I currently have a 2700 sq. ft. "L" shaped ranch house in RI, with oil fired baseboard heat.  The heating is divided into 3 zones.  Two in the living area and one for the bedrooms.  As I have been delving into the possibility of getting a GSHP system installed I have started to wonder about the need for three zones with the geo.  The only reason I would want more than one is to save some money by turning down the heat in unoccupied areas and also in bedrooms at night as I prefer sleeping in a cooler room.  My question is this, is it really necessary to have more than one zone in terms of installation cost, cost to run, and comfort or efficient use of the geo system? 

Thanks for comments.

You can have multiple zones with Geothermal, so instead of one 4 ton unit, you could have two 2 ton units, one for each zone. However, Geo is most efficent when you leave the temperature the same all the time, turning down the heat like you do with Oil or Gas systems at night or when your not home isn't recommended.  I tried the Turning down the heat when your not using it stratagy with my geo system and I found that I saved more when I left the heat at the same temperature all the time. If you enjoy cooler bedrooms at night, there's no reason you can't turn the system down at night, but it's not the most effiecent thing to do and recovery times will proably be longer then with Oil or Gas systems. 
 

Primary reason for zoning is comfort - different parts of a house nearly always have different loads at different times of day. A secondary consideration is the ability to save energy by cutting back some zones sometimes. One can save energy by cutting back some or all zones, but one can lose money doing it if during recovery supplemental strip heaters are operated. Smart thermostats can accommodate this.

Zoning a system and using a 2 speed unit that mostly runs in low delivers additional benefits of quieter and more efficient operation, more even temperatures, and better dehumidification during cooling season

Whether the increased cost of zoning would ever be recovered in energy savings alone (If none of the other considerations matter to you) is certainly debatable, and the additional system complexity is a disadvantage as well.