Pipe spacing from Vertical loop

Well, it was only a little extra cost at the time and could have been (may yet be) helpful.

Sounds like you have a working system that you are happy with ... congrats!


Are DX systems still being installed? Was told that the lubricant that is part of the refrigerant is environmentally toxic and now restricted. (A DX system would be more efficient due to eliminating an heat-exchanger - so there must be some reason water loops are more common)

 

I've never done a link before, but here is an excellent thread about DX by Joe at AMI.

http://www.geoexchange.org/forum/geothermal-heat-pump-discussions/5063-the-trouble-with-dx.html

ChrisJ

 

So in reviewing various responses in this thread (an others of the forum) - there is no practical value for changing the way headers are usually laid out.

Since I have 100' of header (200' total pipe), I viewed this as somewhat a hybrid system (mix of vertical and horizontal loop) and thought of taking the most advantage of this distance.

In one post I read that heat gain in headers is not of concern because of higher velocities in the header pipes. However, I disagree with the statement. Heat exchange is increased with higher velocity (on both sides - but the ground is stationary except during earthquakes ). Any additional heat extracted by higher velocity will be in total BTU ... not in delta T per gallon.

So it comes down to practical use of $$$ ... ROI. This is something best known by experienced installers, not egg-headers like me (pun intended).


The looping in the earlier RI example with five 400' loops (each 200' loop 3' apart), it wasn't necessary to have the header 'split' - that is, the supply connected on one end of the field and the return on the other. To do so would require an additional trench going to the far end of the field with additional cost.

So while it isn't the best heat exchanger design to have supply and return so close, it is totally irrelevant for geothermal performance. With the slow heat transfer occurring in the ground, it simply doesn't really matter (kind of fascinating)

This standard design works well and is more practical to install. The real issue in performance is the total amount of ground covered.

 

David it is the amount of ground covered and the depth. How you get there matters little.

I've not heard the lubricant/toxic DX ban yet, but it sounds like the old "too much refrigerant in the ground" complaint by water source guys. It bugs me when people use propaganda to scare someone away from a technology.
There is no reason to make-up short comings with DX systems as real ones are abundant.

 

Nice summary. You are only one good install and a IGSHPA course away from knowing alot more than the average sub par installer.
Eric

 

Well thank you. Very much want this install to be successful. Its "the lack of the readies" as much as arrogance that has me DIY this project. Was happy to find this forum as there appears to be some actual knowledge here.

Depth is usually just limited by excavation costs ... a 7' trench is a lot more $$$ than a 5' trench.

Is there a rule of thumb for depth? Such as a minimum of 2' below frost line?

 

Generally there is not a standard "below frostline" depth. As a practical matter our depth and loop size are designed to deliver certain EWT parameters.
j

 

ChrisJ, my wife apparently approves of your endorsement as she gave you the thumbs up.
j

 

Would not "as high an EWT as feasible" be the goal? (define 'feasible', lol)

Ok, I have heavy, wet clay soil. Water pipes must be buried at least 4' to be safe from freezing. (Presumably this would be plowed areas, not those with 2' snow during the winter)

If I want 35 F EWT during continuous operation, what loop length and depth / ton?

If I accept 30 F EWT, what loop length and depth / ton?


When I got a quote last year from a Waterfurnace guy, he printed some pages from "GeoLink Design Studio". It shows on coldest days, 100% run time with 32 F 'Avg loop temp'. Comparing the Geo Space BTUs with the units specs, it appears the 'Avg loop temp' is EWT.

Quite frankly, this software confuses me some - mainly as it performance claims (such as average efficiency of 3.47 COP) are higher than I expected and not knowing the logic behind it all. Some errors are obvious, such as using Albany weather, which is 10 F warmer than where I live.

One page is based of OAT and Yrly Hrs with rows of descending temps at 5 F intervals. I presume this data compensates for colder ground temps from months of operations - one page has Ground Lag Time of 38 days. Anyway, the lowest EWT shown is 32 F.

Suspect that a good deal of this is to impress potential customers to get a sale. But would also think that Waterfurnace would produce some legitimately useful information.

 

Cool!! , I have learned a lot from all of the people on this forum!!

ChrisJ

 

What have your EWTs been throughout the year?

 

Lowest winter EWT was mid 30*'s during 2010-2011 season.

Summer high temps only low 70*'s.

What is your location?

ChrisJ

 

Your numbers sound good! Seem to remember lots of snow that year. Of course last year was too warm to evaluate a system, lol.

Near Saratoga Springs

 

Yes a mild winter for sure, EWT never went below 40*.

Originally our loop field trenches were suppose to be 250' X 5 with 3/4" pipe, but only 200' available. Bumped the pipe to 1", I don't know if that got us back all the heat transfer we lost w/500' less.

Drive by a couple times a year on the way to LG.

ChrisJ

 

While a dead thread, I found something interesting. Regarding spacing of vertical boreholes, this has been the response:

"Spacing of vertical boreholes is site specific as well - but usually in the 10-20' range depending on availability of land."
"Being a driller, I can assure you that 50' spacing on vertical loops is not a common practice. The most I have seen or heard of is 25' spacing."
"Spacing on horizontal loops are generally 10-15 apart, same for vertical loops"

According to this [ http://s3.amazonaws.com/suncam/npdocs/091.pdf ] "Net Normalized Annual Ground Load" requires increasing borehole length according to space between boreholes. Only 3 figures are provided in the article: Factor of 1 for single borehole, Factor 1.10 for boreholes 25' apart, Factor 1.21 for borehole 15' apart.

For example, if you needed 1000' vertical loop. Two loops 25' apart would need to be 550' each. Two loops 15' aport would need to be 606' each.

So for borehole 15' apart you have to do 21% more drilling/installing than if you placed 50' apart. Obviously available land makes restrictions, but if this article is accurate, then a lot more work must be done if the industry standard practice is 10-15' apart. Or does the labor for trenching between boreholes cost more that the extra drilling? It would seem that 10' more, going from 15' to 25', would not be significant. If drilling cost $20 / foot, 31' extra would cost $620. And so on...


I realize this could get complicated. For example, a job with 9 boreholes in a grid 3 x 3. The center borehole will be affected (degraded) the most by the others. Then there is setback from other peoples property. "Stealing" heat ... oh, Lord, don't let they lawyers learn about this one.

 

Sorry you missed the mark. Vertical borehole seperation is very site specific. It depends on soil type and groundwater, in conjunction with the ground water flow if present. It is about the transmisivity of the aquifer(s) in play. Your math while entertaining for mental excercise, does not have a very practical impact on residential systems. Only on large commercial fields.

I am involved in another technical discipline. Decompression diving. It is also driven by math and algorithyms but the commonly accepted version of decompression tables math goes something like this:

measured with a micrometer
written in chalk
and cut with an axe

it illustrates the gap between theoretical and actual field execution results while paying due respect to both disciplines, the ethereal and the actual.

I think about geo the same way
Eric

 

Another thing to remember David is we loop in round numbers......150......200....so there is often a little extra. Also 10% loop increase is not likely to have a 1F EWT difference.

 

Thanks.

One thing that interests me is the thermal capacity, not just the exchanger. Heard people say that the heat is from the sun ... and even that loops should be close to the surface for this reason.

But heat must be from the earths core with the mantle being an insulative layer. We just take advantage of some of that heat. The 'R factor' would affect exchanger surface area. On the surface (pun), it would seem that residential systems don't compare to commercial due to relative small heat need and space between houses.

Commissioned my system end of October. Am waiting until spring to post in 'Testimonials' since it has only been two months. However so far it is outstripping my expectations. Typically the home would have used ~250 gal fuel oil. Its been 61 days with 1647 kwh @ 13.5 cents = $222 [ note: this is submetered and includes both source and load circulators ]. Can't complain and even wonder if the submeter is broken (and will investigate this).

Overall the topic of geothermal heating is interesting. The technology has been here a long time. It has been said that 40% of USA energy use is HVAC. It would seem there should be a priority to address this - it should easily be cut in half or to one third. 'Easily' is a caution word though. Failures (like Tamar's system) are too common. One installer around here did 10 systems and has faced 9 lawsuits. It is clear that inside the house/building needs attention and $$$ - not just the loop field.

On another thread, it was posted that some large commercial systems are successful at 100 ft/ton. I won't post further question there because it isn't the purpose of that thread. But the factors in designing heat 'extraction' are fascinating.

Thank you for your responses.

 

Your next stop along the way should be investigating the collection and data gathered during a " conductivity test ", and the subsequent interpretation and implementation of that data. I am very good at collecting the data, everything else is above my pay grade.
Eric

 

<Crunching numbers>

Then you get something like this:

· Average FTC: 1.12 W/m K (0.65 btu/hr ft F)
· Average thermal diffusivity: 8.74 x 10-7 m2/s (0.81 ft2/day)
· Undisturbed formation temperature: 10.8C
· Effective borehole thermal resistance: 0.089 K m/w (0.154 hr F ft/ btu)