Don Lloyd
New Member
Posts:10
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| 18 Apr 2010 11:17 AM |
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Here is a question for you experts. We know that an open loop system is more efficient than a closed loop because water is a more efficient conductor than dirt.
So why is a water to Air system more efficient than a water to water system? |
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docjenser
New Member
Posts:93
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| 18 Apr 2010 12:50 PM |
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Good question....
COP are usually around 3 for water to water, and around 4-4.5 or water to air.
One of the reasons I see that we are putting out 100 degree air, versus 120-125 degree water, meaning lesser delta.
When my water to water unit heats up 60 degree water, my COP is around 6, dropping to 3 when the water it is heating is 125 degrees. |
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| www.buffalogeothermalheating.com |
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Looby
Basic Member
Posts:134
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| 18 Apr 2010 02:22 PM |
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Posted By Don Lloyd on 18 Apr 2010 11:17 AM
... because water is a more efficient conductor than dirt ...
water ............................................ 0.34 BTU/(hr-ft-°F)
slate ............................................. 0.9 - 4.5 BTU/(hr-ft-°F)
schist ........................................... 1.4 - 2.2 BTU/(hr-ft-°F)
sandstone .................................... 1.2 - 2.0 BTU/(hr-ft-°F)
limestone ..................................... 1.4 - 2.2 BTU/(hr-ft-°F)
coarse sand @ 5% moisture .......... 0.8 - 1.4 BTU/(hr-ft-°F)
fine grain clay @ 5% moisture ........ 0.5 - 0.6 BTU/(hr-ft-°F)
Open loop is sometimes more efficient ...despite water's poor conductivity.
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| One measurement is worth a thousand expert opinions. |
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tinoue
New Member
Posts:72
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| 18 Apr 2010 03:47 PM |
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Posted By Looby on 18 Apr 2010 02:22 PM
Posted By Don Lloyd on 18 Apr 2010 11:17 AM
... because water is a more efficient conductor than dirt ...
water ............................................ 0.34 BTU/(hr-ft-°F)
slate ............................................. 0.9 - 4.5 BTU/(hr-ft-°F)
schist ........................................... 1.4 - 2.2 BTU/(hr-ft-°F)
sandstone .................................... 1.2 - 2.0 BTU/(hr-ft-°F)
limestone ..................................... 1.4 - 2.2 BTU/(hr-ft-°F)
coarse sand @ 5% moisture .......... 0.8 - 1.4 BTU/(hr-ft-°F)
fine grain clay @ 5% moisture ........ 0.5 - 0.6 BTU/(hr-ft-°F)
Open loop is sometimes more efficient ...despite water's poor conductivity.
If we're talking about a non-moving volume of water, than the thermal conductivities as listed rule. But if we've got water moving (i.e. through a heat exchanger), it's very different indeed.
The other thing that makes water vastly better in most cases than any solid material is that the surface area of contact is essentially perfect, while embedding tubing in the ground or any other medium, is not. Even if you grout a tube, there's going to be poorer thermal contact.
That said, the primary reason open loop is mostly more efficient than closed is that the source water will usually be at more or less a constant temperature year round. With closed loop, you will (almost always) seasonally vary the ground temperature, pulling it below ground water temperature in the winter and pushing it above water temperature during the summer. However, since it seems we picky for details in these discussions, at the beginning of the heating season, the ground will often be much warmer than ground water temperature because of all the heat dumped during the summer. During this time, the closed loop will be more efficient than open loop. The inverse applies in the cooling season - the ground will be cold after a season of sucking heat out of the ground. |
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jonr
Veteran Member
Posts:1048
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| 18 Apr 2010 03:55 PM |
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Exactly tinoue. |
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Looby
Basic Member
Posts:134
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| 18 Apr 2010 04:43 PM |
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Posted By tinoue on 18 Apr 2010 03:47 PMBut if we've got water moving (i.e. through a heat exchanger), it's very different indeed.
Very different ONLY if the water flow is turbulent. Turbulence drives the "ft" dimension
of "BTU/(hr-ft-°F)" essentially to zero -- so that BTUs don't have to travel through the
water for any appreciable distance. Standing water or slow-flowing water (i.e., laminar
flow regime) are horrid thermal conductors.
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| One measurement is worth a thousand expert opinions. |
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geodean
Veteran Member
Posts:1264
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| 18 Apr 2010 05:06 PM |
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I don't think that laminar and turbulent apply to ground water moving through an aquifer. |
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Dewayne Dean
Do not argue with an idiot. He will drag you down to his level and beat you with experience.
www.PalaceGeothermal.com
Why settle for 90% when you can have 400%
We heat and cool with dirt!
visit- http://welserver.com/WEL0114/- to see my system |
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Looby
Basic Member
Posts:134
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| 18 Apr 2010 05:48 PM |
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Posted By geodean on 18 Apr 2010 05:06 PM
I don't think that laminar and turbulent apply to ground water moving through an aquifer.
Correct, but in that case, heat transfer is largely a matter of bulk transport
and has nothing to do with BTU/(hr-ft-F) coefficients or thermal gradients.
Also, water flowing through an aquifer is in intimate contact with
the rock/soil -- which is a better conductor than the water itself.
Consider that wet sand is a far better thermal conductor than either
dry sand or pure water. That's because BTUs flow through a grain
of sand more quickly than through an equivalent volume of water;
however, a little interstitial water improves grain-to-grain contact
and enhances heat transfer between grains.
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| One measurement is worth a thousand expert opinions. |
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jonr
Veteran Member
Posts:1048
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| 18 Apr 2010 07:38 PM |
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>But if we've got water moving (i.e. through a heat exchanger), it's very different indeed.
>>Very different ONLY if the water flow is turbulent. Turbulence drives the "ft" dimension
Nope, advection or convection at laminar flow rates move heat very well.
It's silly to talk about the thermal conductivity of water as if movement doesn't occur. Your "pure water" is a hypothetical situation that doesn't occur unless you are talking about ice - as soon as you start adding or subtracting heat, the water starts moving.
Air has less thermal conductance than fibrous glass - should one just leave wall cavities empty?
> a little interstitial water improves grain-to-grain contact
No, grain-to-grain contact doesn't change. |
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geotek
Basic Member
Posts:139
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| 18 Apr 2010 09:40 PM |
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Back to the original post.
We know that an open loop system is more efficient than a closed loop
because water is a more efficient conductor than dirt.
That's a misleading statement the efficiency is based on the EWT rather than dirt conductivity.
So why is a water to Air system more efficient than a water to water
system?
Again, the efficiency is based on the EWT both load and source.
A well designed radiant water to water can be as efficient as a water to air system.
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joe.ami
Veteran Member
Posts:2007
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| 18 Apr 2010 09:55 PM |
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My laypersons opinion has always been that radiant heat adds a middle man. If the goal is to heat air, why heat water first?
Much like purchasing most anything. When you can cut out a middle man, it is less expensive.
Conversations about radiant heat advantages usually involve thermal mass. Calculations get fuzzy.
Radiant heating also adds more than a little to install cost.
Comfy? You bet. Luxury vs economic incentive?.....I think so. It is sweet to have, but not easily measured in pure ROI.
'Course as the tag says, I'm just a mechanic....
Joe |
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Just a Mechanic;
Geothermal; Savings Underfoot |
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tinoue
New Member
Posts:72
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| 18 Apr 2010 10:14 PM |
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Word in the "green building" community is that radiant heating combined with massive slabs is starting to work against comfort when the house is very tight and well insulated and has passive solar gain. The daytime solar gain, which you want, combined with the thermal mass of the slab, can lead to overheating. We're getting a lot of people trying to use every technology in their high tech, super "green" homes without looking carefully at the physics, which can get complicated.... |
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jonr
Veteran Member
Posts:1048
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| 19 Apr 2010 12:23 AM |
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The goal is to heat people in a cost effective and comfortable way and that takes some combination of mean radiant temperature and air temperature. I agree on the interior mass issue - it has drawbacks.
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Looby
Basic Member
Posts:134
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| 19 Apr 2010 12:58 AM |
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Posted By jonr on 18 Apr 2010 07:38 PM
Nope, advection or convection at laminar flow rates move heat very well.
Yeah, if you mean moving heat from place to place (in the same way that a bucket
of hot water can move BTUs out to the patio), but to TRANSFER heat across a solid/liquid interface (such as a pipe wall) you really need turbulence. ...or patience.
Why do you suppose loop design manuals worry so much about Reynolds numbers?
Air has less thermal conductance than fibrous glass ...
Speaking of silly, comparing TC's of air and water wins the grand prize.
Gases have molecular diffusion rates about five orders of magnitude
greater than liquids -- not to mention the HUGE differences in viscosity
and temperature-density relationships (and thus, convection).
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| One measurement is worth a thousand expert opinions. |
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Looby
Basic Member
Posts:134
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| 19 Apr 2010 01:41 AM |
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Posted By geotek on 18 Apr 2010 09:40 PM
That's a misleading statement the efficiency is based on the EWT rather than dirt conductivity.
'Zactly! And in the open loop case, thermal conductivity doesn't even enter the picture.
Open loop EWT is at the local (undisturbed) aquifer temperature. However, open loop
pumping costs can have a detrimental effect on overall system efficiency (compared to
a typical Grundfos UP26-99 closed loop circ pump that consumes only 245W). |
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| One measurement is worth a thousand expert opinions. |
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jonr
Veteran Member
Posts:1048
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| 19 Apr 2010 10:53 AM |
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> really need turbulence. ...or patience
Less than 10% loop length difference between laminar and turbulent flow in the case of plastic geothermal loops. Thin wall copper heat exchangers would be quite different. |
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Looby
Basic Member
Posts:134
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| 19 Apr 2010 12:33 PM |
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Posted By jonr on 19 Apr 2010 10:53 AM
Less than 10% loop length difference between laminar and turbulent flow
in the case of plastic geothermal loops.
Measurement conditions? ...GPM? ...temperature? ...pipe diameter? ...antifreeze type? Got references?
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| One measurement is worth a thousand expert opinions. |
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heatoftheearth
New Member
Posts:96
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| 19 Apr 2010 02:29 PM |
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Posted By jonr on 19 Apr 2010 10:53 AM
Less than 10% loop length difference between laminar and turbulent flow in the case of plastic geothermal loops.
10% translates to thousands of $ |
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Don Lloyd
New Member
Posts:10
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| 21 Apr 2010 01:20 PM |
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Still do not see a good answer to the question regarding W to A vs W to W. First of all the new Energy Star minimum COP and EER numbers all have higher numbers for the W to A. But to me it is counter intuitive because my W to A system delivers heat at 165 F, then as it goes thru the ductwork, it delivers at 100F. The W to W only goes to 120F. Seems as if it should take more energy to get to 165F. Something I'm missing! |
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cnygeo
Basic Member
Posts:152
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| 21 Apr 2010 01:33 PM |
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Where are you measuring 165?! If that's your condensing temp I think your compressor will be toast in short order. |
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