8 GPM X 10,000 = 80,000

and 15 / 20 = 0.75

then 80,000 X 0.75 = 60,000

Billie, didn't you insulate the underground run?
 
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Old 02-28-12, 04:30 PM
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NJ, I knew you would have a quicker answer . But now I'm curious. If this is unintended heat loss to the ground, wouldn't the loss decrease as the soil saturates with heat. The exception being the heat that is escaping up to the surface or that may be carried away with water. It's like the China syndrome under a basement slab, where in reality the heat can only go so far as the resistance increases with distance.

billie, how deep in the pex buried?

Bud
 
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Old 02-28-12, 04:59 PM
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It's not necessarily quicker I don't think... more or less the same number of button pushes on the calculator... easier for me to remember though, cuz of that 1 GPM @ 20 delta T = 10K BTU 'rule of thumb'.

( 1 X 60 X 8.33 X 20 = 10,000 )

wouldn't the loss decrease as the soil saturates with heat.
I believe the loss would slow somewhat, probably not as much as one would think though. As the soil around the tubing is heated, the surface area of the heated soil underneath increases at the same time resulting in a larger radiating area. So it's kind of a 'catch 22'. It would reach equilibrium at some point... but still be a constant loss never the less.

Should have used thermopex or equivalent. Or maybe this is a hypothetical question only?
 
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Old 02-28-12, 05:02 PM
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thanks guys..my pex is good i have the logster stuff this question was for another guy on the outdoor wood forum, i kinda got teh same math as you guys but was uncertain if i was doing it right. i told the guy he was losing enough heat to heat another house and not many belived me so i thought i would pop over here and ask the experts. thanks and i will give this forum credit for the answer
 
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Old 02-28-12, 05:09 PM
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Folks believe what they wanna believe... they probably won't believe us either!
 
 

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