The only thing I might add to the previous post is that a 2ton unit might not be the correct size, even though you are only using 2 tons of heat on average the unit has to be designed to meet design load. Designed to meet the homes need at the lowest experienced temps.
   Although some manufacturers and designers are claiming COP of 4.5 I am skeptical of this. On the project we just finished we are data logging the energy being put into the building and logging the energy consumed for the heat pump. This will give me a real COP not calculations from the manufacturers engineering department. From all of my research on systems that have actually being logged and record, a COP of 3 appears to be a good number for cold climates. I hope that by tweeking the system ever so slightly I can bring this up to a COP of 4.
 I use a free program to do a rough heat loss, which also has a cost calculator built into it. After using this program and then having the energy modeling done with a DOE program, I found the calculations for energy loss to be fairly accurate. Then it is a simple matter of putting in your energy costs, fuel, efficiency and the program will give you annual costs and ten year out costs. It also has a co2 calculator that shows your carbon output. I frequently use this to tweek the envelope of the building, by added insulation to a wall this program will quickly show me how much energy and more importantly money will be saved by making this change. upgrading of windows can be quickly calculated as well as different methods of heating. This program is specifically designed for heating and does not calculate cooling loads. I have found the biggest area for savings to be air-infiltration, change the air infiltration rate, recalculate and you can instantly see the difference in Btu's lost and cost difference.
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

check it out, it is the easiest program I have found and after confirming accuracy with DOE-2 energy modeling program. I use this for a first source of reference when computating energy losses and needs for new construction. calculate out an old, poorly insulated, drafty home and then recalculate with an improved envelope and heating system and you can see how your energy costs can change. This is one way I use to justify the way I build, and get a fairly accurate estimate of heating costs associated.

Tom Pittsley
[email protected]
www.eebt.org

I'd go back and look again, at least at the WaterFurnace Envision series.  They advertise at 5.0+ COP.

I also have a data logging system.  (It displays real time energy usage at http://welserver.com/WEL0043/ , include instantaneous COP when in heating mode).  Below is a chart, from historical data, showing that in Feb. 2009, my COP was about 4.8 for my 5 ton WaterFurnace Envision unit, while running in 1st stage.

I'm calculating COP exactly the way WF does. 

So at least for a warm climate (Dallas) a COP of 4.5+ seems reasonable to achieve in the field, not just in the engineering department.  (I'm closed loop.) 

Best regards,

Bill