Anyone know of a reference by geographic location for "feet of vertical bore hole depth per ton of heating/cooling"?  Is it a funtion of soil type?  Water table depth?  I've seen numbers quoted ranging from 125'-225' per ton. 

I'm in southern MN and was planning a 12 hole x 200' bore field for my 12 ton load.  Recently I was thinking of adding another structure to this load which would take my heating needs to ~14 ton.  Any thoughts on the 12 x 200' bore field covering this load (@170'/ton) assuming the heat pumps can cover the 14 ton load?

-=Steve=-

the only way to compare the two products is in a labratory.  Even in the field you could not drill two control boreholes the same to do a comparison, they might be close, but we are talking about math here.

  The ph of the water you use to mix the product is paramount to a good end product.  Yet the ground water content is also an issue.  All bentonite hates rackish or saline water.  It is also very difficult to grout a borehole that has been thinned with TSP to breakdown clay platlets from the formation.

For those that do not know, bentonite is just clay platlets.

I believe that increasing the solids content of your grout will improve conductivity, but it should never be used to shorten the footage of loop.

Most manufacturers preach a highly alkiline make up water for mixing grout. a 9 or better.  This adjustment is made inthe field using soda ash.  If you run out of soda ash on a job, most times the rest of the days grout is mixed without it, and it's ability to suspend anything is called into question.

Eric

I'm just 11.5 miles NE of Minneapolis and drilling for my 4 vertical earth loops started Wednesday.  I have a 4 ton WF dual stage Envision unit going in.  My contractor said he'd need four 185' bores to service our loads.

I spoke to the drilling guy Wednesday evening when I got home.  He said they got into some gravel and water which would help our efficiencies.  They had to cancel drilling Thursday because of cold temperatures but will return today and possibly work Saturday to complete the job.  We have a mass of sub-zero air moving in middle of next week.  Contractor says he can only drill when it's 10ºF or above in the morning.

I posted my design data here a few weeks ago:

That's where we currently have our fossil-fueled beast set during hours of primary occupancy.  When it's at a constant 68º, we're comfortable.  If I bump it up to 69º it feels a bit warm.  Likewise, at 67º it feels a bit cool.  I don't need to act like a Homer Simpson and watch TV wearing only my underwear.

Each bidder sized the same size equipment.  Will we throw things totally out of whack if we do bump up the tstat to 70º?

We tend to use a gas fireplace insert in our family room to augment the temp where everyone spends the majority of their time.  Problem with that is this room gets warmer and our tstat is a half level up (we have a 4-level home) and thinks the entire house is that warm so our furnace doesn't cycle on.  When we retire for the night upstairs, our bedrooms are a tad bit on the cold side.