The suggestion of "replacing" your ductwork, as suggested by a pro should tell you that your comment about working "acceptably" is incorrect. The gshp may require greater airflow than your current system can handle. As well, the vast majority of residential ductwork leaks like a sieve. I would recommend that you totally eliminate your current system ...gulp, and look into mini split heat pumps. No duct work is necessary. Very high efficiency and you can do it for FAR less cost then your gshp. You will have tremendous energy reduction with better comfort than you are currently running and be very close to the operating costs of gshp. Check out www.fujitsugeneral.com and Mitsubishi.com

If you end up going with this gshp and new ductwork, I would strongly recommend that you have a contract that requires third party testing of the ductwork. Do not pay until the system is tight. Otherwise you are not getting what you paid for.

I did as I have suggested in my home and my daughters. Pulled all the old ductwork and equipment and put it Fujitsu heat pumps. Terrific. I'll never put in a piece of ductwork in a home again. Check it out!

Just a couple of points, If you're presently using heat pumps and you say they work fine then your duct work should be OK. It should be checked for leakage and repaired, perhaps some modification may be in order but it doesn't sound like it needs to be ripped out and replaced.

As for the desuperheater for domestic hot water production, I would definitely have this option because you're in an A/C dominated climate, probably very hot and humid. The desuperheater is a BIG plus and will help as you are extracting heat from the envelope which will go into your hot water tank rather than being sunk into the ground. If you were in the North where you would have a heating dominated climate and were trying to heat your home as well as heat the hot water then there may be some merit to the recommendation but not in your case. I would also absolutely have the desuperheater feed a buffer tank that then feeds your active hot water tank. Your hot water should be virtually FREE while you're air-conditioning.

As for your ground conditions, is the ground wet or dry? How deep is the undisturbed earth (overburden) till the bedrock? I would be careful about horizontal ground loops if your soil is mostly clay and sand - not the best conditions! I'm not saying it can't be done, just be careful about what you do. If you do a horizontal ground heat exchange be sure it's properly sized and I would recommend installing a 'soaker hose' so that should you have a very long dry spell you have a method of rehydrating the ground loops otherwise you'll be up a dry creek without any A/C!

As for the mini splits, I'd sooner air-condition in your area with the ground at around 65F to 67F than with the air when it's blistering hot!

SR

Is it hot and humid where you live? If so, having the ducts outside is not the best. Where is the recommended additional insulation going, on the attic floor or under the roof deck? Where I live, often the recommendation for attics with ducts is to seal the attic and insulate the roof deck. Then you could go with mini splits as Jackfre suggested. I like mine and the efficiency rivals that of ground source. You'll have no more pesky HVAC-induced pressure differentials sucking our your precious conditioned air either. Crunch some numbers to see what makes sense.

Thanks for the comments.
Jackfre: The rep did not say that more air flow is needed--just that the existing ducts (especially the ones in the original construction) probably leak. He recommended at least having it tested.
fsq4cw: I'm not sure why the rep said that water heating would not be possible. I think he said that it would be for a self-contained system, but not the split system he would recommend for my application. The ground here is pretty wet in winter; a drilled post hole will fill with water. But I still worry that heat conduction could be compromised by air pockets that remain after pulling the pipes. I don't know if grouting would eliminate that issue.
ionized: The ducts would be in the attic; the insulation is in the attic floor.

There are drill rigs that can drill from one central location out like an upside down sugar cone so the area that the lines area close might be 5' around at the entry and whatever is specified depending on the yard size at the bottom. This allows a person to plant trees or do about anything without being limited by wells across the yard.

Of course in Alabama, it usually wouldn't make financial sense to convert to geothermal.

What do you pay for HVAC now? How old are your heat pumps and what Seer are they?

What does your geo person claim for improved efficiency and cost savings? I'd love to see the numbers....

Minisplits can make more sense but the federal government has picked geo as the winner here so that 30% goes a long way to distort reality.

Regarding how to handle ductwork to achieve higher efficiency, have not heard anyone suggest using a technique to seal the ductwork. The process involves sealing all the outlets then circulating a powder(?) that seals all the joints until leakage is minimal . Has anyone used this process? What was your result.