I have had alot of experience in the hvac trade with water source heatpumps.The outside temp has no effect on the performace with this equipment if installed to the manufacture spec.The cost of installing a secondary heat strips is less than 200.00 for the average house allows you to have a heat if there is some type of failure with the waterpump that supplies the system.Most WHP will supply 125 'F on a 0' day.
We did an ICF basement with 9' walls, 8" core Logix forms. We chose to go geothermal as well. Our site is alternating limestone and shale layers for at least 200' deep, with each limestone layer having more and harder flint in it. Not very conducive to digging trenches for burying a "slinky". We chose the dry well option. Our driller came in and punched 4 wells in our basement before we poured the floor. The wells varied from 170' deep to 195' deep. The varying depth was to keep from cutting the premade loops of 3/4" pipe. If all the pipe runs are the same length, you can dump them all into a manifold and the flow will be equal thru all of them. If you shorten or lengthen them, you have to put balancing valves in to equalize the flow. The driller showed up about 8:00 and if I would have waited till my lunch hour to go out, I would have missed him. They would punch a hole to depth, tape a piece of rebar to the end of the double pipe that has the u-turn fitting, then drop it down the well and lay it out to the place where it emerges from the floor. If the length was right, they would move on to the next hole while they filled the hole with the pipe in it up with a bentonite slurry. About the only requirement on the wells was a separation of at least 10' and the varying depth to make the pipe come out right.
We had bids from 4 contractors for the unit and the install. The one we chose was not the cheapest, but we had the highest comfort level with him. He ran all of our information, such as log diameter, height, width, ceiling material, basement walls, number and type of windows, doors, etc thru a computer program and came up with us needing at least a 3 ton unit, and a 4 ton would not be oversize.(You must be careful to not oversize or undersize the unit, as efficiency goes south if you do). The other three guys kinda scratched their heads, mumbled a little bit, and one said 3 ton, one said 4 ton, and the other said somewhere in between. Although they all came up with pretty much the same results, I felt the guy we chose had a much better approach. He presented us with a veritable book of calculations, results, and recommendations, while the other 3 just gave us a one sheet bid.
We moved into our finished basement last November so have gone thru one winter with it. Our unit is a two speed unit both for the fan and the compressor. Our hvac guy disabled the high speed since we were heating the basement only. It did very well last winter, but we do not really have a good test of it yet and won't have until we finish the upstairs and enable the high speed.
The calculations showed a 3 to 8 year payback when compared to LP. For his calculations, he used $1 propane, which it hasnt' been for a long time. We also found out our rural electric company gives us a better rate since we are total electric. I am putting in a gas fireplace for backup heat and for taking a chill off quickly when needed. Hope that doesn't screw up our rate.
One added benefit I had not counted on was the super deheater option. My unit has a water pump on it that circulates water from a preheat tank when the hvac is running. The water extracts surplus heat from the coolant going back to the ground and stores it in a preheat tank that then feeds the house electric water heater. The preheat tank is just an electric water heater that has not been wired up. When the hvac is running regularly, this preheat tank can reach 120 degrees. We have our water heater set at 130 degrees, so not much electricity required there.
Sorry about the long post, but in a nutshell, I would do it again.