For $50 last I checked you can download a homeowner's version of Don Sleeth's HVAC-CALC software. Use it to run your own load calculation. 10 tons for 2000 SF is completely whacko unless the house is constructed of tinfoil at the north pole.

you have stated that you have large window area with high R value, used for passive heating during the day.
what is the R-value? please note the large glass area is required to be insulated beyond the value of the window itself at night to retain the heat that was collected during the day. when calculating the load calc for heating, the added insulated window value should be used.

What's the U value of the window?

They are argon filled, triple-paned, fiberglass with a 0.18 U value.  Its one of the best on the market as far as I know.

With that U value, it should translate to an R value of 5.56 (just take the reciprocal of the U value)

Buckybanjo -

this sounds like an awesome project and I fully applaud your efforts to be energy conscious - if only more people made as much effort as you do!

My comment (hope its helpful) is that of course it might help to decrease the glazing area some - but if that ain't possible please consider the SHGC as well. I have seen a number of time in this forum that when people talk about "great" windows they only focus on the Rvalue/U value and forget the SHGC. There is a very helpful post by the Thermo Tech (from Canada not the MN company by the same name) engineer where he compares 2 windows - and the one with the WORSE U value (i.e. the higher U value and therefore lower R value) actually wins out in total year round energy performance because it has a higher SHGC.

There is a value called the ER - that actually combines the U and SGHC values - but I rarely see it referred to. Please see extensive post by Stephen T here on 4/17 http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/16/postid/33808/view/topic/tpage/2/Default.aspx

Bottom line is - if you can't shrink the glazing a bit then go for the best combination of high SHGC and low U value.

Good luck with your great project. Todd

I just want to update on our project as promised. We have preliminarily selected an Earthlinked DX system working with their regional rep and a local installer. After doing much research (including great info on this site) I settled on the Earthlinked system because of efficiency (single transfer from ground because of DX achieving 400% efficiency) and cost (better overall but fewer and smaller wells save a lot of money). Our design has a 4 ton unit (as opposed to the 10 ton unit the initial vendor designed) which will run a radiant floor system but also a high velocity air handler doing both AC (not a huge need for us) and hot air. The latter was a great design element introduced by the installer which solves a big problem for us: since it will primarily be a weekend house we didn't want to have the radiant run for hours/days to get it up to temp when we were going there so the hot air will have it come up to temp quicker. We will also be running our DHW off the geo into a buffer tank.

On paper at least, we've achieved our goal of being fossil fuel free in our house and net-zero energy. We slightly upsized our PV array to 5.6kw and that should cover our geo electrical demands (Earthlinked says it should be 2,500-5,000 kwh per year for full time and less for weekend). The first part of our house is framed out and the PV should be installed in a few weeks so the contractor will be running off solar (and I'll be spinning my meter backwards) during construction. Photos here if you'd like to see progress to date:

http://www.flickr.com/photos/distsyst/

I'd like to thank everyone on here but especially Clark for helping me wade through my initial problem and get to a solution that I believe will work. I'll update as we get into geo install and how we did reaching our goals.

Great!

Make sure to get pictures of the drill for the ground loop. I'm curious what type of drill they're using.