First I should perhaps introduce myself. I’ve posted a few other messages, and I am really grateful for the responses. I am just starting out as home-owner, landlord, remodeler. I truly believe in green building- and I am trying to learn and incorporate as much as I can into my projects. I am currently not doing this full time, although I would like to.
At any rate my question now is regarding geothermal heating and cooling systems. I understand that there are 2 models- 1 with a very deep well and 1 with more shallow well. I believe what is needed is enough room to transfer heat along the run of pipe running through the ground. For a shallow system it requires enough room to run pipe back and forth to exchange heat. I have heard this method is cheaper. If there is not enough room to run a shallow system, a deep well is required to exchange heat. How am I doing so far?
I have a few questtions- why is this so expensive- especially in new construction, why cant the excavator dig out the required are- i think for horizontal sysems the piping only needs to be 4′ deep (I live in the northeast). It seems this would be a marginal expense, and then running pex? in this area also seems like a minimal investment. A heat exchanger connected to the system and a pump? How can this be more than $4000? It seems like it should be more like $2000 to me? I’ve heard prices are somewhere around $30,000 for deep well systems, and I didn’t think it was that much cheaper for horizontal systems.
I assume there is a way to calculate heat needs based on SF of living area? and how many feet of pipe one would need?
Does the layout really need to be precise, like spacing between pipes? especially if there is a back-up heating system?
Is this something that is reasonable for me to undertake on my own?
Are there any good resources that people can share?
For a deep-well system I can understand the added cost. Although I’ve heard these systems are $100,000s for commercial projects. I am only doing residential work, so I don’t think I would need a huge system, even if it’s not supplying all the heat- it seems that it could at very least pre-warm water for forced hot water system.
Thank you so much, this forum has been a really great resource for me.
Replies
There are at least 4 different systems that I know of.
Open water source. Uses a well for water, passes through the heat pump coil and then is discharged. Sometimes into another well, some times as surface water.
Vertical bores. Holes are bored and the loops are inserted and grouted in place. In at least one case I know where the hole was bored for water well, but was dry.
Horizontal loops. Dirt is trenched out and coils laid in place and back filled.
I think tht PE (polyehthane tubing is usualy used). And connections are thermoweld. Likewise for the vertical bores.
I have not done any research, but I suspect that in the NE more like 6 ft is needed.
The last method is direct exchange where freon tubing is put into contact with the earth. There is only one or 2 companies that make that type of equipment.
But the first thing is that you need to do is figure the amount of heating and cooling that you need. Then look at the amout of space needed for the horizontal loops or vertical loops.
Then you need to look at the size of your lot and the space that can be used after the required space for the house, septic, etc.
Then look at the surface soils and the deep earth conditions.
THEN decide if a ground contact system is practical and which one.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
#5. The loop is placed in the bottom of a closeby pond that is deep enuff so as not to freeze all the way to the bottom. And then trenched from there to the house.
Bill,
You didn't mention the true geothermal - the kind that they use in Iceland and around the Geysers in Calif. This is the type in which water is brought from the ground at above the temperature needed for space heating. A recent map showed that large areas of the continental U.S. could actually use such geothermal Unfortunately, in most areas of the U.S., the resource is fairly deep - "Typically, geothermal wells are drilled to depths ranging from 200 to 1,500 meters depth for low- and medium-temperature systems, and from 700 to 3,000 meters depth for high-temperature systems." ( http://www.crest.org/geothermal/geothermal_brief_geothermal_resources.html ). A copy of the geothermal resources map is available at:
http://www.geocollaborative.org/publications/Location_of_Geothermal_Resources.pdf
It is a fairly large file, so be warned if you are not on a high speed Internet connection...
I guess this is what the loops are like- I think it takes less room to lay the loops when they overlap like this, and I suppose if you need 400 sq. feet per ton- and I've read an average house would need about 3-5 tons digging 1200-2000 sf 4-6' deep would probably add a fair amount of expense, although, it still seems to me that this should be able to be done for less than $10,000 (much higher than my original estimate). Unless the loops overlapping in this manner save a lot of space.
can't open your pic ... it is in .pict format?
That's 400 lineal feet of well depth per ton ... depending on soil conductivity in your location. Don't recall pipe spacing and lin ft for direct burial/horizontal system. You might be digging up say 2-5Ksqft of area 6 ft deep, though. Seems like you should find some guidance on line for the direct bury systems to give you an idea. I think you can put e.g. 2 layers in ... can't recall the vertical spacing ... I'm thinking 1-1 1/2 ft, though of separation.
I don't know where this picture is from or who is doing the install, but that would not be how it would be done on my site. There may be reasons they're doing it that way, but if'n it were tried on my site, someone would be on the backhoe digging more trench real soon.
What pete said!!
Lotta feet of pipe there, but all it will do is freeze a wad of soil if the soil is at all dry. If there is flowing ground water or that trench is below ground water level in the winter, probably OK. Would not do it that way for my own house if it is above ground water level. Would expect to see a few puddles anyway at this time of year if the ground is wet.
It will work ... if you have a balance of heating and cooling ... you will likely never freeze 'a wad of soil'. Even if you don't have balanced heating/cooling loads. The surrounding ground will/should eat up any chance of freezing. If your coil is too short, that may happen ... that is why you have longer lengths ... to dissipate energy over a larger area.
Right ... true high temp geothermal tends to be very unique and rare ... particlarly for a single residence like this. We are talking about low temperature ground coupled ground source heat pump (GSHP) systems here, I think.
Mr. Hartman is off to a good start ... sans the direct loop in the pond. Never heard of the DX type direct bury in the ground ... I think I would avoid that ... if it exists. Pick the right system for your application. If well drilling is easy, then do that. Consider a 'test' well to check the conductivity of the soil. If you have a nearby body of water ... definately use it.
I think a rule of thumb is about 400+ ft deep per ton of system. I don't think depth matters much probably 150+ minimum, maybe.
Systems expensive ... generally, yes, but again ... choose the right system to minimize the cost. There is no such thing as a more right system as a GENERAL rule. They all have there +/- for YOUR situation. Well drilling can be expensive. So can large area excavation ... which I think should be about 6ft if I recall .. I think someone mentioned that depth.
No backup heat!! Save your money. If you do some of the work yourself, sure, you can save big bucks ... got a well drill in your back pocket?
jack.... we used to sell & install geothermal and water-to-air heat pumps in the mid-80's
we also installed pool heating systems with solar & water-to-air heat pumps
everything about geo thermal is expensive and needs lots of fail-safe
it also is high maintenance and the technicians who service it are top dollar
there is nothing a geothermal system can do for me that i can't do cheaper & better with super insulation and low tech heating and cooling
there is almost nothing green, besides hype, to geothermal
why would a homeowner subject himself to dealing with this for the next 20 - 30 years ? the high initial cost is only the beginning
Just for kicks, did a quick 2D thermal analysis of damp loose sand and ground water saturated soil. Plots are in Deg C. for 400 ft per ton/heat loading, damp sand freezes out to almost 3 ft diameter, ground water only to about 3 inches.
Thus, pipes need to be >> 3 ft apart for dry or only damp soil, but the coil on coil layout would probably work in ground water laden soil.
Top pic is 15C ground temp, damp only soil, bottom is ground water soaked soil.
View Image
Edited 6/1/2008 9:34 pm ET by junkhound
Edited 6/1/2008 9:36 pm ET by junkhound
Not sure, but I'm thinking your analysis is a little too simplistic to truely get the feel for what really happens w/ GSHP ground loop systems ... you could say the same about dry wells w/ loops in them ... except I guess that the wells tend to be spaced further apart.
A good experience GSHP contractor/consultant can help iron out some of the details. Probably not a good system for DIY guestimating to be done ... if you guess wrong, you could end up as you say ... freezing.
GSHP systems are fairly well proven and used in both commercial and residential systems ... successfully.
I disagree ... respectfully. Ground source heat pump system should be very low maintenance since the equipment is not sitting outside in the swelter heat or freezing temps of the weather ... which relatively speaking, will shorten the life of your equipment.
I've designed GSHP systems for commercial that were cheaper to install than the conventional counterparts ... but that is far from the rule. GSHP systems tend to be expensive, but if you choose the right system for the right site, you will minimize the cost. Bottom line ... you can't make any generalizations of the applications.
GSHP are FAR and away MUCH more energy efficient than conventional heating/cooling systems. For summer, I'd rather dump heat to a 50-60 degF source than the ambient air temp at say 90-100+ degF. The reverse is true for heating ... I can continue to use my heat pump well below 0 degF when my conventional heat pump has crapped out and I'm forced to use my back up heating at say 80 % efficiency.
Green is relative ... and I'm tired of hearing the green buzzward at every intersection. But GSHP are as green as anything and far better than conventional ... albeit they tend to be expensive to install (hence they adhere to the laws of thermo ... there ain't no free lunch).
clew...
if you insulate the structure all the technology becomes superfluousMike Smith Rhode Island : Design / Build / Repair / Restore
11,000 posts congraulations.
I agree with you on this one.
Start off with the all the basics, insulation, infiltration, solar gain, etc... - all the things that have no moving parts and require liittle or no maintenance.
Get those right for the climate and everything else is a lot easier.
Bill pretty much covered the system types.
GSHP systems use a poly propylene piping, not pex. It is charged with a antifreeze mix, and should be checked annually. A horizontal loop depth of 6 and 4 feet are generally used for out going and incoming loop coils. The separation is important and may be greater depending on soil type. Soil type also determines the loop lengths, thus the amount of space needed for installation. The same thing applies to loop depth in vertical install, although the in/out loop shares the same bore hole.
In terms of heat/cool exchange think thermodynamics. Water source is first, followed by solid rock, then stratified rock and clay. After that is dense clays and then loose type soils. Sand and sandy soils are pretty far down the list IIRC.
Like poly gas line, the loops in GSHP systems require heat fussing or welding for poly to poly joints and fittings. The loops are then pressure and lead tested before the completion of the installation. The most common problems with early loop installations was leaks that developed because of fill settlement around poorly fussed joints. Installers need to be certified and have the correct equipment to insure a leak free system. That equates to more cost, just as it does with any proprietary product.
I had a 5 ton GSHP vertical loop system installed in a house I built 16 years ago. The loops have never leaked and the system has provided lower energy cost per square foot than any other home in the development. Had I built the home to higher standards that Mike Smith is talking about the operating cost would be even lower and I could have used a smaller unit size.
I'm not sure what is used for load calculations currently, but manual J is based on "standard or normal" construction techniques. Build better and you save on future energy cost period, no matter what HVAC system you choose. GSHP just happen to be one of the most efficient system available although also one of the most expensive.