Hello, I'm looking for some advice on whether I should go for pond or ground closed loop. New construction in Cincinnati, OH 3,722 sqft living space / 4,222 sqft of conditioned space in 1st and 2nd floor. 2,100 sqft in basement. Distance between center of pond and back of the proposed house -- 250 ft. Pond about 1/2 acre to 3/4 acre; 20 ft deep. Should I go with a ground or pond loop? Why? If pond loop, hyperloop or slim jim? why? What are the insulation requirements for walls and attic? Thank you in advance.

Personally I'd go with the horizontal ground loop if that is an option, but really the best thing to do is have some contractors out for quotes. They are the ones that will know your area and what works best.

If a contractor were to push the pond loop option I'd question them about the possibility for muskrats or other animals damaging the system, and whether that would be covered by the warranty. Also, a drought would not be a good thing if you had a pond loop. Not likely, but there is a slight risk.

Hopefully some of the experts on here have some experience with pond loops and can guide you further.

I defer to others on the pond vs. ground loop considerations. But in-re insulation:

The code-min requirements for Cincinnati are lower than what would still be economically viable in a long term present value financial analysis for new construction. Code-min walls would be R19 batts or blown in a 2x6 studwall, but after the thermal shorts from framing are factored in that works out to be ~ R14 for a whole-wall value. Similarly for attics code-min spec is R38 between joists, which is closer to R32 after framing fraction. With low density batts actual performance will be well below that during the hot & cold temperature extremes. Blown higher-density fiber stabilizes the actual in-situ performance over a wide temperature range. (Cellulose is usually a good value, but new-school fiberglass such as Optima or Spider "dense-packed" to 1.8lbs per cubic foot density will outperform it slightly, bringing your wall-R up to ~ R15ish after framing factors.) Going code-min and using low density batts are not in your best interests.

For a rough idea as to what makes economic sense in your area see table 2 on p10 of this document:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

Cinci is on the warm edge of US climate zone 5, so look at the whole-assembly (not center-cavity) values, so with a high-efficiency system such a geo the target values that make long-term financial sense are probably closer to those recommended for zone 4 than zone 5. An example of an ~R25 studwall after the thermal bridging of the framing is factored in would be 2x6 studs with cellulose cavity fill, and 2" of exterior rigid polyisocyanurate foam sheathing. (With that much exterior foam interior vapor barriers are neither necessary nor desirable in your climate- latex paint is more than sufficient to protect the framing & sheathing from winter moisture drives, and allows the structural wood to dry toward the interior) This type of construction is common in colder climates where R19 cavity + R5 sheathing is code-min, but for an idea of what it takes to go even thicker on the foam see:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

To hit R60 in the attic takes an initial blow of ~18-20" of cellulose that covers the joist tops or truss chords by at least 8". That much insulation has some heft to it (~2.5lbs per square foot). Be sure that any truss chords would be rated for the load, and don't try to support the fiber with half-inch gypsum. It's better to put up a layer of OSB between the gypsum & joist/truss for mechanical support to keep it from sagging over time, and detail the OSB as the primary air-barrier. using duct mastic/caulk/foam as appropriate for sealing seams and electrical/vent/plumbing penetrations, etc. Be sure that the framing/trusses allow for full depth insulation all the way out to the outer edge of the top plate of the studwall below.

Also, avoid ducts & air-handlers in attics above the insulation- it's a huge efficiency hit that also makes air sealing more difficult. If you need to put ducts overhead, increase the framing height of the upper floor 6"-12" to accommodate them keeping them fully within the pressure & insulation boundary of the envelope. The additional heat gain/loss of that extra foot of insulated wall is minimal compared to the losses of ducts in unconditioned attics.

Building the basement walls with insulated concrete forms (ICF) is an easy way of hitting the R15 (R16 is usually the lowest ICF offered.) Floating the basement slab so that there is no direct connection between the slab and the footing or wall removes a large thermal short circuit. Sub-slab insulation can either be 1.5" of XPS (pink, blue, green, gray- doesn't matter), but it's often cheaper to use 2" of EPS (white bead-board) to hit the recommended R7.5-ish. (Doing the upper floor walls with R22 ICFs would have good performance too, but it's usually at a price premium over other wall structures. A code-min would be met with R16 ICFs, but the upcharge for going to R22 isn't huge- it's the minimum offering from some manufacturers.)

With R20+ walls and R50+ attic the windows & doors start to dominate the heat load. Using lower-U-value windows than the U-0.34 code max is cost effective as is reducing total glass area on E,N, & W sides, and ad,usting the S facing glass type & area for optimal wintertime solar tempering. Avoid double/single-hungs & sliders too, since they're far more prone to leaking, with inherently more weatherstripping edge to fail. Fixed windows are the tightest, but push-out casement, awning, & hopper windows work better on windows that you want to be able to open. They offer more ventilation & egress area per square foot of glass than double-hungs & sliders, and the weatherstripping is typically more secure. (Similarly, swing french or patio doors are more functional than glass sliders.) With sufficient interior thermal mass (3/4" wall board instead of half-inch, etc.), you can probably do a significant share of the heating with passive solar by adusting the total area of the south windows up 25% and using high SGHC windows there, while reducing elsewhere by a similar amount. If the south facing glass exceeds 5% of the total floor area it's time to do the careful-math on just how much thermal mass you'd need to keep it from overheating, and figure out how to get there (stone/concrete/ceramic floors, etc.)

With a bit of design and good air sealing on the building envelope it's possible to get the peak loads under 3 tons in a Cincinnati climate without breaking the bank, even sub-2-tons isn't out of the question.

I would take the pond loop every day over the ground loop. We install about 85% horizontal slinkies, 10% vertical drill and 5% pond.

Lesser impact on the lawn, higher entering water temperature (always above 35F), thus higher performance and efficiency. Much better heat exchange.
We have one online
http://welserver.com/WEL0384/
and you can compare it with the performance of other systems (http://www.buffalogeothermalheating.com/sample_diagram.html)

We spead the slnkies out for better heat exchange, so I don't see a role for slim jim in our configurations. Make sure you have the right header pipe, pressure drop calsc and the right pump for your 250 feet to the pond. This is crucial for overall system performance.

Geo does not have specific insulation requirements. But it always pays out to go the extra mile.

I also like the pond loop option IF it can be done properly and is installed by a contractor with experience, however as others have said there are environmental factors that can change the dynamics of a pond and the piping.

As far as insulation do not forget about ventilation. If you are insulating and sealing you will have no air exchanges and it is vital to get fresh air into a house. Check out HRV's or ERV's with your geothermal contractor, they will know what is best for your climate and specific conditions.

listen to the pros pond is always more efficient.

Posted By rikmeister on 14 Oct 2011 02:19 AM
listen to the pros pond is always more efficient.

"Always"........no, and they come with other possible problems.
I wouldn't want someone to leave with the impression that other systems are less satisfactory, and one must use or dig a pond for an efficient system.

Joe - What are the other possible problems? One thing I though of was algae buildup, but I wouldn't think that would affect performance. Muskrats and other rodents would be my only other thought, other than some major environmental change causing the pond to empty.

I had considered a pond loop but my pond wasn't big/deep enough. Seems like I read other issues too, but I don't remember now.

If you like to fish in your pond could be a problem......:)

Or throw an anker out of the boat....

Dana,

What you suggest sounds about halfway toward a Passive House.I agree that the approach would dramatically cut heat load.

Posted By engineer on 15 Oct 2011 11:35 PM
Dana,

What you suggest sounds about halfway toward a Passive House.I agree that the approach would dramatically cut heat load.

It's all a balancing act of costs & benefits, and that's a fair assessment.

At the PassiveHouse extreme sometimes the cost of the last 20-30% of insulation (or better stated, the last 20-30% reduction in average overall U-value) can be more expensive than supplying that otherwise-avoided energy cost with photovoltaic-powered heat pumps.  At the PassiveHouse extreme they tend to use resistance-elements in the ventilation air stream to support peak heating loads which is very cheap up front, but doesn't fully eliminate the need for heat-pump mechanical systems for dehumidification in areas with outdoor dew points that average 60F+, at which point the higher efficiency of heat pumps for heating & cooling point takes the edge off the envelope-efficiency requirements.   The PassiveHouse spec makes more sense for the drier air of Europe than it does in the eastern or southeastern US, but it's a good starting point for the analysis. In some ways NetZero is a more useful spec, but that can also lead one off the lifecycle-cost path if you're not careful.

The upfront costs of geo are significant, and you have to balance the costs of envelope efficiency against the reduction in size on the geo, as well as the reduced energy costs.  The current levels of subsidy for geo make sense in retrofits, where the cost of retrofitting the envelope to high performance are quite high, but for new construction that subsidy is misplaced IMHO, since the lifecycle costs of higher-efficiency building envelopes tend to be lower than that of geo, which has maintenance & replacement costs. 

There are also inherent design risks with geo that affect it's as-used efficiency, as compared to pre-engineered (comparatively) cheap & low-risk ductless air source heat pumps.  With the right floor plan & climate, high efficiency envelopes and mini/multi-split air source heat pumps often make better economic sense, than geo, given the dramatic difference in upfront costs for the high-efficiency air-source heat pumps.  While somewhat less efficient than best-in-class geo, the cost of going mini-splite + photovoltaics to offset that difference in efficiency will often be cheaper than the ground-source system, when all subsidies are off the table (including that for the PV.)

If you have algae forming on the pond loops, I would expect a significant reduction in heat transfer. At least consider a generally higher performing option - a well with open loop dumping into the pond.

Stuart many of us here offer services, but we don't use every blog as an opportunity to advertise.

Find a local contractor who has done a few ponds in your climate. To me 20' deep with your load is perfect. Pump and dumps are higher energy costs and higher maintenance at least in my area. I service a 6 ton in a pond close to the same size but not as deep thats over 20 years old...duct work is marginal but its still working great. I don't think you will have enough load to affect the pond adversely, more so if you follow Dana's advice.

New here and to geothermal. I've been in talks with Halco for installation of a Geothermal system (and solar PV). I mentioned to them a pond loop after they initially proposed a trench or well. They've come back to me about the pond loop stating there is a lower COP as well as possibility of the water freezing around the pipes. I am located near Whitney Point, NY, my pond is 22 feet deep, size is approximately half of an ice rink (I should get actual measurements). Do they have a point here or not so much? Everywhere that I've read about pond loops has pretty much stated "Do it if you can".

Posted By pvalenta on 14 Nov 2011 11:08 PM
Everywhere that I've read about pond loops has pretty much stated "Do it if you can".

Hmm, I don't recall that vibe from my reading anywhere. Without dimensions suitability of your pond is hard to determine though manufacturers generally ask for a half acre and 8' deep minimum. Pond loops can have higher COPs if properly installed and designed. Pond loops also offer opportunity for trouble from anchors and wildlife and UV rays. Do it if you can? No, not if you pass enough earth on the way to the pond to bury adequate horizontal loops.

Joe -  The last sentence of your last post struck me and made me pause.  Initially I thought well yeah.... but then upon reflection I realized with a "great pond heat exchanger in mind" I would have ignored the conneting pipe underground (or probably realized it when it was being installed!!). 
This is the voice of experience giving the all imortant perspective.  Shows what can happne in the heat of the design, chasing the elusive ultimate system!

Thanks for sharing! 

Yes, you will have 500' of header pipe. You do need to do a pressure drop calculation, since the entire flow will go through these 500'!

Our ponds perform great, EWT in winter does not drop below 35F (vs 30F for horizontal loopfields). However, in the summer the EWT is higher, costing you COP in A/C mode. EWT tops 80F in ponds, while horizontal lops do not go above 65F at the peak of the summer.

The header pipe will not contribute as much to the heat exchange since the stratified pond is slightly warmer than the ground. While the pipe going out to the pond will get "pre-heated' by the ground, the water coming back will be too warm to benefit from any further ground contact.

The return through the ground will add heat in the Winter. The pond return temp will be around 35F and the ground temp around 55F (to start). Of course the ground will drop as it supplies heat, but it can only approach 35F, never reach it or drop below it.

It is possible that you would benefit by opening a valve to bypass the pond loop in the summer. The ground loop alone might stay below what the pond temp is.