Some Geothermal installers are suggesting 3/4" heat transfer tubing and others 1 1/4".  They both seem to have their valid reasons but would really appreciate some more input.  It makes it hard to make the right decision.

Hi bimbo,
And welcome

The major differance in "tubing size" is friction loss. The larger the tubing the less friction loss you have over a given distance. Less friction loss equals smaller circulator pump to move the fluid. Smaller circulator pump equals a minutia less electric consumed to circulate the fluid that actually does the work.

In some cases in vertical borings the increase in "tubing" size and depth can reduce the number of bores required to meet the objective.

Hope this helps

Frankly, of all the things to worry about when shopping, this is the last item i would expect to clarify. It's not a criticism for asking, so much as a curiousity....
What do the contractor's refrences tell you, what are the operating cost projections.....
I will tell you that 11/4" is very uncommon for a loop field in my area (3/4" is preferred), but different areas have different habits as many area contractors are often trained by the same person.
Good luck,
Joe

Bimbo,

As long as a contractor's design optimizes for both pumping power AND maintaining turbulent flow for good heat transfer tubing size is not an important criterion upon which to judge a system or contractor.

For any given situation there is nearly always more than one 'right' answer.