Question on closed loop well depth and double u bends
Last Post 25 Jul 2014 01:42 AM by docjenser. 17 Replies.
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mbgeoUser is Offline
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16 Jul 2014 01:44 PM
Hi, I'm contemplating a retrofit geothermal system for my 2500sq ft house in Portland, Maine. I have a quote for a 4 ton Bosch CDI Greensource SM048, electric backup, desuperheater, and two zoned ductwork (upstairs and downstairs). I have another quote for a 4 ton 5 Series from Waterfurnace. My main question is on well sizing for closed loop well. The Bosch quote wants to use a single 350ft well with double u bends. I've read this is around 35% more efficient than a single ubend, but am wondering if the depth is enough? My friend in town has a similar 4 ton with the same 350 ft, double u bend well, but he doesn't have enough data yet to analyze. The Waterfurnace quote proposes an 800ft well so there's quite a bit of difference. If the well isn't deep enough will the system not be as efficient? My primary use of the system will be heating, as I am in Maine. I appreciate the help!
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16 Jul 2014 04:15 PM
What data does your friend have available? For example, the temperatures going in and out of the well when the system has been in use for many hours.
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16 Jul 2014 09:11 PM
installers should have software that will give a report showing how well the loop will perform with your heating load. If they don't, I would be wary. I also think that 35% is a bit rich for the benefit of double ubends
Dewayne Dean

<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system
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16 Jul 2014 09:12 PM
I also bet that the 800' is two 400' bores. I doubt they are planning on one 800' bore. Never heard of any one doing that
Dewayne Dean

<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system
docjenserUser is Offline
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16 Jul 2014 11:50 PM
The double u-bend often used in Europe increases performance of a single borehole usually by about 15%, the surrounding ground is more the limiting factor. Not knowing the local geology, but there is not a scenario I can think about where the ground conductivity and diffusivity would be high enough to have a 350ft borehole support a 4 ton system. Water temps would drop too low for reasonable efficiency, if they would work at all. The other concern would be pressure drop with 2 circuits. What pipe size do they propose?
www.buffalogeothermalheating.com
mbgeoUser is Offline
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17 Jul 2014 09:06 PM
I believe with a double u bend it would be 1 inch as it is a 6 inch bore. We do have a lot of ledge. I have another quote for a 4 ton, but with a horizontal loop. They are proposing 850ft of trench. The trench will be 8ft deep with a a loop of pipe, back filled 2 ft, then another loop of pipe. So the system will have 3,400 ft of pipe. Does that sound more reasonable?
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17 Jul 2014 11:14 PM
Posted By mbgeo on 17 Jul 2014 09:06 PM
I believe with a double u bend it would be 1 inch as it is a 6 inch bore. We do have a lot of ledge. I have another quote for a 4 ton, but with a horizontal loop. They are proposing 850ft of trench. The trench will be 8ft deep with a a loop of pipe, back filled 2 ft, then another loop of pipe. So the system will have 3,400 ft of pipe. Does that sound more reasonable?


3,400 ft of pipe between 6 and 8 ft sounds more realistic. Again, what size pipe? What is the pressure drop? 1700 ft per circuit is a lot of flow resistance.
www.buffalogeothermalheating.com
mbgeoUser is Offline
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18 Jul 2014 08:10 AM
Thank you, I really appreciate the help. It is 1 inch straight pipe (not slinky). Hmm, I don't see the pressure drop on the design sheet. It's GeoLink Design Studio by WaterFurnance. I can ask the contractor if it's not on there. Are you asking for concern of power consumption to circulate through the length of pipe? I'm not familiar with pressure drop. Is there other information from Geolink that would be helpful? Soil is estimated at silt/clay (damp) with conductivity of .75 and diffusivity of .6. The performance data says when the outside air is betweeen 12 and -23, the avg loop temp will be 35 degrees. When the air gets up to 32, the loop is 39 degrees. By the way, I found your website earlier while researching WEL server, your systems look very impressive. I asked my contractor if he could install one. Sounds like Waterfurnance may be coming out with something similar???
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19 Jul 2014 12:53 AM
Geolink does not provide pressure drop data. Yes, the longer the pipe, the higher the flow resistance, the more power is needed to circulate the water. Something you don't interpret right with the geo-link. Looptemps are not influenced very much by the outside temperature. That is the whole point with geo. Yes, Wf plans a similar system called "Symphony". 1" pipe going out and coming back in a 850' trench makes 2x1700' loops (circuits), if I understand you correctly.

With that design the pressure drop is very high, requiring a lot of pumping power. 4 circuits in 2 trenches, or even in 4 trenches, would work much better. Are you pressed for space?

Both loop designs have issues, either not enough borehole, or too much water needs to go through too small pipe.
www.buffalogeothermalheating.com
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19 Jul 2014 09:42 AM
Thanks for the reply. What is the issue with longer horizontal loops? Power usage? Would it be noticeably more expensive to run the circulator pump? The Geolink software said 840 ft of 1 inch pipe. It doesn't say anything about linear amount of pipe. The contractor said two 420 ft trenches with the two layers of loop (at 6 and 8ft) for a total linear length of 3,3360ft. Is the two loops of 420ft of 1 inch pipe bad? Would three loops of 280ft be better? Or four loops of 210ft? Or should a bigger pipe be used? I'm afraid I don't know what pressure drop is or how it is calculated (even after some Googling).
docjenserUser is Offline
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19 Jul 2014 01:40 PM
Yes, power usage. You need to run the pipes in parallel in order to reduce the flow per pipe. You heatpump (4 tons) needs between 10 and 12 gpm. If you double your flow the flow resistance goes up in a square function, meaning it is 4 times as much. Double it again and the pressure drop is 16 times as much. Plus you need the same amount of pipe in the ground. 1700 ft of pipe has twice the flow resistance than 850ft of pipe.

If you run 12 gpm through (2) 1" pipes (6 gpm per pipe) of 1700' you loop field pressure drop is 50 ft of head (FT HD). If you run the same amount (12 gpm) through (4) 1" pipes (3 gpm/pipe) of 850ft, you have the same amount of pipe in the ground but you loop field pressure drop goes down to 7.4 FT HD, which is roughly 1/7 of the pumping power needed.

If you stack up the pipes you loose significant performance due to the second loop not getting enough heat from the ground below (because the first loop already catches the majority of the heat). How significant? Well, about 50% of the performance than the loop below. I have only done so if I was pressed for space.

So together your loop field design only performs at 75% and needs 7 times the pumping power versus a parallel designed loop appropriately spaced at the same depth (not stacked). You have all the room in the world, do it right. For the sake of this discussion, put (6) 600 ft slinkies in 6 trenches 10 ft apart, 8 ft deep. Your loop field would be 60x75ft and cover your entire load.
www.buffalogeothermalheating.com
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19 Jul 2014 10:12 PM
Just curious...on horizontal  fields why isn't slinky used most of the time?
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20 Jul 2014 12:19 AM
Posted By robinnc on 19 Jul 2014 10:12 PM
Just curious...on horizontal  fields why isn't slinky used most of the time?


You are there within a few days where most contractors will never get to.....I ask myself that every time I see someone do a non slinky horizontal loop field. They must have a reason, I just have not found it yet.
www.buffalogeothermalheating.com
Palace GeothermalUser is Offline
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20 Jul 2014 10:56 AM
slinkies are standard for us
Dewayne Dean

<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system
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21 Jul 2014 11:41 AM
Criticisms of slinkies include more feet/ft in trench over taxing soil TC and contact between pipes that rub against one another (during expansion and contraction due to heating or cooling the brine) that might one day breach the pipe wall.
I know of no evidence to support the later and ask someone to explain why a 6 pipe racetrack has less feet/ft than my 6 foot/ft slinky.
Preferences are what they are and sometimes like subjective things people will not always agree. That said as long as everyone uses enough pipe I don't care which system they use. I'll stick with slinkies.
Joe Hardin
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mbgeoUser is Offline
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21 Jul 2014 06:47 PM
For the design my contractor is proposing (straight pipe at 8 ft and 6ft) is more parallel, short trenches more efficient than less, longer trenches? I measured my backyard and it looks like I can fit 5 trenches at 168ft each, spaced 20 ft away from each other. Was wondering if this would be more efficient or if there was a magic number of length of trench or number of parallel trenches? Thanks again! One other thing, what style slinky do you use? I'm looking in the LoopLink software now and it has 18 or 36 on center (both of which are 36 wide coils). The 36 oc coils do not overlap each other. Do you use 3/4 or 1 inch?
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25 Jul 2014 01:42 AM
We use 3/4", 18" pitch, but you can use 36ft pitch, which doubles the length of the trench but improves efficiency.

Stacking the loops 2' apart is much less efficient than laying them out next to each other 2' apart. 20' apart is good but unnecessary, 10 ft is plenty.
www.buffalogeothermalheating.com
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25 Jul 2014 01:42 AM
We use 3/4", 18" pitch, but you can use 36ft pitch, which doubles the length of the trench but improves efficiency.

Stacking the loops 2' apart is much less efficient than laying them out next to each other 2' apart. 20' apart is good but unnecessary, 10 ft is plenty.
www.buffalogeothermalheating.com
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