Today I spoke with a heating/cooling guy about a split system geothermal heat system.  He described it as a 2 piece unit, a geothermal pump/cube unit and a separate air handler.  All by waterfurnace.  I've been researching geothermals off and on for 2 years but have never heard of this (but I am in WV, so it's entirely possible that we just don't know).  Any thoughts??

Most manufacturers make split systems. They work fine and in some situations work better than packaged units.

A split allows locating the compressor and geo water lines well away from the air handler, though at a slight increase in complexity and a slight loss in system efficiency. Noise, space, plumbing and air duct considerations may make a split a better fit for some situations.

Note that conventional air source central air conditioners / heat pumps are nearly all split systems - a split is by far the most conventional setup.

In our area, splits are most popular where folks wish to maintain a fossil back-up system. We have many aging power grids and automatic generators are very popular but often not large enough to operate the geo systems.
J

If the customer wants to have a fossil fuel back up ( as Joe mentioned ) then the geo system is split, with the air handler going inline with the fossil system. With this type of setup, the geo system does not run while the backup is running.

On some two story homes, a split is used. This keeps the compressor in the basement ( less noise and vibration ) and allows the air handler to go upstairs ( which eliminates having to run ductwork from the basement)

Thank you for the information when to use split system.  In retrofits do you reuse the old line set?  

Depends. If you have R22 and get a new R22 unit then reuse is fine as long as the lines are not leaking or corroded. If you change to a R410 unit then the good HVAC companies will replace all line sets. The two different freon units use differing lublicating oils.

Alex

How many tons do I need for an almost 1600 square foot house? The installer said 3 tons, but I read on here about someone getting a 4 ton for a 3300 sq footer...

On average you can get between 600-800sf per ton.  However, you can not go on this number alone to size the equipment.  This nyumber can be used to get a preliminary estimate of the system.  However, to properly size your system, a Manual J load calculation needs to be performed and then a duct system designed.  The orientation of the house alone can affect a system by 1-2 tons.  Another key component is to make sure that the system can handle the CFM requirements of the duct system.  We have done a few estimates where we had to increase the system by about 2 tons in order to get proper airflow through the duct system. 

Agreed, your contractor should do a proper load.
I've never had to upsize a system to fit duct work, but I have had to make them smaller and use a larger aux. coil.
Type of foundation not just sf matters to load.
J