Thanks Dick,

Certainly something I had not heard yet.

Bill

Even if a pond loop were silted in, saturated mud will exchange better than similarly composed dry to damp dirt. Remember all the discussion and emphasis on the benefit of soaking a horizontal field?

If nothing else, a pond loop should never ever need to be soaked...

Any water movement is a bonus. Even still water will experience natural convection via heating or cooling of the tubing or heat exchanger

As with everything else proper design informed by local experience will win the day.

We use about the same length pipe for ground or pond loops. This easily mitigates the "crud factor." We don't calculate any advantage in pond loops other than savings on excavation.
I must reply to the ROI calculation comments;
We compare geo vs conventional or ASHP operating costs. It is based on a 20 year weather average and followed by disclaimers; but it is fairly accurate. Time and again I've asked retrofit customers not to tell me what they've spent heating their home until we see what the cost calculator predicts. In that way we have a control group that helps to determine the veracity of our calculation. Large disparities are usually data entry errors (most often forgetting to enter MI, it seems Tulsa OK uses a lot less heat and that is the default weather area).
Cost calculators are helpful in illustrating savings from different kinds and sizes of equipment. It is a tangible, dollars and cents, break down of 90% (afue) vs 400% (4.0 C.O.P.) or whatever the numbers happen to be. It also can help select equipment size.
A R.O.I. projection should be part of any sales presentation. I do it right after my heat load (of course some don't do that either). Without data......;)
J

I'll agree that inserting high quality data results in a high quality result. I didn't say not to do ROIs, just a suggestion to take the output with several grains of salt.

I like your 'blind' way of generating a predicted present cost BEFORE verifying it against historical data.

The biggest single uncertainty no one can predict is future costs of fuels and electricity. That injects huge uncertainty into ROI calcs when they span across different fuel choices.

The other problem is that so many systems underperform against SEER / EER ratings, in large part due to duct issues.

Then there is 'moral hazard' "My HVAC system is so efficient I'll worry less about backing down the stat and wearing a sweater", or "My tankless water heater is so much more efficient so I can take longer showers - besides, I now have endless hot water - no need to save some for the next user"

Here's an example on how 'slow' heat moves through earth.

This example comes from looking at heat transfer to/from a vertical closed loop.

In this case, heat is being 'sucked' from the earth, as the GSHP is in heat mode.

The base line (zero) is the 'down deep' earth temp - the 'undisturbed' temp that would be present if no loop, extracting heat, was present.  (This temp, BTW, varies geographically.  For my Dallas climate, it's 68°F.)

Here we see the EWT dives below the 'down deep' earth temp as the loop is trying to suck heat from the earth (blue line - 3 ton unit, black line - 5 T unit).  In this example, the 'gap' between 'down deep' earth temp and EWT gets to about 2°.

(The area between the red line and zero-axis represents the amount of heat in KBTU in real time being extracted from the closed loop.)

Since right now my heating need is only about 10 - 20 KBTU each day, with only a day here or there needing about 125 KBTU, the earth immediately adjacent to the pipe has time to recover.  I.e., there's enough time for heat to slowly propagate to the cooler earth immediately adjacent to the pipe by the next day such that EWT will again start out near 'down deep' earth temp.

As Winter settles in, more and more heat will be sucked from the earth, eventually getting to the point where there's not enough time in each day for the EWT to recover to 'down deep' earth temp.  Hence, on a weekly and monthly time scale basis, EWT will gradually decline to a low of about 8° below 'down deep' earth temp.  Then, as Spring arrives, the amount of heat needing to be sucked from the earth is less and less, allowing enough time for the temp of the earth immediately adjacent to the loop to recover to 'down deep' earth temp.

Parenthetically, seeing how slow heat moves through the earth (i.e. to places that are temporarily hotter/colder than the rest of the surrounding earth like the earth immediately adjacent to the loop pipe), it's apparent why the average drilling depth for a vertical borehole field, in the Dallas area, is 300 ft/ton.

Two things have to be fundamentally accounted for.  (1)  It takes a lot of pipe surface area in contact with the earth to suck in enough heat to the water.  And (2) it takes a long time for heat to move from the 'down deep' infinite heat source into the area immediately adjacent to the pipe and 'replace' the heat that was earlier sucked into the loop water.

I would think all of this is equally applicable to a pond loop.  My guess is a pond loop may even be a little more complex, to account for how much pond water current may be present, and how much 'fouling' of the outside of the loop pipe may occur over time.  Sounds like, absent the more complex design, the rule of thumb to ensure a well functioning loop is to design as if it's not a pond.

Best regards,

Bill

Bill,

Thanks for taking the time 'taking us to school'.  Very interesting stuff.

Over the past weekend, I received a call from a guy that did some asking around for me reference the above geothermal system.  His two 'sources' were not very keen on this system as they have heard numerous instances of corrosion in the Water Furnace it self.  They also mentioned the amount of buildup that would occur on the pipe under the water.

Since this will be a vacation home for at least the next five years, I am now thinking maybe the risk and expense are not warranted at this time.  I may, (still searching for systems) just go with the highest efficiency heat pump that I can find.  This might be the toughest decision I have to make on this project.  I will keep looking, but not much longer!!

Thanks all for your input on this thread, it is much appreciated. 

Bill

* The potential for build-up on the exterior of the pipe is what prompted me to say be sure you put in enough pipe to start this thread. One poster responded similarly with the comment that they would trreat it just like a dirt install. Since there is no ditch/backfill labor involved I would think of that suggestion as a being a minimum.

* Corrosion inside a Closed Loop system will not be a problem. The water is a one time fill that you can treat w/ suitable inhibitor-antifreeze additive. Your "sources" must have been referring to some experience with an open loop.

Dick

I would agree with Dick.....corrosion on a closed loop is never a concern as long as you start with a clean solution.

Bill, You definitely should crunch the numbers to see if the geo system will pay for itself given your usage expectations. But I wouldn't consider the geo system to be any more risk than any other system. Your contractor appears to have his act together and there's no reason to believe he won't size the loops properly. If this technology is popular in your lake area it is likely your contractor has experience - and should be able to give you some references. In an unlikely worst case scenario it wouldn't be very expensive to add loops.

The interesting thing about pond loops is that the physical characteristics of water really work well with the geo system. Lakes are coldest at the bottom during the summer when you need cooling and the warm water generated by the coils will naturally rise to the surface and be replaced by cooler water. The really neat part is that during the winter the lakes are actually warmest at the bottom and the water cooled by the coils will also rise to the surface and be replaced by warmer water. This is because water is most dense at 39 F.

Thanks everyone for responding. I agree that my buddy must have talked to someone that thought it was open looped. I will do a some more digging and try to make a decision soon. My contractor has done a handful of these projects, not on my lake, and certainly seems pretty knowledgeable. Continueing to study this part of my project.

Bill

I have read / heard that some earlier WaterFurnaces had an issue with "Formicary Corrosion" I believe it affected the evaporator, so the question of open vs closed loop is not relevant.

This is strictly heresay, no personal knowledge or experience.

Given the proximity of the lake and high end (SIP, ICF) technologies you've employed, it would be a shame to abandon geo. You appear to have another rarity - an experienced installer with fair prices.

If ROI justifies it there is no performance reason to avoid geo. Pond loops (sized like earth loops) have an advantage over traditional earth loops if size is similar. Even with silt or barnacle or tiger mussels...... your worst case performance is akin to saturated sand/gravel which works well.
J

Thanks J.

Bill