Hello, I am an energy efficient home builder who is also looking to expand my company to include geothermal systems. I have been putting together a team of subcontractors for the ductwork, system install and well drilling required for the systems I plan to install on the new homes that I build. I have been interested in questioning some of the "rules of thumb" that most geo installers in my area abide by. In eastern CT everyone seems to like installing vertical bore systems with 1.25" diameter pipe, however, I have had several conversations with engineers and sales people of some of the big heat pump manufactures and they all seem to reccomend .75" or 1" pipe. My understanding is that 1.25" pipe doesn't offer nearly the same thermal conductivity because of the reduced amount of surface area of pipe to loop fluid in a larger diameter pipe. Every installer around here also seems to have their own feelings on the correct number of feet of loop per ton to drill. This leads to my second question: Is 150' per ton of loop sufficient for both .75 or 1.25" piping or is it for one or the other? Lots of conflicting information has passed my way. I would be interested in some professional opinions outside my area. Thanks for any reply.
1-1/4" loops Another thing with 1-1/4" loops is the much higher volume than 3/4" or 1" this equates to significantly more antifreeze required in a system. You would also need a larger bore which would cost more to drill and more grout to fill around the loops. I have run many scenarios comparing different diameter loops and 1-1/4" is a small amount more efficient than 1", like about 10%: not really worth the extra expense.
The best rule of thumb for geothermal is to not use rules of thumb. The ground loop should be sized based on the heat load of the house, specific heat pump model efficiency, and percent of load covered with an evaluation of various ground loop lengths and corresponding entering water loop temperatures to determine the optimal configuration to extract/inject the necessary BTU's from the ground. The diameter of the pipe chosen is generally based on the length of the borehole as long runs (>800') of small diameter pipes (3/4") require large inefficient circulation pumps. Hard rock drillers (typical of CT) like to set up their rig on one hole and drill deep and thus the deep borehole and corresponding 1.25" pipe. Other geologies lend themselves to lighter rigs and shallower boreholes. However, the borehole depths, antifreeze solution, frictional losses, and circulation pumps all should be modeled in Geo Software for each job. -Adam
Software Suggestions? Thanks for the replies. I attended a 2-day seminar last year put on by Heat Controller and they were demonstrating a geo software not yet on the market called Loop Logix. I used the trial version a little and it seemed ok, but I was hopeful that there was a better program on the market. Any reccomendations?
Not true. The thermal conductivity of HDPE geo pipe is essentially independent of pipe diameter. Heat flow through the pipe wall is directly proportional to surface area -- but inversely proportional to wall thickness. For standard diameter ratio (SDR) pipe these two effects exactly cancel, and heat transfer per linear foot is the same for any diameter. Surface-to-volume ratio is also unimportant in a correctly sized system. The key here is to maintain turbulent flow -- which is a function of GPM and antifreeze type as well as pipe diameter. You have fewer choices than you think: - The heating/cooling load establishes the flow rate requirements; just follow the heat pump manufacturer's GPM recommendations. - Local geology determines the total loop length; lean on the experience of successful loop installers in your area. - Finally, run some calculations for various loop layouts, pipe diameters, and antifreeze types to find a configuration with satisfactorily high turbulence (Reynolds#) and low pumping cost (dynamic head). .
If you use 1.25" pipe the software shows you about 10% lesser length than for a 0.75" pipe to satisfy the same heatload. We use 400ft of 1.25" pipe for each 3 ton, or 150ft of 0.75" for each ton in the Buffalo, NY area. With 0.75" you are limited to a maximum of 800ft of pipe per loop, otherwise your pressure drop is too high. I prefer lesser boreholes, besides needing 10% lesser lengths. Not repeated casing until you hit the bedrock, lesser mess in the yard, lesser header trench to connect all the boreholes, lesser repositioning of the rig, speedier installation. So as a con, more antifreeze due to more volume, more $ per foot of pipe, and you have to calculate the flow to ensure you are having enough flow for turbulence. Our drillers rig has 5 1/2 inch diameter anyway, so he could do both. For me the 1.25" pipe has more advantages.
Don't understand where the 10% comes from, but I won't quibble. 60% more surface area, nearly 3x increase in cross section, only 10% difference in length. Pretty close to diameter-independent. My system has about 1100' of 1.25" HDPE -- a single 450' bore plus 60' trench-to-house, plus indoor plumbing. One UP26-99 circ pump is good for 8 GPM to the 3-ton Envision -- anything under 1.25" would have required more pump or two bore holes running in parallel ...and I didn't really have space for two bores. Simple, easy-flush loop with no manifold. One 20' length of casing was more than sufficient -- solid rock from 12' down. Exactly the same situation that down2earth described for CT.
O.K. lets address the real issue here. It is not about the software, or diameter, or the increase or decrease in loop length. It is about the machine. Unless you are fortunate enough to live where geo thermal drilling is booming what you get from a drilling contractor is a water well rig used for geothermal. I am going to assume that in eastern CT we are talking about air rotary due to geology. Typically the smallest diameter rods for air rotary water well work are 4 1/2". The smallest hammer is 6", yielding a smallest bore possible of 6". Regardless of the size loop you insert into the bore, the cost per foot to construct the bore remains the same. So you will insert the largest standard size loop that will fit in the bore. The more space the loop takes up, the less volume of grout required to provide the thermal link. While the software and the educatures bear out facts and data they lack the feild experiance to help their students apply the software in a meaningfull way that directly applies to the region of the country that they are offering the course this week. As allways the answer is "get the local talent". What is commonly reffered to as rules of thumb have a huge grounding in reality and success for any given locality.
First off, any trench should be (must be!) well below frost line. Mine is 5' to 6' deep and within inches of the water table for about half its length. So, insulating it would yield a reduction in efficiency -- both winter and summer. Also, a 5% "difference" is far, far down in the noise compared to the numerous other uncertainties in any geo system. There's no way that anyone can estimate/predict heat load, soil conductivity, GPM, COP, EER, etc., etc. to that degree of accuracy. Finally, heat exchange for SDR 11 HDPE pipe runs about 7.0 BTU/hr-LinearFoot-°F (independent of pipe diameter). That should be all you need to answer the question of whether it's worth insulating your favorite trench. A wise customer will require the installer to deliver (a reasonable approximation of) whatever was promised in the contract. My contract said nothing about EER or COP, but it DID specify heating/cooling temperature performance (indoor versus outdoor), room-to-room temperature balance, and a guaranteed maximum operating cost. As a measurement weenie, I like to keep eye on temperature/pressure/flow -- but SWMBO only needs to compare this month's electric bill to our pre-geothermal utility bills. If sumpin' ain't working up to spec, she'll discover it before I do.
You'll find WF's comments on the subject in the following (page 20): WF Flow Center Installation Manual Be aware that WF recommends and sells Environol (ethanol) antifreeze, so their opinion of PG might be somewhat slanted -- OTOH, they didn't say anything negative about methanol. Also, try the forum's search utility. This subject has been discussed before. .
I am with you, I looked at it from a theoretical question, how much more heat transfer to you get with a 1.25" pipe vs. 0.75" pipe. The answer is 10%. However, there are more practical reasons to choose one over the other. Sometimes it comes down to the fact that you are out of pipe, and your supplier's truck broke down, and you can finish the job and go home if you take 0.75" pipe and fuse your own u-bend... As always in geo, you need to know hoe to adopt. Many things then can work, all you need is knowing which are the ones which do.
