air vs geo heat pump

I am doing some prelimary research prior to building my retirement home. Home will be about 1200 sq ft in located in Ga. about 15 miles from fla. border(inland near Tallahassie Fla). Very hot prolonged summers and short mild winters. I will probably get a heat pump with high efficiency and was wondering about air vs fluid well. House will be extremely well insulated etc but of course I have not finalized plans enough to do heat loss/gain worksheet so I can not size unit. In general what will be the cost differential between the 2 types assuming use of upper middle class equipment.
Thanks stan
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On Mon, 31 Jan 2005 02:38:56 -0500, snipped-for-privacy@nospam.net <uriah> wrote:

Most people avoid talking $ because there are so many variables. You'll find many reports that indicate a GSHP (ground source heat pump) will run $2k-$3k/ton. In my area, that figure was more like $4k-$6k/ton. However, in general, because of the need for excavation or drilling, you will have that cost on top of the cost of the heat pump. It's quite labor intensive. In my specific case, for a 4-ton system, I was replacing a furnace and an A/C. Top shelf units for those two would have run me about $14k installed. The GSHP system quotes I got ranged from $18k-$23k.
Since you're in a southern climate, the cooling needs will dominate the heating. For a ground source heat pump, you have to be very careful about the loop field, especially in such conditions. What happens is this - when you have an imbalance between heating and cooling, the ground around the loop field can raise (when cooling) or lower (when heating) every year because you're pumping more heat into the ground than you are extracting (in the case of cooling). If the loop field isn't designed properly, then this will cause your system output and efficiency to drop each year.
If you do go with a GSHP, you need to cover your butt and get a written guarantee from the installer for the long term system performance. For example, if you're buying a 4-ton system, it might actually have a rated cooling BTU output at "design temperature" of 40,000BTUH. Based on this, and the cooling load calculations for your house, you might decide that this is perfect for you. Now, say that the loop field was designed wrong. After the first summer, the system is moving 35,000BTUH. After the second, only 30,000BTUH etc.
With an air source heat pump you have no such issue. The heat pump will bring heat/cooling to your house in a manner that's directly related to the temperature of the outside air. These curves are published. So if you buy a 4-ton system, you can look at the chart and see that at 47F outside temp, the system can move 48,000 BTUH (or whatever the value is). At 20F or 100F, the system may only move 30,000 BTUH, but if you know this in advance, you can properly size the unit for your home's needs. There shouldn't be surprises.
In your particular case, that's a very small house and if it's well insulated and tight, your heating / cooling needs would be quite modest. Let's say, for example, the load analysis shows that at 100F, you will need 10000BTUH to keep your house interior at 75F. (Again, keep in mind that these are totally made up numbers for example only). You might think that you could buy a 1-ton rated heat pump and be all set. However, if you look at the curves, you might find that at 100F, the heat pump will only deliver 6000BTUH of cooling to your house. The system would then be undersized at 100F even though it is perfect for, say, 80F.
Your heating contractor should be able to explain how the system they're providing is the right sized based on the projected heating/cooling needs of your home.
Note too that dual compressors and variable speed air handlers, it's possible to get systems that have multiple output capacities. On moderate days, they can run with a lower capacity, which will be more efficient. Then, on the tough days, they use their second stage to give you that extra capacity. In this way, the delivered heating/cooling can more match the actual needs of your home.
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" snipped-for-privacy@nospam.net" wrote:

In addition to other cautions...had one in E TN until moved back to farm. Worked <very> well, much better cost efficiency operating. Initial cost differential can probably be minimized w/ proper timing of excvation work if building a new house rather than retrofit.
If you're in TVA or Southern service area, check w/ them--they both had extensive programs evaluating geothermal in their service regions...
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