geothermal heating/cooling question

[Adahn]

We're in the planning stages of adding a freestanding garage. The project will require moving the septic tank and leaching pools and therefore require excavation of a hole 16'd x 35'l x 15'w.

Is this deep/large enough for closed loop geothermal use? It would be nice to at least bury the tubing with the ground open, even if I don't complete the install at this time.

The home is on Long Island, NY, with about 2500 sq ft under heat/air.

 

[mayhem]

I'm not positive, but I think you need to go way, way deeper than 16' deep. Like another couple hundred feet or so.

 

[granpajohn]

I'm not positive, but I think you need to go way, way deeper than 16' deep. Like another couple hundred feet or so.

That's only true of the vertical well style. I'm not much of an expert on these, but by my estimation, if you use the horizontal type of heat sink piping, you shouldn't have to go more than 5' deep. It is at this point that the earth's temperature stabilizes. I have only dabbled in the vertical style and the economic preference (for a residential house) is to sink 2 wells about 75', (I think, don't quote me on the depth). This should provide about 4 ton of cooling. Might be able to get by with less on LI; I dunno.
In a wooded area, the deep wells save trees. In a golf course type area, the horizontal avoids drilling cooling pipes into the aquifer. Like most engineered structures, it may be best to talk to an installer to see how they prefer to do it. That will be least expensive.

16' seems like an unusually deep septic tank hole to me.

 

[d.n.f.]

My neighbour installed one last year in his backyard by digging a trench and laying pipe in it. Trench is below the frost line (5 or 6'). Area (I am looking out the window and guesstimating) is maybe 70' by 120'. Don't know how many loops he has in it. Don't know how much that area will heat or cool either.

The horizontal installs are way cheaper than the vertical ones if you have the space. He has his own back hoe which made things way cheaper.

btw the cost is not the pipe or the digging, but the heat exchanger. It is big big bucks. Payback is like 10 to 20 years depending on climate.

 

[Adahn]

16' depth comes from 12' tall tanks that have to be 4' below grade.

If the cost is mostly for the exhangers, perhaps that cost will go down over time. Excavation costs will only go up.

Some cursory research shows systems of closed and open loop, and types with vertical, trench, and 'slinky' tubing.
'Slinky' is likely best for the excavation I'm looking at. The question is hole depth/area.

 

[d.n.f.]

I asked him how much it was going to cost with him doing all the digging work and hiring a plumber for the fittings to the exchanger and such. He said it was in the 12 to 15k range (Cdn).

Digging the hole is the cheap part.

There is lots of stuff on the internet about this topic. Closed loop gets the environmental nod as you are not discharging warm water into the lake/river.

 

[granpajohn]

Closed loop gets the environmental nod as you are not discharging warm water into the lake/river.

The heat is still being discharged, by design. What is known as "thermal pollution". (Uh, that's during cooling season primarily.)

 

[reaperman]

I run heavy equiptment for a living. I have dug in probably a dozen geothermal units in the past. I have dug in some trench style, which consisted four runs 100' long and app 4' wide, and tried to maintain 10'deep. And a few bed systems, approx 30x40x8'deep. Whatever the installer mainly called for. Its kind of a "wives tale" that the earth is a constant temp at a give depth. At least to a certain degree. Anyone who has a private well can see for themselves what I mean. The water temp coming into your home in the winter is much colder than the water coming into your home in the summer. Here in Mn. the frost depth varies depending on many variables. But building code says water lines must be at least 7 feet deep. I buried the water lines at my house 9 feet deep, inside a pvc pipe. And just for a expermint a few weeks ago, I ran water from a yard hydrant that hadn't been turned on since late fall. I measured the water temp with my IR gun. It registerd 40 degrees! So in saying all of this, the deeper the better for a geothermal unit. The upfront cost of the system is expensive. You want this unit to work as best as possible. So I'd get the lines as deep as possible for maxium benefit. I'd say 16' would be great if it is possible, probably overkill a bit, but hay.

Another note: Your tanks may be that deep, but chances are your drainfield is most likely no where near that deep.

 

[d.n.f.]

Closed loop gets the environmental nod as you are not discharging warm water into the lake/river.

The heat is still being discharged, by design. What is known as "thermal pollution". (Uh, that's during cooling season primarily.)

Yeah but I think the fisheries concern is that you are supplying a constant hot water output at source, where in a closed loop it is over a larger area and in theory the water is less effected by volume. If that makes any sense. Not a bunch of fish/algae/plants hanging out at your discharge.